From 734a61a4aaf2f139f72c183cdcaf0a62416a19f9 Mon Sep 17 00:00:00 2001
From: johnny850807 <johnny850807@gmail.com>
Date: Fri, 6 Nov 2020 00:33:22 +0800
Subject: [PATCH 01/24] :beer: implement DiscoveryClient design

---
 commons/__init__.py                          |   0
 commons/client/ServiceInstance.py            |  90 ++++++
 commons/client/__init__.py                   |   0
 commons/client/discovery/__init__.py         |   0
 commons/client/discovery/composite.py        |  23 ++
 commons/client/discovery/discovery_client.py |  49 +++
 commons/client/loadbalancer/__init__.py      |   0
 commons/client/service_registry/__init__.py  |   0
 commons/utils/__init__.py                    |   0
 commons/utils/functional_operators.py        |  13 +
 commons/utils/list_utils.py                  |   6 +
 context/__init__.py                          |   0
 poetry.lock                                  | 306 +++++++++++++++----
 pyproject.toml                               |   2 +
 src/__init__.py                              |  13 -
 src/stub.py                                  |  17 --
 tests/__init__.py                            |  13 -
 tests/client/__init__.py                     |   0
 tests/client/discovery/__init__.py           |   0
 tests/client/discovery/composite_test.py     |  30 ++
 tests/stub_test.py                           |  20 --
 21 files changed, 465 insertions(+), 117 deletions(-)
 create mode 100644 commons/__init__.py
 create mode 100644 commons/client/ServiceInstance.py
 create mode 100644 commons/client/__init__.py
 create mode 100644 commons/client/discovery/__init__.py
 create mode 100644 commons/client/discovery/composite.py
 create mode 100644 commons/client/discovery/discovery_client.py
 create mode 100644 commons/client/loadbalancer/__init__.py
 create mode 100644 commons/client/service_registry/__init__.py
 create mode 100644 commons/utils/__init__.py
 create mode 100644 commons/utils/functional_operators.py
 create mode 100644 commons/utils/list_utils.py
 create mode 100644 context/__init__.py
 delete mode 100644 src/__init__.py
 delete mode 100644 src/stub.py
 create mode 100644 tests/client/__init__.py
 create mode 100644 tests/client/discovery/__init__.py
 create mode 100644 tests/client/discovery/composite_test.py
 delete mode 100644 tests/stub_test.py

diff --git a/commons/__init__.py b/commons/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/commons/client/ServiceInstance.py b/commons/client/ServiceInstance.py
new file mode 100644
index 0000000..c716605
--- /dev/null
+++ b/commons/client/ServiceInstance.py
@@ -0,0 +1,90 @@
+# standard library
+from abc import ABC, abstractmethod
+from urllib.parse import urlparse
+
+__author__ = "Waterball (johnny850807@gmail.com)"
+__license__ = "Apache 2.0"
+
+
+class ServiceInstance(ABC):
+    @property
+    @abstractmethod
+    def instance_id(self):
+        pass
+
+    @property
+    @abstractmethod
+    def service_id(self):
+        pass
+
+    @property
+    @abstractmethod
+    def host(self):
+        pass
+
+    @property
+    @abstractmethod
+    def port(self):
+        pass
+
+    @property
+    @abstractmethod
+    def secure(self):
+        pass
+
+    @property
+    @abstractmethod
+    def uri(self):
+        pass
+
+    @property
+    @abstractmethod
+    def scheme(self):
+        pass
+
+
+class StaticServiceInstance(ServiceInstance):
+    """
+    A service instance that is initialized with its basic properties
+    """
+
+    def __init__(self, uri, service_id, instance_id):
+        """
+        :param uri: the url in the string type
+        """
+        url_obj = urlparse(uri)
+        self._uri = uri
+        self._scheme = url_obj.scheme
+        self._secure = self._scheme == "https"
+        self._host = url_obj.netloc
+        self._port = url_obj.port
+        self._service_id = service_id
+        self._instance_id = instance_id
+
+    @property
+    def service_id(self):
+        return self._service_id
+
+    @property
+    def instance_id(self):
+        return self._instance_id
+
+    @property
+    def host(self):
+        return self._host
+
+    @property
+    def port(self):
+        return self._port
+
+    @property
+    def secure(self):
+        return self._secure
+
+    @property
+    def uri(self):
+        return self._uri
+
+    @property
+    def scheme(self):
+        return self._scheme
diff --git a/commons/client/__init__.py b/commons/client/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/commons/client/discovery/__init__.py b/commons/client/discovery/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/commons/client/discovery/composite.py b/commons/client/discovery/composite.py
new file mode 100644
index 0000000..cb39845
--- /dev/null
+++ b/commons/client/discovery/composite.py
@@ -0,0 +1,23 @@
+# scip plugin
+from commons.client.discovery.discovery_client import DiscoveryClient
+from commons.utils.functional_operators import flat_map
+from commons.utils.list_utils import not_null_nor_empty
+
+__author__ = "Waterball (johnny850807@gmail.com)"
+__license__ = "Apache 2.0"
+
+
+class CompositeDiscoveryClient(DiscoveryClient):
+    def __init__(self, *discovery_clients):
+        self.discovery_clients = discovery_clients
+
+    def get_instances(self, service_id):
+        for client in self.discovery_clients:
+            services = client.get_instances(service_id)
+            if not_null_nor_empty(services):
+                return services
+        return []
+
+    @property
+    def services(self):
+        return flat_map(lambda d: d.services, self.discovery_clients)
diff --git a/commons/client/discovery/discovery_client.py b/commons/client/discovery/discovery_client.py
new file mode 100644
index 0000000..a3bca2e
--- /dev/null
+++ b/commons/client/discovery/discovery_client.py
@@ -0,0 +1,49 @@
+# standard library
+from abc import ABC, abstractmethod
+
+# scip plugin
+from commons.client.ServiceInstance import StaticServiceInstance
+from commons.utils.functional_operators import filter_get_first
+
+__author__ = "Waterball (johnny850807@gmail.com)"
+__license__ = "Apache 2.0"
+
+
+class DiscoveryClient(ABC):
+    @abstractmethod
+    def get_instances(self, service_id):
+        """
+        Gets all ServiceInstances associated with a particular serviceId.
+        :param service_id: The serviceId to query.
+        :return: A List of ServiceInstance.
+        """
+        pass
+
+    @property
+    @abstractmethod
+    def services(self):
+        """
+        :return: All known service IDs.
+        """
+        pass
+
+
+class StaticDiscoveryClient(DiscoveryClient):
+    """
+    A DiscoveryClient initialized with the services
+    """
+
+    def __init__(self, services):
+        self._services = services
+
+    def get_instances(self, service_id):
+        return list(filter(lambda s: s.service_id == service_id, self.services))
+
+    @property
+    def services(self):
+        return self._services
+
+
+def static_discovery_client(uri, service_id, instance_ids):
+    services = [StaticServiceInstance(uri, service_id, instance_id) for instance_id in instance_ids]
+    return StaticDiscoveryClient(services)
diff --git a/commons/client/loadbalancer/__init__.py b/commons/client/loadbalancer/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/commons/client/service_registry/__init__.py b/commons/client/service_registry/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/commons/utils/__init__.py b/commons/utils/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/commons/utils/functional_operators.py b/commons/utils/functional_operators.py
new file mode 100644
index 0000000..013b851
--- /dev/null
+++ b/commons/utils/functional_operators.py
@@ -0,0 +1,13 @@
+# standard library
+from functools import reduce
+
+__author__ = "Waterball (johnny850807@gmail.com)"
+__license__ = "Apache 2.0"
+
+
+def flat_map(f, xs):
+    return reduce(lambda a, b: a + b, map(f, xs))
+
+
+def filter_get_first(f, the_list):
+    return next(filter(f, the_list), None)
diff --git a/commons/utils/list_utils.py b/commons/utils/list_utils.py
new file mode 100644
index 0000000..6d403ff
--- /dev/null
+++ b/commons/utils/list_utils.py
@@ -0,0 +1,6 @@
+__author__ = "Waterball (johnny850807@gmail.com)"
+__license__ = "Apache 2.0"
+
+
+def not_null_nor_empty(the_list):
+    return the_list is not None and len(the_list) != 0
diff --git a/context/__init__.py b/context/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/poetry.lock b/poetry.lock
index c8880eb..181b4e7 100644
--- a/poetry.lock
+++ b/poetry.lock
@@ -1,54 +1,94 @@
 [[package]]
-category = "dev"
-description = "A small Python module for determining appropriate platform-specific dirs, e.g. a \"user data dir\"."
 name = "appdirs"
+version = "1.4.4"
+description = "A small Python module for determining appropriate platform-specific dirs, e.g. a \"user data dir\"."
+category = "dev"
 optional = false
 python-versions = "*"
-version = "1.4.4"
 
 [[package]]
-category = "dev"
-description = "Validate configuration and produce human readable error messages."
+name = "atomicwrites"
+version = "1.4.0"
+description = "Atomic file writes."
+category = "main"
+optional = false
+python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*"
+
+[[package]]
+name = "attrs"
+version = "20.3.0"
+description = "Classes Without Boilerplate"
+category = "main"
+optional = false
+python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*"
+
+[package.extras]
+docs = ["furo", "sphinx", "zope.interface"]
+tests = ["coverage[toml] (>=5.0.2)", "hypothesis", "pympler", "pytest (>=4.3.0)", "six", "zope.interface"]
+tests_no_zope = ["coverage[toml] (>=5.0.2)", "hypothesis", "pympler", "pytest (>=4.3.0)", "six"]
+dev = ["coverage[toml] (>=5.0.2)", "hypothesis", "pympler", "pytest (>=4.3.0)", "six", "zope.interface", "furo", "sphinx", "pre-commit"]
+
+[[package]]
 name = "cfgv"
+version = "3.2.0"
+description = "Validate configuration and produce human readable error messages."
+category = "dev"
 optional = false
 python-versions = ">=3.6.1"
-version = "3.2.0"
 
 [[package]]
-category = "dev"
-description = "Distribution utilities"
+name = "colorama"
+version = "0.4.4"
+description = "Cross-platform colored terminal text."
+category = "main"
+optional = false
+python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*"
+
+[[package]]
+name = "coverage"
+version = "5.3"
+description = "Code coverage measurement for Python"
+category = "main"
+optional = false
+python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*, <4"
+
+[package.extras]
+toml = ["toml"]
+
+[[package]]
 name = "distlib"
+version = "0.3.1"
+description = "Distribution utilities"
+category = "dev"
 optional = false
 python-versions = "*"
-version = "0.3.1"
 
 [[package]]
-category = "dev"
-description = "A platform independent file lock."
 name = "filelock"
+version = "3.0.12"
+description = "A platform independent file lock."
+category = "dev"
 optional = false
 python-versions = "*"
-version = "3.0.12"
 
 [[package]]
-category = "dev"
-description = "File identification library for Python"
 name = "identify"
+version = "1.5.5"
+description = "File identification library for Python"
+category = "dev"
 optional = false
 python-versions = "!=3.0.*,!=3.1.*,!=3.2.*,!=3.3.*,>=2.7"
-version = "1.5.5"
 
 [package.extras]
 license = ["editdistance"]
 
 [[package]]
-category = "dev"
-description = "Read metadata from Python packages"
-marker = "python_version < \"3.8\""
 name = "importlib-metadata"
+version = "2.0.0"
+description = "Read metadata from Python packages"
+category = "main"
 optional = false
 python-versions = "!=3.0.*,!=3.1.*,!=3.2.*,!=3.3.*,!=3.4.*,>=2.7"
-version = "2.0.0"
 
 [package.dependencies]
 zipp = ">=0.5"
@@ -58,105 +98,235 @@ docs = ["sphinx", "rst.linker"]
 testing = ["packaging", "pep517", "importlib-resources (>=1.3)"]
 
 [[package]]
-category = "dev"
-description = "Node.js virtual environment builder"
-name = "nodeenv"
+name = "iniconfig"
+version = "1.1.1"
+description = "iniconfig: brain-dead simple config-ini parsing"
+category = "main"
 optional = false
 python-versions = "*"
-version = "1.5.0"
 
 [[package]]
+name = "nodeenv"
+version = "1.5.0"
+description = "Node.js virtual environment builder"
 category = "dev"
-description = "A framework for managing and maintaining multi-language pre-commit hooks."
+optional = false
+python-versions = "*"
+
+[[package]]
+name = "packaging"
+version = "20.4"
+description = "Core utilities for Python packages"
+category = "main"
+optional = false
+python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*"
+
+[package.dependencies]
+pyparsing = ">=2.0.2"
+six = "*"
+
+[[package]]
+name = "pluggy"
+version = "0.13.1"
+description = "plugin and hook calling mechanisms for python"
+category = "main"
+optional = false
+python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*"
+
+[package.dependencies]
+importlib-metadata = {version = ">=0.12", markers = "python_version < \"3.8\""}
+
+[package.extras]
+dev = ["pre-commit", "tox"]
+
+[[package]]
 name = "pre-commit"
+version = "2.7.1"
+description = "A framework for managing and maintaining multi-language pre-commit hooks."
+category = "dev"
 optional = false
 python-versions = ">=3.6.1"
-version = "2.7.1"
 
 [package.dependencies]
+virtualenv = ">=20.0.8"
+pyyaml = ">=5.1"
 cfgv = ">=2.0.0"
-identify = ">=1.0.0"
 nodeenv = ">=0.11.1"
-pyyaml = ">=5.1"
+importlib-metadata = {version = "*", markers = "python_version < \"3.8\""}
+identify = ">=1.0.0"
 toml = "*"
-virtualenv = ">=20.0.8"
 
-[package.dependencies.importlib-metadata]
-python = "<3.8"
-version = "*"
+[[package]]
+name = "py"
+version = "1.9.0"
+description = "library with cross-python path, ini-parsing, io, code, log facilities"
+category = "main"
+optional = false
+python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*"
 
 [[package]]
-category = "dev"
-description = "YAML parser and emitter for Python"
-name = "pyyaml"
+name = "pyparsing"
+version = "2.4.7"
+description = "Python parsing module"
+category = "main"
+optional = false
+python-versions = ">=2.6, !=3.0.*, !=3.1.*, !=3.2.*"
+
+[[package]]
+name = "pytest"
+version = "6.1.2"
+description = "pytest: simple powerful testing with Python"
+category = "main"
+optional = false
+python-versions = ">=3.5"
+
+[package.dependencies]
+packaging = "*"
+py = ">=1.8.2"
+iniconfig = "*"
+importlib-metadata = {version = ">=0.12", markers = "python_version < \"3.8\""}
+colorama = {version = "*", markers = "sys_platform == \"win32\""}
+attrs = ">=17.4.0"
+toml = "*"
+pluggy = ">=0.12,<1.0"
+atomicwrites = {version = ">=1.0", markers = "sys_platform == \"win32\""}
+
+[package.extras]
+testing = ["argcomplete", "hypothesis (>=3.56)", "mock", "nose", "requests", "xmlschema"]
+checkqa_mypy = ["mypy (==0.780)"]
+
+[[package]]
+name = "pytest-cov"
+version = "2.10.1"
+description = "Pytest plugin for measuring coverage."
+category = "main"
 optional = false
 python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*"
-version = "5.3.1"
+
+[package.dependencies]
+pytest = ">=4.6"
+coverage = ">=4.4"
+
+[package.extras]
+testing = ["fields", "hunter", "process-tests (==2.0.2)", "six", "pytest-xdist", "virtualenv"]
 
 [[package]]
+name = "pyyaml"
+version = "5.3.1"
+description = "YAML parser and emitter for Python"
 category = "dev"
-description = "Python 2 and 3 compatibility utilities"
+optional = false
+python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*"
+
+[[package]]
 name = "six"
+version = "1.15.0"
+description = "Python 2 and 3 compatibility utilities"
+category = "main"
 optional = false
 python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*"
-version = "1.15.0"
 
 [[package]]
-category = "dev"
-description = "Python Library for Tom's Obvious, Minimal Language"
 name = "toml"
+version = "0.10.1"
+description = "Python Library for Tom's Obvious, Minimal Language"
+category = "main"
 optional = false
 python-versions = "*"
-version = "0.10.1"
 
 [[package]]
-category = "dev"
-description = "Virtual Python Environment builder"
 name = "virtualenv"
+version = "20.0.33"
+description = "Virtual Python Environment builder"
+category = "dev"
 optional = false
 python-versions = "!=3.0.*,!=3.1.*,!=3.2.*,!=3.3.*,>=2.7"
-version = "20.0.33"
 
 [package.dependencies]
-appdirs = ">=1.4.3,<2"
+importlib-metadata = {version = ">=0.12,<3", markers = "python_version < \"3.8\""}
 distlib = ">=0.3.1,<1"
 filelock = ">=3.0.0,<4"
 six = ">=1.9.0,<2"
-
-[package.dependencies.importlib-metadata]
-python = "<3.8"
-version = ">=0.12,<3"
+appdirs = ">=1.4.3,<2"
 
 [package.extras]
 docs = ["proselint (>=0.10.2)", "sphinx (>=3)", "sphinx-argparse (>=0.2.5)", "sphinx-rtd-theme (>=0.4.3)", "towncrier (>=19.9.0rc1)"]
 testing = ["coverage (>=4)", "coverage-enable-subprocess (>=1)", "flaky (>=3)", "pytest (>=4)", "pytest-env (>=0.6.2)", "pytest-freezegun (>=0.4.1)", "pytest-mock (>=2)", "pytest-randomly (>=1)", "pytest-timeout (>=1)", "pytest-xdist (>=1.31.0)", "packaging (>=20.0)", "xonsh (>=0.9.16)"]
 
 [[package]]
-category = "dev"
-description = "Backport of pathlib-compatible object wrapper for zip files"
-marker = "python_version < \"3.8\""
 name = "zipp"
+version = "3.3.0"
+description = "Backport of pathlib-compatible object wrapper for zip files"
+category = "main"
 optional = false
 python-versions = ">=3.6"
-version = "3.3.0"
 
 [package.extras]
 docs = ["sphinx", "jaraco.packaging (>=3.2)", "rst.linker (>=1.9)"]
-testing = ["pytest (>=3.5,<3.7.3 || >3.7.3)", "pytest-checkdocs (>=1.2.3)", "pytest-flake8", "pytest-cov", "jaraco.test (>=3.2.0)", "jaraco.itertools", "func-timeout", "pytest-black (>=0.3.7)", "pytest-mypy"]
+testing = ["pytest (>=3.5,!=3.7.3)", "pytest-checkdocs (>=1.2.3)", "pytest-flake8", "pytest-cov", "jaraco.test (>=3.2.0)", "jaraco.itertools", "func-timeout", "pytest-black (>=0.3.7)", "pytest-mypy"]
 
 [metadata]
-content-hash = "6082dab94ba09f8cac3bf2c2a65ed2ed32c8b385ce34e2a81e683f325ed55e13"
+lock-version = "1.1"
 python-versions = "^3.7"
+content-hash = "1789c0c6dfba8dea8f6c09917f6f30aad10173e904761db257b0dd790dfb5543"
 
 [metadata.files]
 appdirs = [
     {file = "appdirs-1.4.4-py2.py3-none-any.whl", hash = "sha256:a841dacd6b99318a741b166adb07e19ee71a274450e68237b4650ca1055ab128"},
     {file = "appdirs-1.4.4.tar.gz", hash = "sha256:7d5d0167b2b1ba821647616af46a749d1c653740dd0d2415100fe26e27afdf41"},
 ]
+atomicwrites = [
+    {file = "atomicwrites-1.4.0-py2.py3-none-any.whl", hash = "sha256:6d1784dea7c0c8d4a5172b6c620f40b6e4cbfdf96d783691f2e1302a7b88e197"},
+    {file = "atomicwrites-1.4.0.tar.gz", hash = "sha256:ae70396ad1a434f9c7046fd2dd196fc04b12f9e91ffb859164193be8b6168a7a"},
+]
+attrs = [
+    {file = "attrs-20.3.0-py2.py3-none-any.whl", hash = "sha256:31b2eced602aa8423c2aea9c76a724617ed67cf9513173fd3a4f03e3a929c7e6"},
+    {file = "attrs-20.3.0.tar.gz", hash = "sha256:832aa3cde19744e49938b91fea06d69ecb9e649c93ba974535d08ad92164f700"},
+]
 cfgv = [
     {file = "cfgv-3.2.0-py2.py3-none-any.whl", hash = "sha256:32e43d604bbe7896fe7c248a9c2276447dbef840feb28fe20494f62af110211d"},
     {file = "cfgv-3.2.0.tar.gz", hash = "sha256:cf22deb93d4bcf92f345a5c3cd39d3d41d6340adc60c78bbbd6588c384fda6a1"},
 ]
+colorama = [
+    {file = "colorama-0.4.4-py2.py3-none-any.whl", hash = "sha256:9f47eda37229f68eee03b24b9748937c7dc3868f906e8ba69fbcbdd3bc5dc3e2"},
+    {file = "colorama-0.4.4.tar.gz", hash = "sha256:5941b2b48a20143d2267e95b1c2a7603ce057ee39fd88e7329b0c292aa16869b"},
+]
+coverage = [
+    {file = "coverage-5.3-cp27-cp27m-macosx_10_13_intel.whl", hash = "sha256:bd3166bb3b111e76a4f8e2980fa1addf2920a4ca9b2b8ca36a3bc3dedc618270"},
+    {file = "coverage-5.3-cp27-cp27m-macosx_10_9_x86_64.whl", hash = "sha256:9342dd70a1e151684727c9c91ea003b2fb33523bf19385d4554f7897ca0141d4"},
+    {file = "coverage-5.3-cp27-cp27m-manylinux1_i686.whl", hash = "sha256:63808c30b41f3bbf65e29f7280bf793c79f54fb807057de7e5238ffc7cc4d7b9"},
+    {file = "coverage-5.3-cp27-cp27m-manylinux1_x86_64.whl", hash = "sha256:4d6a42744139a7fa5b46a264874a781e8694bb32f1d76d8137b68138686f1729"},
+    {file = "coverage-5.3-cp27-cp27m-win32.whl", hash = "sha256:86e9f8cd4b0cdd57b4ae71a9c186717daa4c5a99f3238a8723f416256e0b064d"},
+    {file = "coverage-5.3-cp27-cp27m-win_amd64.whl", hash = "sha256:7858847f2d84bf6e64c7f66498e851c54de8ea06a6f96a32a1d192d846734418"},
+    {file = "coverage-5.3-cp27-cp27mu-manylinux1_i686.whl", hash = "sha256:530cc8aaf11cc2ac7430f3614b04645662ef20c348dce4167c22d99bec3480e9"},
+    {file = "coverage-5.3-cp27-cp27mu-manylinux1_x86_64.whl", hash = "sha256:381ead10b9b9af5f64646cd27107fb27b614ee7040bb1226f9c07ba96625cbb5"},
+    {file = "coverage-5.3-cp35-cp35m-macosx_10_13_x86_64.whl", hash = "sha256:71b69bd716698fa62cd97137d6f2fdf49f534decb23a2c6fc80813e8b7be6822"},
+    {file = "coverage-5.3-cp35-cp35m-manylinux1_i686.whl", hash = "sha256:1d44bb3a652fed01f1f2c10d5477956116e9b391320c94d36c6bf13b088a1097"},
+    {file = "coverage-5.3-cp35-cp35m-manylinux1_x86_64.whl", hash = "sha256:1c6703094c81fa55b816f5ae542c6ffc625fec769f22b053adb42ad712d086c9"},
+    {file = "coverage-5.3-cp35-cp35m-win32.whl", hash = "sha256:cedb2f9e1f990918ea061f28a0f0077a07702e3819602d3507e2ff98c8d20636"},
+    {file = "coverage-5.3-cp35-cp35m-win_amd64.whl", hash = "sha256:7f43286f13d91a34fadf61ae252a51a130223c52bfefb50310d5b2deb062cf0f"},
+    {file = "coverage-5.3-cp36-cp36m-macosx_10_13_x86_64.whl", hash = "sha256:c851b35fc078389bc16b915a0a7c1d5923e12e2c5aeec58c52f4aa8085ac8237"},
+    {file = "coverage-5.3-cp36-cp36m-manylinux1_i686.whl", hash = "sha256:aac1ba0a253e17889550ddb1b60a2063f7474155465577caa2a3b131224cfd54"},
+    {file = "coverage-5.3-cp36-cp36m-manylinux1_x86_64.whl", hash = "sha256:2b31f46bf7b31e6aa690d4c7a3d51bb262438c6dcb0d528adde446531d0d3bb7"},
+    {file = "coverage-5.3-cp36-cp36m-win32.whl", hash = "sha256:c5f17ad25d2c1286436761b462e22b5020d83316f8e8fcb5deb2b3151f8f1d3a"},
+    {file = "coverage-5.3-cp36-cp36m-win_amd64.whl", hash = "sha256:aef72eae10b5e3116bac6957de1df4d75909fc76d1499a53fb6387434b6bcd8d"},
+    {file = "coverage-5.3-cp37-cp37m-macosx_10_13_x86_64.whl", hash = "sha256:e8caf961e1b1a945db76f1b5fa9c91498d15f545ac0ababbe575cfab185d3bd8"},
+    {file = "coverage-5.3-cp37-cp37m-manylinux1_i686.whl", hash = "sha256:29a6272fec10623fcbe158fdf9abc7a5fa032048ac1d8631f14b50fbfc10d17f"},
+    {file = "coverage-5.3-cp37-cp37m-manylinux1_x86_64.whl", hash = "sha256:2d43af2be93ffbad25dd959899b5b809618a496926146ce98ee0b23683f8c51c"},
+    {file = "coverage-5.3-cp37-cp37m-win32.whl", hash = "sha256:c3888a051226e676e383de03bf49eb633cd39fc829516e5334e69b8d81aae751"},
+    {file = "coverage-5.3-cp37-cp37m-win_amd64.whl", hash = "sha256:9669179786254a2e7e57f0ecf224e978471491d660aaca833f845b72a2df3709"},
+    {file = "coverage-5.3-cp38-cp38-macosx_10_9_x86_64.whl", hash = "sha256:0203acd33d2298e19b57451ebb0bed0ab0c602e5cf5a818591b4918b1f97d516"},
+    {file = "coverage-5.3-cp38-cp38-manylinux1_i686.whl", hash = "sha256:582ddfbe712025448206a5bc45855d16c2e491c2dd102ee9a2841418ac1c629f"},
+    {file = "coverage-5.3-cp38-cp38-manylinux1_x86_64.whl", hash = "sha256:0f313707cdecd5cd3e217fc68c78a960b616604b559e9ea60cc16795c4304259"},
+    {file = "coverage-5.3-cp38-cp38-win32.whl", hash = "sha256:78e93cc3571fd928a39c0b26767c986188a4118edc67bc0695bc7a284da22e82"},
+    {file = "coverage-5.3-cp38-cp38-win_amd64.whl", hash = "sha256:8f264ba2701b8c9f815b272ad568d555ef98dfe1576802ab3149c3629a9f2221"},
+    {file = "coverage-5.3-cp39-cp39-macosx_10_13_x86_64.whl", hash = "sha256:50691e744714856f03a86df3e2bff847c2acede4c191f9a1da38f088df342978"},
+    {file = "coverage-5.3-cp39-cp39-manylinux1_i686.whl", hash = "sha256:9361de40701666b034c59ad9e317bae95c973b9ff92513dd0eced11c6adf2e21"},
+    {file = "coverage-5.3-cp39-cp39-manylinux1_x86_64.whl", hash = "sha256:c1b78fb9700fc961f53386ad2fd86d87091e06ede5d118b8a50dea285a071c24"},
+    {file = "coverage-5.3-cp39-cp39-win32.whl", hash = "sha256:cb7df71de0af56000115eafd000b867d1261f786b5eebd88a0ca6360cccfaca7"},
+    {file = "coverage-5.3-cp39-cp39-win_amd64.whl", hash = "sha256:47a11bdbd8ada9b7ee628596f9d97fbd3851bd9999d398e9436bd67376dbece7"},
+    {file = "coverage-5.3.tar.gz", hash = "sha256:280baa8ec489c4f542f8940f9c4c2181f0306a8ee1a54eceba071a449fb870a0"},
+]
 distlib = [
     {file = "distlib-0.3.1-py2.py3-none-any.whl", hash = "sha256:8c09de2c67b3e7deef7184574fc060ab8a793e7adbb183d942c389c8b13c52fb"},
     {file = "distlib-0.3.1.zip", hash = "sha256:edf6116872c863e1aa9d5bb7cb5e05a022c519a4594dc703843343a9ddd9bff1"},
@@ -173,14 +343,42 @@ importlib-metadata = [
     {file = "importlib_metadata-2.0.0-py2.py3-none-any.whl", hash = "sha256:cefa1a2f919b866c5beb7c9f7b0ebb4061f30a8a9bf16d609b000e2dfaceb9c3"},
     {file = "importlib_metadata-2.0.0.tar.gz", hash = "sha256:77a540690e24b0305878c37ffd421785a6f7e53c8b5720d211b211de8d0e95da"},
 ]
+iniconfig = [
+    {file = "iniconfig-1.1.1-py2.py3-none-any.whl", hash = "sha256:011e24c64b7f47f6ebd835bb12a743f2fbe9a26d4cecaa7f53bc4f35ee9da8b3"},
+    {file = "iniconfig-1.1.1.tar.gz", hash = "sha256:bc3af051d7d14b2ee5ef9969666def0cd1a000e121eaea580d4a313df4b37f32"},
+]
 nodeenv = [
     {file = "nodeenv-1.5.0-py2.py3-none-any.whl", hash = "sha256:5304d424c529c997bc888453aeaa6362d242b6b4631e90f3d4bf1b290f1c84a9"},
     {file = "nodeenv-1.5.0.tar.gz", hash = "sha256:ab45090ae383b716c4ef89e690c41ff8c2b257b85b309f01f3654df3d084bd7c"},
 ]
+packaging = [
+    {file = "packaging-20.4-py2.py3-none-any.whl", hash = "sha256:998416ba6962ae7fbd6596850b80e17859a5753ba17c32284f67bfff33784181"},
+    {file = "packaging-20.4.tar.gz", hash = "sha256:4357f74f47b9c12db93624a82154e9b120fa8293699949152b22065d556079f8"},
+]
+pluggy = [
+    {file = "pluggy-0.13.1-py2.py3-none-any.whl", hash = "sha256:966c145cd83c96502c3c3868f50408687b38434af77734af1e9ca461a4081d2d"},
+    {file = "pluggy-0.13.1.tar.gz", hash = "sha256:15b2acde666561e1298d71b523007ed7364de07029219b604cf808bfa1c765b0"},
+]
 pre-commit = [
     {file = "pre_commit-2.7.1-py2.py3-none-any.whl", hash = "sha256:810aef2a2ba4f31eed1941fc270e72696a1ad5590b9751839c90807d0fff6b9a"},
     {file = "pre_commit-2.7.1.tar.gz", hash = "sha256:c54fd3e574565fe128ecc5e7d2f91279772ddb03f8729645fa812fe809084a70"},
 ]
+py = [
+    {file = "py-1.9.0-py2.py3-none-any.whl", hash = "sha256:366389d1db726cd2fcfc79732e75410e5fe4d31db13692115529d34069a043c2"},
+    {file = "py-1.9.0.tar.gz", hash = "sha256:9ca6883ce56b4e8da7e79ac18787889fa5206c79dcc67fb065376cd2fe03f342"},
+]
+pyparsing = [
+    {file = "pyparsing-2.4.7-py2.py3-none-any.whl", hash = "sha256:ef9d7589ef3c200abe66653d3f1ab1033c3c419ae9b9bdb1240a85b024efc88b"},
+    {file = "pyparsing-2.4.7.tar.gz", hash = "sha256:c203ec8783bf771a155b207279b9bccb8dea02d8f0c9e5f8ead507bc3246ecc1"},
+]
+pytest = [
+    {file = "pytest-6.1.2-py3-none-any.whl", hash = "sha256:4288fed0d9153d9646bfcdf0c0428197dba1ecb27a33bb6e031d002fa88653fe"},
+    {file = "pytest-6.1.2.tar.gz", hash = "sha256:c0a7e94a8cdbc5422a51ccdad8e6f1024795939cc89159a0ae7f0b316ad3823e"},
+]
+pytest-cov = [
+    {file = "pytest-cov-2.10.1.tar.gz", hash = "sha256:47bd0ce14056fdd79f93e1713f88fad7bdcc583dcd7783da86ef2f085a0bb88e"},
+    {file = "pytest_cov-2.10.1-py2.py3-none-any.whl", hash = "sha256:45ec2d5182f89a81fc3eb29e3d1ed3113b9e9a873bcddb2a71faaab066110191"},
+]
 pyyaml = [
     {file = "PyYAML-5.3.1-cp27-cp27m-win32.whl", hash = "sha256:74809a57b329d6cc0fdccee6318f44b9b8649961fa73144a98735b0aaf029f1f"},
     {file = "PyYAML-5.3.1-cp27-cp27m-win_amd64.whl", hash = "sha256:240097ff019d7c70a4922b6869d8a86407758333f02203e0fc6ff79c5dcede76"},
diff --git a/pyproject.toml b/pyproject.toml
index 0594a6b..e642ff9 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -7,6 +7,8 @@ license = "Apache-2.0"
 
 [tool.poetry.dependencies]
 python = "^3.7"
+pytest = "^6.1.2"
+pytest-cov = "^2.10.1"
 
 [tool.poetry.dev-dependencies]
 pre-commit = "^2.7.1"
diff --git a/src/__init__.py b/src/__init__.py
deleted file mode 100644
index b9b3c0c..0000000
--- a/src/__init__.py
+++ /dev/null
@@ -1,13 +0,0 @@
-#  Copyright 2020 Waterball (johnny850807@gmail.com)
-#
-#     Licensed under the Apache License, Version 2.0 (the "License");
-#     you may not use this file except in compliance with the License.
-#     You may obtain a copy of the License at
-#
-#         http://www.apache.org/licenses/LICENSE-2.0
-#
-#     Unless required by applicable law or agreed to in writing, software
-#     distributed under the License is distributed on an "AS IS" BASIS,
-#     WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-#     See the License for the specific language governing permissions and
-#     limitations under the License.
diff --git a/src/stub.py b/src/stub.py
deleted file mode 100644
index 56d99ca..0000000
--- a/src/stub.py
+++ /dev/null
@@ -1,17 +0,0 @@
-#  Copyright 2020 Waterball (johnny850807@gmail.com)
-#
-#     Licensed under the Apache License, Version 2.0 (the "License");
-#     you may not use this file except in compliance with the License.
-#     You may obtain a copy of the License at
-#
-#         http://www.apache.org/licenses/LICENSE-2.0
-#
-#     Unless required by applicable law or agreed to in writing, software
-#     distributed under the License is distributed on an "AS IS" BASIS,
-#     WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-#     See the License for the specific language governing permissions and
-#     limitations under the License.
-
-
-def add(a, b):
-    return a + b
diff --git a/tests/__init__.py b/tests/__init__.py
index b9b3c0c..e69de29 100644
--- a/tests/__init__.py
+++ b/tests/__init__.py
@@ -1,13 +0,0 @@
-#  Copyright 2020 Waterball (johnny850807@gmail.com)
-#
-#     Licensed under the Apache License, Version 2.0 (the "License");
-#     you may not use this file except in compliance with the License.
-#     You may obtain a copy of the License at
-#
-#         http://www.apache.org/licenses/LICENSE-2.0
-#
-#     Unless required by applicable law or agreed to in writing, software
-#     distributed under the License is distributed on an "AS IS" BASIS,
-#     WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-#     See the License for the specific language governing permissions and
-#     limitations under the License.
diff --git a/tests/client/__init__.py b/tests/client/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/tests/client/discovery/__init__.py b/tests/client/discovery/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/tests/client/discovery/composite_test.py b/tests/client/discovery/composite_test.py
new file mode 100644
index 0000000..39a4990
--- /dev/null
+++ b/tests/client/discovery/composite_test.py
@@ -0,0 +1,30 @@
+# scip plugin
+from commons.client.discovery.composite import CompositeDiscoveryClient
+from commons.client.discovery.discovery_client import static_discovery_client
+
+__author__ = "Waterball (johnny850807@gmail.com)"
+__license__ = "Apache"
+
+client = CompositeDiscoveryClient(
+    static_discovery_client("url-1", "service-1", ["1-1", "1-2", "1-3"]),
+    static_discovery_client("url-2", "service-2", ["2-1", "2-2", "2-3"]),
+    static_discovery_client("url-3", "service-3", ["3-1", "3-2", "3-3"]),
+)
+
+
+def test_get_instances():
+    for service_id in range(1, 4):
+        instances = client.get_instances("service-{}".format(service_id))
+        for instance_id in range(1, 4):
+            instance = instances[instance_id - 1]
+            assert instance.instance_id == "{}-{}".format(service_id, instance_id)
+
+
+def test_get_services():
+    instances = client.services
+    i = 0
+    for service_id in range(1, 4):
+        for instance_id in range(1, 4):
+            assert instances[i].service_id == "service-{}".format(service_id)
+            assert instances[i].instance_id == "{}-{}".format(service_id, instance_id)
+            i += 1
diff --git a/tests/stub_test.py b/tests/stub_test.py
deleted file mode 100644
index de90136..0000000
--- a/tests/stub_test.py
+++ /dev/null
@@ -1,20 +0,0 @@
-#  Copyright 2020 Waterball (johnny850807@gmail.com)
-#
-#     Licensed under the Apache License, Version 2.0 (the "License");
-#     you may not use this file except in compliance with the License.
-#     You may obtain a copy of the License at
-#
-#         http://www.apache.org/licenses/LICENSE-2.0
-#
-#     Unless required by applicable law or agreed to in writing, software
-#     distributed under the License is distributed on an "AS IS" BASIS,
-#     WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-#     See the License for the specific language governing permissions and
-#     limitations under the License.
-
-# scip plugin
-from src import stub
-
-
-def test():
-    assert 10 == stub.add(4, 6)

From a94c3b3d41b661a615da3bf0c20e668494f954cd Mon Sep 17 00:00:00 2001
From: johnny850807 <johnny850807@gmail.com>
Date: Fri, 6 Nov 2020 01:06:38 +0800
Subject: [PATCH 02/24] Add more comments

---
 commons/client/discovery/composite.py        | 4 ++++
 commons/client/discovery/discovery_client.py | 4 ++++
 2 files changed, 8 insertions(+)

diff --git a/commons/client/discovery/composite.py b/commons/client/discovery/composite.py
index cb39845..ddc3432 100644
--- a/commons/client/discovery/composite.py
+++ b/commons/client/discovery/composite.py
@@ -8,6 +8,10 @@
 
 
 class CompositeDiscoveryClient(DiscoveryClient):
+    """
+    Composite pattern application: aggregate the service sources from a list of discovery client
+    """
+
     def __init__(self, *discovery_clients):
         self.discovery_clients = discovery_clients
 
diff --git a/commons/client/discovery/discovery_client.py b/commons/client/discovery/discovery_client.py
index a3bca2e..c5fb607 100644
--- a/commons/client/discovery/discovery_client.py
+++ b/commons/client/discovery/discovery_client.py
@@ -45,5 +45,9 @@ def services(self):
 
 
 def static_discovery_client(uri, service_id, instance_ids):
+    """
+    Usage:
+        static_discovery_client("url-1", "service-1", ["id-1", "id-2", "id-3"])
+    """
     services = [StaticServiceInstance(uri, service_id, instance_id) for instance_id in instance_ids]
     return StaticDiscoveryClient(services)

From fc2dbb4ee569ee7257ab1064d6b1340ae301e7f1 Mon Sep 17 00:00:00 2001
From: johnny850807 <johnny850807@gmail.com>
Date: Fri, 6 Nov 2020 11:32:19 +0800
Subject: [PATCH 03/24] Modify some functional operation concerning performance

---
 README.md                                |  6 +++++
 commons/client/ServiceInstance.py        |  4 +++
 commons/client/discovery/composite.py    |  4 +--
 commons/utils/functional_operators.py    |  7 ++++--
 commons/utils/list_utils.py              |  7 +++++-
 poetry.lock                              | 32 ++++++++++++------------
 pyproject.toml                           |  4 +--
 tests/client/discovery/composite_test.py |  6 ++---
 tests/utils/__init__.py                  |  2 ++
 tests/utils/functional_operators_test.py | 17 +++++++++++++
 10 files changed, 63 insertions(+), 26 deletions(-)
 create mode 100644 tests/utils/__init__.py
 create mode 100644 tests/utils/functional_operators_test.py

diff --git a/README.md b/README.md
index 36e8225..bf1e281 100644
--- a/README.md
+++ b/README.md
@@ -49,3 +49,9 @@ poetry run pre-commit install
 ```
 
 6. Start to commit :)
+
+## Test
+
+1. Show test coverage report on terminal
+
+`poetry run pytest --cov-report term --cov ${package} tests/`
diff --git a/commons/client/ServiceInstance.py b/commons/client/ServiceInstance.py
index c716605..0edc3ba 100644
--- a/commons/client/ServiceInstance.py
+++ b/commons/client/ServiceInstance.py
@@ -7,6 +7,10 @@
 
 
 class ServiceInstance(ABC):
+    """
+
+    """
+
     @property
     @abstractmethod
     def instance_id(self):
diff --git a/commons/client/discovery/composite.py b/commons/client/discovery/composite.py
index ddc3432..c3be2fa 100644
--- a/commons/client/discovery/composite.py
+++ b/commons/client/discovery/composite.py
@@ -1,7 +1,7 @@
 # scip plugin
 from commons.client.discovery.discovery_client import DiscoveryClient
 from commons.utils.functional_operators import flat_map
-from commons.utils.list_utils import not_null_nor_empty
+from commons.utils.list_utils import not_none_nor_empty
 
 __author__ = "Waterball (johnny850807@gmail.com)"
 __license__ = "Apache 2.0"
@@ -18,7 +18,7 @@ def __init__(self, *discovery_clients):
     def get_instances(self, service_id):
         for client in self.discovery_clients:
             services = client.get_instances(service_id)
-            if not_null_nor_empty(services):
+            if not_none_nor_empty(services):
                 return services
         return []
 
diff --git a/commons/utils/functional_operators.py b/commons/utils/functional_operators.py
index 013b851..54463f6 100644
--- a/commons/utils/functional_operators.py
+++ b/commons/utils/functional_operators.py
@@ -6,8 +6,11 @@
 
 
 def flat_map(f, xs):
-    return reduce(lambda a, b: a + b, map(f, xs))
+    results = []
+    for element in xs:
+        results += f(element)
+    return results
 
 
 def filter_get_first(f, the_list):
-    return next(filter(f, the_list), None)
+    return [x for x in the_list if f(x)][0]
diff --git a/commons/utils/list_utils.py b/commons/utils/list_utils.py
index 6d403ff..8e2c8e3 100644
--- a/commons/utils/list_utils.py
+++ b/commons/utils/list_utils.py
@@ -2,5 +2,10 @@
 __license__ = "Apache 2.0"
 
 
-def not_null_nor_empty(the_list):
+def not_none_nor_empty(the_list):
+    """
+    :param the_list: a list
+    :return: true if the list is neither none nor empty
+    """
+    assert not the_list or isinstance(the_list, list)
     return the_list is not None and len(the_list) != 0
diff --git a/poetry.lock b/poetry.lock
index 181b4e7..26d957a 100644
--- a/poetry.lock
+++ b/poetry.lock
@@ -10,7 +10,7 @@ python-versions = "*"
 name = "atomicwrites"
 version = "1.4.0"
 description = "Atomic file writes."
-category = "main"
+category = "dev"
 optional = false
 python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*"
 
@@ -18,7 +18,7 @@ python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*"
 name = "attrs"
 version = "20.3.0"
 description = "Classes Without Boilerplate"
-category = "main"
+category = "dev"
 optional = false
 python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*"
 
@@ -40,7 +40,7 @@ python-versions = ">=3.6.1"
 name = "colorama"
 version = "0.4.4"
 description = "Cross-platform colored terminal text."
-category = "main"
+category = "dev"
 optional = false
 python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*"
 
@@ -48,7 +48,7 @@ python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*"
 name = "coverage"
 version = "5.3"
 description = "Code coverage measurement for Python"
-category = "main"
+category = "dev"
 optional = false
 python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*, <4"
 
@@ -86,7 +86,7 @@ license = ["editdistance"]
 name = "importlib-metadata"
 version = "2.0.0"
 description = "Read metadata from Python packages"
-category = "main"
+category = "dev"
 optional = false
 python-versions = "!=3.0.*,!=3.1.*,!=3.2.*,!=3.3.*,!=3.4.*,>=2.7"
 
@@ -101,7 +101,7 @@ testing = ["packaging", "pep517", "importlib-resources (>=1.3)"]
 name = "iniconfig"
 version = "1.1.1"
 description = "iniconfig: brain-dead simple config-ini parsing"
-category = "main"
+category = "dev"
 optional = false
 python-versions = "*"
 
@@ -117,7 +117,7 @@ python-versions = "*"
 name = "packaging"
 version = "20.4"
 description = "Core utilities for Python packages"
-category = "main"
+category = "dev"
 optional = false
 python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*"
 
@@ -129,7 +129,7 @@ six = "*"
 name = "pluggy"
 version = "0.13.1"
 description = "plugin and hook calling mechanisms for python"
-category = "main"
+category = "dev"
 optional = false
 python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*"
 
@@ -160,7 +160,7 @@ toml = "*"
 name = "py"
 version = "1.9.0"
 description = "library with cross-python path, ini-parsing, io, code, log facilities"
-category = "main"
+category = "dev"
 optional = false
 python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*"
 
@@ -168,7 +168,7 @@ python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*"
 name = "pyparsing"
 version = "2.4.7"
 description = "Python parsing module"
-category = "main"
+category = "dev"
 optional = false
 python-versions = ">=2.6, !=3.0.*, !=3.1.*, !=3.2.*"
 
@@ -176,7 +176,7 @@ python-versions = ">=2.6, !=3.0.*, !=3.1.*, !=3.2.*"
 name = "pytest"
 version = "6.1.2"
 description = "pytest: simple powerful testing with Python"
-category = "main"
+category = "dev"
 optional = false
 python-versions = ">=3.5"
 
@@ -199,7 +199,7 @@ checkqa_mypy = ["mypy (==0.780)"]
 name = "pytest-cov"
 version = "2.10.1"
 description = "Pytest plugin for measuring coverage."
-category = "main"
+category = "dev"
 optional = false
 python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*"
 
@@ -222,7 +222,7 @@ python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*, !=3.4.*"
 name = "six"
 version = "1.15.0"
 description = "Python 2 and 3 compatibility utilities"
-category = "main"
+category = "dev"
 optional = false
 python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*"
 
@@ -230,7 +230,7 @@ python-versions = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*"
 name = "toml"
 version = "0.10.1"
 description = "Python Library for Tom's Obvious, Minimal Language"
-category = "main"
+category = "dev"
 optional = false
 python-versions = "*"
 
@@ -257,7 +257,7 @@ testing = ["coverage (>=4)", "coverage-enable-subprocess (>=1)", "flaky (>=3)",
 name = "zipp"
 version = "3.3.0"
 description = "Backport of pathlib-compatible object wrapper for zip files"
-category = "main"
+category = "dev"
 optional = false
 python-versions = ">=3.6"
 
@@ -268,7 +268,7 @@ testing = ["pytest (>=3.5,!=3.7.3)", "pytest-checkdocs (>=1.2.3)", "pytest-flake
 [metadata]
 lock-version = "1.1"
 python-versions = "^3.7"
-content-hash = "1789c0c6dfba8dea8f6c09917f6f30aad10173e904761db257b0dd790dfb5543"
+content-hash = "190192c42431e322402cdda9bf4820d92afcffbaca203bb403e552118ff6eb7c"
 
 [metadata.files]
 appdirs = [
diff --git a/pyproject.toml b/pyproject.toml
index e642ff9..de09a11 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -7,11 +7,11 @@ license = "Apache-2.0"
 
 [tool.poetry.dependencies]
 python = "^3.7"
-pytest = "^6.1.2"
-pytest-cov = "^2.10.1"
 
 [tool.poetry.dev-dependencies]
 pre-commit = "^2.7.1"
+pytest = "^6.1.2"
+pytest-cov = "^2.10.1"
 
 [build-system]
 requires = ["poetry>=0.12"]
diff --git a/tests/client/discovery/composite_test.py b/tests/client/discovery/composite_test.py
index 39a4990..758c4e3 100644
--- a/tests/client/discovery/composite_test.py
+++ b/tests/client/discovery/composite_test.py
@@ -17,7 +17,7 @@ def test_get_instances():
         instances = client.get_instances("service-{}".format(service_id))
         for instance_id in range(1, 4):
             instance = instances[instance_id - 1]
-            assert instance.instance_id == "{}-{}".format(service_id, instance_id)
+            assert "{}-{}".format(service_id, instance_id) == instance.instance_id
 
 
 def test_get_services():
@@ -25,6 +25,6 @@ def test_get_services():
     i = 0
     for service_id in range(1, 4):
         for instance_id in range(1, 4):
-            assert instances[i].service_id == "service-{}".format(service_id)
-            assert instances[i].instance_id == "{}-{}".format(service_id, instance_id)
+            assert "service-{}".format(service_id) == instances[i].service_id
+            assert "{}-{}".format(service_id, instance_id) == instances[i].instance_id
             i += 1
diff --git a/tests/utils/__init__.py b/tests/utils/__init__.py
new file mode 100644
index 0000000..d7b1237
--- /dev/null
+++ b/tests/utils/__init__.py
@@ -0,0 +1,2 @@
+__author__ = "Waterball (johnny850807@gmail.com)"
+__license__ = "Apache 2.0"
diff --git a/tests/utils/functional_operators_test.py b/tests/utils/functional_operators_test.py
new file mode 100644
index 0000000..cf40349
--- /dev/null
+++ b/tests/utils/functional_operators_test.py
@@ -0,0 +1,17 @@
+# scip plugin
+from commons.utils.functional_operators import filter_get_first, flat_map
+
+__author__ = "Waterball (johnny850807@gmail.com)"
+__license__ = "Apache 2.0"
+
+
+def test_flat_map():
+    the_list = ["a,b,c", "1,2,3", ""]
+    results = flat_map(lambda e: e.split(","), the_list)
+    assert ["a", "b", "c", "1", "2", "3", ""] == results
+
+
+def test_filter_get_first():
+    the_list = [1, 2, 3, 4, 5, 6, 7, 8]
+    result = filter_get_first(lambda e: e > 4, the_list)
+    assert 5 == result

From 02c1d470bcf74f2c5aac9e9287307fa744c748f5 Mon Sep 17 00:00:00 2001
From: johnny850807 <johnny850807@gmail.com>
Date: Fri, 6 Nov 2020 11:54:22 +0800
Subject: [PATCH 04/24] :memo: add typing and docstrings

---
 commons/__init__.py                          |  1 +
 commons/client/ServiceInstance.py            | 11 +++++++----
 commons/client/__init__.py                   |  1 +
 commons/client/discovery/__init__.py         |  1 +
 commons/client/discovery/composite.py        | 14 ++++++++++----
 commons/client/discovery/discovery_client.py | 18 +++++++++++++++---
 commons/client/loadbalancer/__init__.py      |  1 +
 commons/client/service_registry/__init__.py  |  1 +
 commons/utils/__init__.py                    |  1 +
 commons/utils/functional_operators.py        | 17 +++++++++++++----
 commons/utils/list_utils.py                  |  2 ++
 context/__init__.py                          |  1 +
 tests/__init__.py                            |  1 +
 tests/client/__init__.py                     |  1 +
 tests/client/discovery/__init__.py           |  1 +
 tests/client/discovery/composite_test.py     |  8 +++++---
 16 files changed, 62 insertions(+), 18 deletions(-)

diff --git a/commons/__init__.py b/commons/__init__.py
index e69de29..40a96af 100644
--- a/commons/__init__.py
+++ b/commons/__init__.py
@@ -0,0 +1 @@
+# -*- coding: utf-8 -*-
diff --git a/commons/client/ServiceInstance.py b/commons/client/ServiceInstance.py
index 0edc3ba..1d2f701 100644
--- a/commons/client/ServiceInstance.py
+++ b/commons/client/ServiceInstance.py
@@ -1,14 +1,17 @@
-# standard library
-from abc import ABC, abstractmethod
-from urllib.parse import urlparse
+# -*- coding: utf-8 -*-
 
 __author__ = "Waterball (johnny850807@gmail.com)"
 __license__ = "Apache 2.0"
 
 
+# standard library
+from abc import ABC, abstractmethod
+from urllib.parse import urlparse
+
+
 class ServiceInstance(ABC):
     """
-
+    A service's instance that owns basic HTTP info.
     """
 
     @property
diff --git a/commons/client/__init__.py b/commons/client/__init__.py
index e69de29..40a96af 100644
--- a/commons/client/__init__.py
+++ b/commons/client/__init__.py
@@ -0,0 +1 @@
+# -*- coding: utf-8 -*-
diff --git a/commons/client/discovery/__init__.py b/commons/client/discovery/__init__.py
index e69de29..40a96af 100644
--- a/commons/client/discovery/__init__.py
+++ b/commons/client/discovery/__init__.py
@@ -0,0 +1 @@
+# -*- coding: utf-8 -*-
diff --git a/commons/client/discovery/composite.py b/commons/client/discovery/composite.py
index c3be2fa..7bc38b0 100644
--- a/commons/client/discovery/composite.py
+++ b/commons/client/discovery/composite.py
@@ -1,18 +1,24 @@
+# -*- coding: utf-8 -*-
+
+__author__ = "Waterball (johnny850807@gmail.com)"
+__license__ = "Apache 2.0"
+
 # scip plugin
 from commons.client.discovery.discovery_client import DiscoveryClient
 from commons.utils.functional_operators import flat_map
 from commons.utils.list_utils import not_none_nor_empty
 
-__author__ = "Waterball (johnny850807@gmail.com)"
-__license__ = "Apache 2.0"
-
 
 class CompositeDiscoveryClient(DiscoveryClient):
     """
-    Composite pattern application: aggregate the service sources from a list of discovery client
+    Composite pattern application:
+        aggregate the service sources from a list of discovery client
     """
 
     def __init__(self, *discovery_clients):
+        """
+        :param discovery_clients: a list of DiscoveryClient
+        """
         self.discovery_clients = discovery_clients
 
     def get_instances(self, service_id):
diff --git a/commons/client/discovery/discovery_client.py b/commons/client/discovery/discovery_client.py
index c5fb607..6606b9e 100644
--- a/commons/client/discovery/discovery_client.py
+++ b/commons/client/discovery/discovery_client.py
@@ -1,3 +1,11 @@
+# -*- coding: utf-8 -*-
+"""
+DiscoveryClient is responsible for providing a list of service's instances.
+There may be many services, for example: 'user-service', 'order-service', 'product-service', and so on.
+Where each service may have several instances registered (e.g. user-service-1, user-service-2, ...),
+and the registered instances are called ServiceInstances.
+"""
+
 # standard library
 from abc import ABC, abstractmethod
 
@@ -14,7 +22,7 @@ class DiscoveryClient(ABC):
     def get_instances(self, service_id):
         """
         Gets all ServiceInstances associated with a particular serviceId.
-        :param service_id: The serviceId to query.
+        :param service_id: (str) The serviceId to query.
         :return: A List of ServiceInstance.
         """
         pass
@@ -23,14 +31,14 @@ def get_instances(self, service_id):
     @abstractmethod
     def services(self):
         """
-        :return: All known service IDs.
+        :return: All known service IDs (*str).
         """
         pass
 
 
 class StaticDiscoveryClient(DiscoveryClient):
     """
-    A DiscoveryClient initialized with the services
+    A DiscoveryClient initialized with the services.
     """
 
     def __init__(self, services):
@@ -46,6 +54,10 @@ def services(self):
 
 def static_discovery_client(uri, service_id, instance_ids):
     """
+    A helper method that helps create a list of instances from the same service.
+    :param uri the uri of every service's
+    :param service_id the service_id of the service
+    :param instance_ids a list of ids (*str) of the instances'
     Usage:
         static_discovery_client("url-1", "service-1", ["id-1", "id-2", "id-3"])
     """
diff --git a/commons/client/loadbalancer/__init__.py b/commons/client/loadbalancer/__init__.py
index e69de29..40a96af 100644
--- a/commons/client/loadbalancer/__init__.py
+++ b/commons/client/loadbalancer/__init__.py
@@ -0,0 +1 @@
+# -*- coding: utf-8 -*-
diff --git a/commons/client/service_registry/__init__.py b/commons/client/service_registry/__init__.py
index e69de29..40a96af 100644
--- a/commons/client/service_registry/__init__.py
+++ b/commons/client/service_registry/__init__.py
@@ -0,0 +1 @@
+# -*- coding: utf-8 -*-
diff --git a/commons/utils/__init__.py b/commons/utils/__init__.py
index e69de29..40a96af 100644
--- a/commons/utils/__init__.py
+++ b/commons/utils/__init__.py
@@ -0,0 +1 @@
+# -*- coding: utf-8 -*-
diff --git a/commons/utils/functional_operators.py b/commons/utils/functional_operators.py
index 54463f6..084fb45 100644
--- a/commons/utils/functional_operators.py
+++ b/commons/utils/functional_operators.py
@@ -1,16 +1,25 @@
-# standard library
-from functools import reduce
+# -*- coding: utf-8 -*-
 
 __author__ = "Waterball (johnny850807@gmail.com)"
 __license__ = "Apache 2.0"
 
 
 def flat_map(f, xs):
+    """
+    :param f: a mapping function
+    :param xs: (Iterable)
+    :return: list
+    """
     results = []
     for element in xs:
         results += f(element)
     return results
 
 
-def filter_get_first(f, the_list):
-    return [x for x in the_list if f(x)][0]
+def filter_get_first(f, xs):
+    """
+    :param f: a mapping function
+    :param xs: (Iterable)
+    :return: an element
+    """
+    return [x for x in xs if f(x)][0]
diff --git a/commons/utils/list_utils.py b/commons/utils/list_utils.py
index 8e2c8e3..5185483 100644
--- a/commons/utils/list_utils.py
+++ b/commons/utils/list_utils.py
@@ -1,3 +1,5 @@
+# -*- coding: utf-8 -*-
+
 __author__ = "Waterball (johnny850807@gmail.com)"
 __license__ = "Apache 2.0"
 
diff --git a/context/__init__.py b/context/__init__.py
index e69de29..40a96af 100644
--- a/context/__init__.py
+++ b/context/__init__.py
@@ -0,0 +1 @@
+# -*- coding: utf-8 -*-
diff --git a/tests/__init__.py b/tests/__init__.py
index e69de29..40a96af 100644
--- a/tests/__init__.py
+++ b/tests/__init__.py
@@ -0,0 +1 @@
+# -*- coding: utf-8 -*-
diff --git a/tests/client/__init__.py b/tests/client/__init__.py
index e69de29..40a96af 100644
--- a/tests/client/__init__.py
+++ b/tests/client/__init__.py
@@ -0,0 +1 @@
+# -*- coding: utf-8 -*-
diff --git a/tests/client/discovery/__init__.py b/tests/client/discovery/__init__.py
index e69de29..40a96af 100644
--- a/tests/client/discovery/__init__.py
+++ b/tests/client/discovery/__init__.py
@@ -0,0 +1 @@
+# -*- coding: utf-8 -*-
diff --git a/tests/client/discovery/composite_test.py b/tests/client/discovery/composite_test.py
index 758c4e3..f9eb46d 100644
--- a/tests/client/discovery/composite_test.py
+++ b/tests/client/discovery/composite_test.py
@@ -1,10 +1,12 @@
+# -*- coding: utf-8 -*-
+
+__author__ = "Waterball (johnny850807@gmail.com)"
+__license__ = "Apache 2.0"
+
 # scip plugin
 from commons.client.discovery.composite import CompositeDiscoveryClient
 from commons.client.discovery.discovery_client import static_discovery_client
 
-__author__ = "Waterball (johnny850807@gmail.com)"
-__license__ = "Apache"
-
 client = CompositeDiscoveryClient(
     static_discovery_client("url-1", "service-1", ["1-1", "1-2", "1-3"]),
     static_discovery_client("url-2", "service-2", ["2-1", "2-2", "2-3"]),

From 617c03eefe2a53a363fc2f89a55caaee817571c5 Mon Sep 17 00:00:00 2001
From: johnny850807 <johnny850807@gmail.com>
Date: Fri, 6 Nov 2020 12:31:09 +0800
Subject: [PATCH 05/24] Make some fields private

---
 commons/client/discovery/composite.py        | 6 +++---
 commons/client/discovery/discovery_client.py | 4 ++--
 2 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/commons/client/discovery/composite.py b/commons/client/discovery/composite.py
index 7bc38b0..bd4ecb2 100644
--- a/commons/client/discovery/composite.py
+++ b/commons/client/discovery/composite.py
@@ -19,10 +19,10 @@ def __init__(self, *discovery_clients):
         """
         :param discovery_clients: a list of DiscoveryClient
         """
-        self.discovery_clients = discovery_clients
+        self.__discovery_clients = discovery_clients
 
     def get_instances(self, service_id):
-        for client in self.discovery_clients:
+        for client in self.__discovery_clients:
             services = client.get_instances(service_id)
             if not_none_nor_empty(services):
                 return services
@@ -30,4 +30,4 @@ def get_instances(self, service_id):
 
     @property
     def services(self):
-        return flat_map(lambda d: d.services, self.discovery_clients)
+        return flat_map(lambda d: d.services, self.__discovery_clients)
diff --git a/commons/client/discovery/discovery_client.py b/commons/client/discovery/discovery_client.py
index 6606b9e..f013fc7 100644
--- a/commons/client/discovery/discovery_client.py
+++ b/commons/client/discovery/discovery_client.py
@@ -42,14 +42,14 @@ class StaticDiscoveryClient(DiscoveryClient):
     """
 
     def __init__(self, services):
-        self._services = services
+        self.__services = services
 
     def get_instances(self, service_id):
         return list(filter(lambda s: s.service_id == service_id, self.services))
 
     @property
     def services(self):
-        return self._services
+        return self.__services
 
 
 def static_discovery_client(uri, service_id, instance_ids):

From cdfa9a726b5ef5607d090be8038cf48f2ea09ea4 Mon Sep 17 00:00:00 2001
From: johnny850807 <johnny850807@gmail.com>
Date: Fri, 6 Nov 2020 12:32:27 +0800
Subject: [PATCH 06/24] Fix naming

---
 commons/client/discovery/discovery_client.py               | 2 +-
 commons/client/{ServiceInstance.py => service_instance.py} | 0
 2 files changed, 1 insertion(+), 1 deletion(-)
 rename commons/client/{ServiceInstance.py => service_instance.py} (100%)

diff --git a/commons/client/discovery/discovery_client.py b/commons/client/discovery/discovery_client.py
index f013fc7..dc94e94 100644
--- a/commons/client/discovery/discovery_client.py
+++ b/commons/client/discovery/discovery_client.py
@@ -10,7 +10,7 @@
 from abc import ABC, abstractmethod
 
 # scip plugin
-from commons.client.ServiceInstance import StaticServiceInstance
+from commons.client.service_instance import StaticServiceInstance
 from commons.utils.functional_operators import filter_get_first
 
 __author__ = "Waterball (johnny850807@gmail.com)"
diff --git a/commons/client/ServiceInstance.py b/commons/client/service_instance.py
similarity index 100%
rename from commons/client/ServiceInstance.py
rename to commons/client/service_instance.py

From 70c8aa1867d7524ec364088edf38a15de901cb45 Mon Sep 17 00:00:00 2001
From: johnny850807 <johnny850807@gmail.com>
Date: Fri, 6 Nov 2020 14:57:32 +0800
Subject: [PATCH 07/24] pending discussion: next+filter vs list comprehension

---
 commons/utils/functional_operators.py | 6 +++---
 commons/utils/list_utils.py           | 1 -
 2 files changed, 3 insertions(+), 4 deletions(-)

diff --git a/commons/utils/functional_operators.py b/commons/utils/functional_operators.py
index 084fb45..386d3da 100644
--- a/commons/utils/functional_operators.py
+++ b/commons/utils/functional_operators.py
@@ -18,8 +18,8 @@ def flat_map(f, xs):
 
 def filter_get_first(f, xs):
     """
-    :param f: a mapping function
-    :param xs: (Iterable)
+    :param f: a predicate function that returns a boolean
+    :param xs: (Iterable
     :return: an element
     """
-    return [x for x in xs if f(x)][0]
+    return next(filter(f, xs), None)
diff --git a/commons/utils/list_utils.py b/commons/utils/list_utils.py
index 5185483..044008d 100644
--- a/commons/utils/list_utils.py
+++ b/commons/utils/list_utils.py
@@ -9,5 +9,4 @@ def not_none_nor_empty(the_list):
     :param the_list: a list
     :return: true if the list is neither none nor empty
     """
-    assert not the_list or isinstance(the_list, list)
     return the_list is not None and len(the_list) != 0

From 41a8c648439aece6803d950ec53bcb73507fed13 Mon Sep 17 00:00:00 2001
From: johnny850807 <johnny850807@gmail.com>
Date: Fri, 6 Nov 2020 20:22:58 +0800
Subject: [PATCH 08/24] :art: Add type checking syntax

---
 commons/client/discovery/composite.py        | 16 ++++++-----
 commons/client/discovery/discovery_client.py | 21 +++++++-------
 commons/client/service_instance.py           | 30 ++++++++++----------
 commons/utils/functional_operators.py        | 17 ++++++-----
 commons/utils/list_utils.py                  |  3 +-
 tests/client/discovery/composite_test.py     | 13 +++------
 6 files changed, 48 insertions(+), 52 deletions(-)

diff --git a/commons/client/discovery/composite.py b/commons/client/discovery/composite.py
index bd4ecb2..5541ec0 100644
--- a/commons/client/discovery/composite.py
+++ b/commons/client/discovery/composite.py
@@ -1,4 +1,9 @@
 # -*- coding: utf-8 -*-
+# standard library
+from typing import List, Set
+
+# scip plugin
+from commons.client.service_instance import ServiceInstance
 
 __author__ = "Waterball (johnny850807@gmail.com)"
 __license__ = "Apache 2.0"
@@ -15,13 +20,10 @@ class CompositeDiscoveryClient(DiscoveryClient):
         aggregate the service sources from a list of discovery client
     """
 
-    def __init__(self, *discovery_clients):
-        """
-        :param discovery_clients: a list of DiscoveryClient
-        """
+    def __init__(self, *discovery_clients: DiscoveryClient):
         self.__discovery_clients = discovery_clients
 
-    def get_instances(self, service_id):
+    def get_instances(self, service_id: str) -> List[ServiceInstance]:
         for client in self.__discovery_clients:
             services = client.get_instances(service_id)
             if not_none_nor_empty(services):
@@ -29,5 +31,5 @@ def get_instances(self, service_id):
         return []
 
     @property
-    def services(self):
-        return flat_map(lambda d: d.services, self.__discovery_clients)
+    def services(self) -> Set[str]:
+        return set(flat_map(lambda d: d.services, self.__discovery_clients))
diff --git a/commons/client/discovery/discovery_client.py b/commons/client/discovery/discovery_client.py
index dc94e94..d65fa78 100644
--- a/commons/client/discovery/discovery_client.py
+++ b/commons/client/discovery/discovery_client.py
@@ -8,9 +8,10 @@
 
 # standard library
 from abc import ABC, abstractmethod
+from typing import List, Set
 
 # scip plugin
-from commons.client.service_instance import StaticServiceInstance
+from commons.client.service_instance import ServiceInstance, StaticServiceInstance
 from commons.utils.functional_operators import filter_get_first
 
 __author__ = "Waterball (johnny850807@gmail.com)"
@@ -19,7 +20,7 @@
 
 class DiscoveryClient(ABC):
     @abstractmethod
-    def get_instances(self, service_id):
+    def get_instances(self, service_id: str) -> List[ServiceInstance]:
         """
         Gets all ServiceInstances associated with a particular serviceId.
         :param service_id: (str) The serviceId to query.
@@ -29,7 +30,7 @@ def get_instances(self, service_id):
 
     @property
     @abstractmethod
-    def services(self):
+    def services(self) -> List[str]:
         """
         :return: All known service IDs (*str).
         """
@@ -41,18 +42,18 @@ class StaticDiscoveryClient(DiscoveryClient):
     A DiscoveryClient initialized with the services.
     """
 
-    def __init__(self, services):
-        self.__services = services
+    def __init__(self, instances: List[ServiceInstance]):
+        self.__instances = instances
 
-    def get_instances(self, service_id):
-        return list(filter(lambda s: s.service_id == service_id, self.services))
+    def get_instances(self, service_id: str) -> List[ServiceInstance]:
+        return list(filter(lambda s: s.service_id == service_id, self.__instances))
 
     @property
-    def services(self):
-        return self.__services
+    def services(self) -> Set[str]:
+        return {s.service_id for s in self.__instances}
 
 
-def static_discovery_client(uri, service_id, instance_ids):
+def static_discovery_client(uri: str, service_id: str, instance_ids: List[str]) -> StaticDiscoveryClient:
     """
     A helper method that helps create a list of instances from the same service.
     :param uri the uri of every service's
diff --git a/commons/client/service_instance.py b/commons/client/service_instance.py
index 1d2f701..a7103e6 100644
--- a/commons/client/service_instance.py
+++ b/commons/client/service_instance.py
@@ -16,37 +16,37 @@ class ServiceInstance(ABC):
 
     @property
     @abstractmethod
-    def instance_id(self):
+    def instance_id(self) -> str:
         pass
 
     @property
     @abstractmethod
-    def service_id(self):
+    def service_id(self) -> str:
         pass
 
     @property
     @abstractmethod
-    def host(self):
+    def host(self) -> str:
         pass
 
     @property
     @abstractmethod
-    def port(self):
+    def port(self) -> int:
         pass
 
     @property
     @abstractmethod
-    def secure(self):
+    def secure(self) -> bool:
         pass
 
     @property
     @abstractmethod
-    def uri(self):
+    def uri(self) -> str:
         pass
 
     @property
     @abstractmethod
-    def scheme(self):
+    def scheme(self) -> str:
         pass
 
 
@@ -55,7 +55,7 @@ class StaticServiceInstance(ServiceInstance):
     A service instance that is initialized with its basic properties
     """
 
-    def __init__(self, uri, service_id, instance_id):
+    def __init__(self, uri: str, service_id: str, instance_id: str):
         """
         :param uri: the url in the string type
         """
@@ -69,29 +69,29 @@ def __init__(self, uri, service_id, instance_id):
         self._instance_id = instance_id
 
     @property
-    def service_id(self):
+    def service_id(self) -> str:
         return self._service_id
 
     @property
-    def instance_id(self):
+    def instance_id(self) -> str:
         return self._instance_id
 
     @property
-    def host(self):
+    def host(self) -> str:
         return self._host
 
     @property
-    def port(self):
+    def port(self) -> int:
         return self._port
 
     @property
-    def secure(self):
+    def secure(self) -> bool:
         return self._secure
 
     @property
-    def uri(self):
+    def uri(self) -> str:
         return self._uri
 
     @property
-    def scheme(self):
+    def scheme(self) -> str:
         return self._scheme
diff --git a/commons/utils/functional_operators.py b/commons/utils/functional_operators.py
index 386d3da..78ac357 100644
--- a/commons/utils/functional_operators.py
+++ b/commons/utils/functional_operators.py
@@ -1,25 +1,24 @@
 # -*- coding: utf-8 -*-
+# standard library
+from typing import Iterable
 
 __author__ = "Waterball (johnny850807@gmail.com)"
 __license__ = "Apache 2.0"
 
 
-def flat_map(f, xs):
+def flat_map(f, xs: Iterable):
     """
     :param f: a mapping function
     :param xs: (Iterable)
     :return: list
     """
-    results = []
-    for element in xs:
-        results += f(element)
-    return results
+    return [item for element in xs for item in f(element)]
 
 
-def filter_get_first(f, xs):
+def filter_get_first(f, xs: Iterable):
     """
     :param f: a predicate function that returns a boolean
-    :param xs: (Iterable
-    :return: an element
+    :param xs: (Iterable)
+    :return: the first element matches the predicate
     """
-    return next(filter(f, xs), None)
+    return [x for x in xs if f(x)][0]
diff --git a/commons/utils/list_utils.py b/commons/utils/list_utils.py
index 044008d..f9433f3 100644
--- a/commons/utils/list_utils.py
+++ b/commons/utils/list_utils.py
@@ -4,9 +4,8 @@
 __license__ = "Apache 2.0"
 
 
-def not_none_nor_empty(the_list):
+def not_none_nor_empty(the_list: list):
     """
-    :param the_list: a list
     :return: true if the list is neither none nor empty
     """
     return the_list is not None and len(the_list) != 0
diff --git a/tests/client/discovery/composite_test.py b/tests/client/discovery/composite_test.py
index f9eb46d..c1fbaed 100644
--- a/tests/client/discovery/composite_test.py
+++ b/tests/client/discovery/composite_test.py
@@ -16,17 +16,12 @@
 
 def test_get_instances():
     for service_id in range(1, 4):
-        instances = client.get_instances("service-{}".format(service_id))
+        instances = client.get_instances(f"service-{service_id}")
         for instance_id in range(1, 4):
             instance = instances[instance_id - 1]
-            assert "{}-{}".format(service_id, instance_id) == instance.instance_id
+            assert f"{service_id}-{instance_id}" == instance.instance_id
 
 
 def test_get_services():
-    instances = client.services
-    i = 0
-    for service_id in range(1, 4):
-        for instance_id in range(1, 4):
-            assert "service-{}".format(service_id) == instances[i].service_id
-            assert "{}-{}".format(service_id, instance_id) == instances[i].instance_id
-            i += 1
+    services = client.services
+    assert {"service-1", "service-2", "service-3"} == services

From b05986444dd82e2347761c9d4bbb0e7865031fe3 Mon Sep 17 00:00:00 2001
From: johnny850807 <johnny850807@gmail.com>
Date: Fri, 6 Nov 2020 20:41:38 +0800
Subject: [PATCH 09/24] improve filter_get_first utils

---
 commons/utils/functional_operators.py    | 2 +-
 tests/utils/functional_operators_test.py | 8 +++++++-
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/commons/utils/functional_operators.py b/commons/utils/functional_operators.py
index 78ac357..9c04da7 100644
--- a/commons/utils/functional_operators.py
+++ b/commons/utils/functional_operators.py
@@ -21,4 +21,4 @@ def filter_get_first(f, xs: Iterable):
     :param xs: (Iterable)
     :return: the first element matches the predicate
     """
-    return [x for x in xs if f(x)][0]
+    return next((x for x in xs if f(x)), None)
diff --git a/tests/utils/functional_operators_test.py b/tests/utils/functional_operators_test.py
index cf40349..8aa999f 100644
--- a/tests/utils/functional_operators_test.py
+++ b/tests/utils/functional_operators_test.py
@@ -11,7 +11,13 @@ def test_flat_map():
     assert ["a", "b", "c", "1", "2", "3", ""] == results
 
 
-def test_filter_get_first():
+def test_filter_get_first_Given_numbers():
     the_list = [1, 2, 3, 4, 5, 6, 7, 8]
     result = filter_get_first(lambda e: e > 4, the_list)
     assert 5 == result
+
+
+def test_filter_get_first_Given_empty_list():
+    the_list = []
+    result = filter_get_first(lambda e: e > 4, the_list)
+    assert result is None

From 3115215526a941f0558a03f3f3ef522f1672ddaf Mon Sep 17 00:00:00 2001
From: johnny850807 <johnny850807@gmail.com>
Date: Fri, 6 Nov 2020 12:49:39 +0800
Subject: [PATCH 10/24] :sparkles: FixedServiceInstanceListSupplier completed

---
 .../client/loadbalancer/supplier/__init__.py  |  1 +
 .../service_instance_list_supplier.py         | 58 +++++++++++++++++++
 commons/client/service_instance.py            | 15 ++++-
 tests/client/loadbalancer/__init__.py         |  1 +
 .../client/loadbalancer/supplier/__init__.py  |  1 +
 .../service_instance_list_supplier_test.py    | 19 ++++++
 6 files changed, 94 insertions(+), 1 deletion(-)
 create mode 100644 commons/client/loadbalancer/supplier/__init__.py
 create mode 100644 commons/client/loadbalancer/supplier/service_instance_list_supplier.py
 create mode 100644 tests/client/loadbalancer/__init__.py
 create mode 100644 tests/client/loadbalancer/supplier/__init__.py
 create mode 100644 tests/client/loadbalancer/supplier/service_instance_list_supplier_test.py

diff --git a/commons/client/loadbalancer/supplier/__init__.py b/commons/client/loadbalancer/supplier/__init__.py
new file mode 100644
index 0000000..40a96af
--- /dev/null
+++ b/commons/client/loadbalancer/supplier/__init__.py
@@ -0,0 +1 @@
+# -*- coding: utf-8 -*-
diff --git a/commons/client/loadbalancer/supplier/service_instance_list_supplier.py b/commons/client/loadbalancer/supplier/service_instance_list_supplier.py
new file mode 100644
index 0000000..203f92d
--- /dev/null
+++ b/commons/client/loadbalancer/supplier/service_instance_list_supplier.py
@@ -0,0 +1,58 @@
+# -*- coding: utf-8 -*-
+"""
+Since the load-balancer is responsible for choosing one instance
+per service request from a list of instances. We need a ServiceInstanceListSupplier for
+each service to decouple the source of the instances from load-balancers.
+"""
+# standard library
+from abc import ABC, abstractmethod
+
+__author__ = "Waterball (johnny850807@gmail.com)"
+__license__ = "Apache 2.0"
+
+
+class ServiceInstanceListSupplier(ABC):
+    """
+    Non-Reactive version of ServiceInstanceListSupplier.
+    (Spring Cloud implement the supplier in the reactive way, means that
+    its supplier returns an Observable which broadcasts the instances on every change.)
+
+    We may consider to adopt reactive programming in the future.
+    """
+
+    @property
+    @abstractmethod
+    def service_id(self):
+        """
+        :return: (str) the service's id
+        """
+        pass
+
+    @abstractmethod
+    def get(self, request=None):
+        """
+        :param request (opt) TODO not sure will we need this,
+                        this extension was designed by spring-cloud.
+        """
+        pass
+
+
+class FixedServiceInstanceListSupplier(ServiceInstanceListSupplier):
+    """
+    A supplier that is initialized with fixed instances. (i.e. they won't be changed)
+    """
+
+    def __init__(self, service_id, instances):
+        """
+        :param service_id: (str)
+        :param instances: (*ServiceInstance)
+        """
+        self._service_id = service_id
+        self._instances = instances
+
+    def get(self, request=None):
+        return self._instances
+
+    @property
+    def service_id(self):
+        return self._service_id
diff --git a/commons/client/service_instance.py b/commons/client/service_instance.py
index a7103e6..4951070 100644
--- a/commons/client/service_instance.py
+++ b/commons/client/service_instance.py
@@ -3,7 +3,6 @@
 __author__ = "Waterball (johnny850807@gmail.com)"
 __license__ = "Apache 2.0"
 
-
 # standard library
 from abc import ABC, abstractmethod
 from urllib.parse import urlparse
@@ -95,3 +94,17 @@ def uri(self) -> str:
     @property
     def scheme(self) -> str:
         return self._scheme
+
+    def __eq__(self, o):
+        if isinstance(o, ServiceInstance):
+            return (
+                self.uri == o.uri
+                and self.service_id == o.service_id
+                and self.instance_id == o.instance_id
+                and self.host == o.host
+                and self.port == o.port
+                and self.secure == o.secure
+                and self.scheme == o.scheme
+            )
+
+        return False
diff --git a/tests/client/loadbalancer/__init__.py b/tests/client/loadbalancer/__init__.py
new file mode 100644
index 0000000..40a96af
--- /dev/null
+++ b/tests/client/loadbalancer/__init__.py
@@ -0,0 +1 @@
+# -*- coding: utf-8 -*-
diff --git a/tests/client/loadbalancer/supplier/__init__.py b/tests/client/loadbalancer/supplier/__init__.py
new file mode 100644
index 0000000..40a96af
--- /dev/null
+++ b/tests/client/loadbalancer/supplier/__init__.py
@@ -0,0 +1 @@
+# -*- coding: utf-8 -*-
diff --git a/tests/client/loadbalancer/supplier/service_instance_list_supplier_test.py b/tests/client/loadbalancer/supplier/service_instance_list_supplier_test.py
new file mode 100644
index 0000000..5bb5610
--- /dev/null
+++ b/tests/client/loadbalancer/supplier/service_instance_list_supplier_test.py
@@ -0,0 +1,19 @@
+# -*- coding: utf-8 -*-
+# scip plugin
+from commons.client.loadbalancer.supplier.service_instance_list_supplier import FixedServiceInstanceListSupplier
+from commons.client.service_instance import StaticServiceInstance
+
+__author__ = "Waterball (johnny850807@gmail.com)"
+__license__ = "Apache 2.0"
+
+SERVICE_ID = "serviceId"
+instances = [StaticServiceInstance("uri", SERVICE_ID, instance_id) for instance_id in ["1", "2", "3"]]
+supplier = FixedServiceInstanceListSupplier(SERVICE_ID, instances)
+
+
+def test_get_service_id():
+    assert SERVICE_ID == supplier.service_id
+
+
+def test_get_instances():
+    assert instances == supplier.get()

From 560fe21f89fbc81f83b18dd27c5e0a9a9b76415a Mon Sep 17 00:00:00 2001
From: johnny850807 <johnny850807@gmail.com>
Date: Fri, 6 Nov 2020 16:28:42 +0800
Subject: [PATCH 11/24] :sparkles: CachingServiceInstanceListSupplier completed

---
 .../client/loadbalancer/supplier/decorator.py | 35 ++++++++++
 .../service_instance_list_supplier.py         | 25 +++++++
 {context => external}/__init__.py             |  0
 external/cache/__init__.py                    |  1 +
 external/cache/cache_manager.py               | 68 +++++++++++++++++++
 poetry.lock                                   | 20 +++++-
 pyproject.toml                                |  1 +
 .../loadbalancer/supplier/decorator_test.py   | 23 +++++++
 .../service_instance_list_supplier_test.py    | 36 +++++++---
 tests/client/loadbalancer/supplier/stubs.py   | 10 +++
 10 files changed, 209 insertions(+), 10 deletions(-)
 create mode 100644 commons/client/loadbalancer/supplier/decorator.py
 rename {context => external}/__init__.py (100%)
 create mode 100644 external/cache/__init__.py
 create mode 100644 external/cache/cache_manager.py
 create mode 100644 tests/client/loadbalancer/supplier/decorator_test.py
 create mode 100644 tests/client/loadbalancer/supplier/stubs.py

diff --git a/commons/client/loadbalancer/supplier/decorator.py b/commons/client/loadbalancer/supplier/decorator.py
new file mode 100644
index 0000000..1f84079
--- /dev/null
+++ b/commons/client/loadbalancer/supplier/decorator.py
@@ -0,0 +1,35 @@
+# -*- coding: utf-8 -*-
+__author__ = "Waterball (johnny850807@gmail.com)"
+__license__ = "Apache 2.0"
+# standard library
+from abc import ABC
+
+# scip plugin
+from commons.client.loadbalancer.supplier.service_instance_list_supplier import ServiceInstanceListSupplier
+from external.cache.cache_manager import CacheManager
+
+
+class DelegatingServiceInstanceListSupplier(ServiceInstanceListSupplier, ABC):
+    """
+    An application of decorator pattern that adds behaviors to ServiceInstanceListSupplier.
+    The decorators should inherit this class.
+    """
+
+    def __init__(self, delegate: ServiceInstanceListSupplier):
+        assert delegate is not None
+        self.delegate = delegate
+
+    @property
+    def service_id(self):
+        return self.delegate.service_id
+
+
+class CachingServiceInstanceListSupplier(DelegatingServiceInstanceListSupplier):
+    CACHE_NAME = "CacheKey"
+
+    def __init__(self, cache_manager: CacheManager, delegate: ServiceInstanceListSupplier):
+        super().__init__(delegate)
+        self.__cache_manager = cache_manager
+
+    def get(self, request=None):
+        return self.__cache_manager.get(self.CACHE_NAME).on_cache_miss(self.delegate.get)
diff --git a/commons/client/loadbalancer/supplier/service_instance_list_supplier.py b/commons/client/loadbalancer/supplier/service_instance_list_supplier.py
index 203f92d..8b9aa12 100644
--- a/commons/client/loadbalancer/supplier/service_instance_list_supplier.py
+++ b/commons/client/loadbalancer/supplier/service_instance_list_supplier.py
@@ -7,6 +7,9 @@
 # standard library
 from abc import ABC, abstractmethod
 
+# scip plugin
+from commons.client.discovery.discovery_client import DiscoveryClient
+
 __author__ = "Waterball (johnny850807@gmail.com)"
 __license__ = "Apache 2.0"
 
@@ -33,6 +36,7 @@ def get(self, request=None):
         """
         :param request (opt) TODO not sure will we need this,
                         this extension was designed by spring-cloud.
+        :return: (*ServiceInstance) a list of instances
         """
         pass
 
@@ -56,3 +60,24 @@ def get(self, request=None):
     @property
     def service_id(self):
         return self._service_id
+
+
+class DiscoveryClientServiceInstanceListSupplier(ServiceInstanceListSupplier):
+    """
+    The adapter delegating to discovery client for querying instances
+    """
+
+    def __init__(self, service_id, discovery_client):
+        """
+        :param service_id: (str)
+        :param discovery_client: (DiscoveryClient)
+        """
+        self.__service_id = service_id
+        self.__delegate = discovery_client
+
+    @property
+    def service_id(self):
+        return self.__service_id
+
+    def get(self, request=None):
+        return self.__delegate.get_instances()
diff --git a/context/__init__.py b/external/__init__.py
similarity index 100%
rename from context/__init__.py
rename to external/__init__.py
diff --git a/external/cache/__init__.py b/external/cache/__init__.py
new file mode 100644
index 0000000..40a96af
--- /dev/null
+++ b/external/cache/__init__.py
@@ -0,0 +1 @@
+# -*- coding: utf-8 -*-
diff --git a/external/cache/cache_manager.py b/external/cache/cache_manager.py
new file mode 100644
index 0000000..595826b
--- /dev/null
+++ b/external/cache/cache_manager.py
@@ -0,0 +1,68 @@
+# -*- coding: utf-8 -*-
+
+"""
+A cache manager wrapper that supports some syntax sugar.
+
+Usage:
+    value = cache_manager.get(cache_key) \
+                .on_cache_miss(lambda: retrieve_value(key))
+"""
+
+
+__author__ = "Waterball (johnny850807@gmail.com)"
+__license__ = "Apache 2.0"
+
+# standard library
+from abc import ABC, abstractmethod
+
+
+class OnCacheMiss:
+    def __init__(self, cache_manager, key, value):
+        self.__cache_manager = cache_manager
+        self.__key = key
+        self.__value = value
+
+    @abstractmethod
+    def on_cache_miss(self, cache_miss_func):
+        """
+        :param cache_miss_func: (lambda ()->value)
+        """
+        if not self.__value:
+            value = cache_miss_func()
+            self.__cache_manager.put(self.__key, value)
+            return value
+        return self.__value
+
+
+class CacheManager(ABC):
+    """
+    Service Provider Interface (SPI) for basic caching.
+    We might want to extend this class with many features in the future.
+    """
+
+    def get(self, key) -> OnCacheMiss:
+        value = self.retrieve_value(key)
+        return OnCacheMiss(self, key, value)
+
+    @abstractmethod
+    def retrieve_value(self, key):
+        pass
+
+    @abstractmethod
+    def put(self, key, value):
+        pass
+
+
+class NaiveCacheManager(CacheManager):
+    """
+    A very simple cache implementation that is not concerned about evict-and-replacement.
+    """
+
+    def __init__(self):
+        self.cache_dict = {}
+
+    def retrieve_value(self, key):
+        return self.cache_dict.get(key, None)
+
+    def put(self, key, value):
+        self.cache_dict[key] = value
diff --git a/poetry.lock b/poetry.lock
index 26d957a..d6b5c99 100644
--- a/poetry.lock
+++ b/poetry.lock
@@ -210,6 +210,20 @@ coverage = ">=4.4"
 [package.extras]
 testing = ["fields", "hunter", "process-tests (==2.0.2)", "six", "pytest-xdist", "virtualenv"]
 
+[[package]]
+name = "pytest-mock"
+version = "3.3.1"
+description = "Thin-wrapper around the mock package for easier use with pytest"
+category = "dev"
+optional = false
+python-versions = ">=3.5"
+
+[package.dependencies]
+pytest = ">=5.0"
+
+[package.extras]
+dev = ["pre-commit", "tox", "pytest-asyncio"]
+
 [[package]]
 name = "pyyaml"
 version = "5.3.1"
@@ -268,7 +282,7 @@ testing = ["pytest (>=3.5,!=3.7.3)", "pytest-checkdocs (>=1.2.3)", "pytest-flake
 [metadata]
 lock-version = "1.1"
 python-versions = "^3.7"
-content-hash = "190192c42431e322402cdda9bf4820d92afcffbaca203bb403e552118ff6eb7c"
+content-hash = "64c77a7ee3f4c43d2487b8f572e71cdfbadc7d90d31c9b204739cc426db78ba2"
 
 [metadata.files]
 appdirs = [
@@ -379,6 +393,10 @@ pytest-cov = [
     {file = "pytest-cov-2.10.1.tar.gz", hash = "sha256:47bd0ce14056fdd79f93e1713f88fad7bdcc583dcd7783da86ef2f085a0bb88e"},
     {file = "pytest_cov-2.10.1-py2.py3-none-any.whl", hash = "sha256:45ec2d5182f89a81fc3eb29e3d1ed3113b9e9a873bcddb2a71faaab066110191"},
 ]
+pytest-mock = [
+    {file = "pytest-mock-3.3.1.tar.gz", hash = "sha256:a4d6d37329e4a893e77d9ffa89e838dd2b45d5dc099984cf03c703ac8411bb82"},
+    {file = "pytest_mock-3.3.1-py3-none-any.whl", hash = "sha256:024e405ad382646318c4281948aadf6fe1135632bea9cc67366ea0c4098ef5f2"},
+]
 pyyaml = [
     {file = "PyYAML-5.3.1-cp27-cp27m-win32.whl", hash = "sha256:74809a57b329d6cc0fdccee6318f44b9b8649961fa73144a98735b0aaf029f1f"},
     {file = "PyYAML-5.3.1-cp27-cp27m-win_amd64.whl", hash = "sha256:240097ff019d7c70a4922b6869d8a86407758333f02203e0fc6ff79c5dcede76"},
diff --git a/pyproject.toml b/pyproject.toml
index de09a11..beb3e95 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -12,6 +12,7 @@ python = "^3.7"
 pre-commit = "^2.7.1"
 pytest = "^6.1.2"
 pytest-cov = "^2.10.1"
+pytest-mock = "^3.3.1"
 
 [build-system]
 requires = ["poetry>=0.12"]
diff --git a/tests/client/loadbalancer/supplier/decorator_test.py b/tests/client/loadbalancer/supplier/decorator_test.py
new file mode 100644
index 0000000..8f03098
--- /dev/null
+++ b/tests/client/loadbalancer/supplier/decorator_test.py
@@ -0,0 +1,23 @@
+# -*- coding: utf-8 -*-
+# standard library
+from unittest.mock import Mock
+
+# scip plugin
+from commons.client.loadbalancer.supplier.decorator import CachingServiceInstanceListSupplier
+from external.cache.cache_manager import NaiveCacheManager
+from tests.client.loadbalancer.supplier.stubs import INSTANCES
+
+__author__ = "Waterball (johnny850807@gmail.com)"
+__license__ = "Apache 2.0"
+
+
+class TestCachingServiceInstanceListSupplier:
+    def setup_class(self):
+        self.delegate = Mock()
+        self.delegate.get = Mock(return_value=INSTANCES)
+        self.supplier = CachingServiceInstanceListSupplier(NaiveCacheManager(), self.delegate)
+
+    def test_Given_cache_When_10_invocations_Then_only_1_cache_miss_and_delegate(self):
+        for i in range(1, 10):
+            self.supplier.get()
+        assert self.delegate.get.call_count == 1
diff --git a/tests/client/loadbalancer/supplier/service_instance_list_supplier_test.py b/tests/client/loadbalancer/supplier/service_instance_list_supplier_test.py
index 5bb5610..ea8ce93 100644
--- a/tests/client/loadbalancer/supplier/service_instance_list_supplier_test.py
+++ b/tests/client/loadbalancer/supplier/service_instance_list_supplier_test.py
@@ -1,19 +1,37 @@
 # -*- coding: utf-8 -*-
+# standard library
+from unittest.mock import Mock
+
 # scip plugin
-from commons.client.loadbalancer.supplier.service_instance_list_supplier import FixedServiceInstanceListSupplier
-from commons.client.service_instance import StaticServiceInstance
+from commons.client.loadbalancer.supplier.service_instance_list_supplier import (
+    DiscoveryClientServiceInstanceListSupplier,
+    FixedServiceInstanceListSupplier,
+)
+from tests.client.loadbalancer.supplier.stubs import INSTANCES, SERVICE_ID
 
 __author__ = "Waterball (johnny850807@gmail.com)"
 __license__ = "Apache 2.0"
 
-SERVICE_ID = "serviceId"
-instances = [StaticServiceInstance("uri", SERVICE_ID, instance_id) for instance_id in ["1", "2", "3"]]
-supplier = FixedServiceInstanceListSupplier(SERVICE_ID, instances)
+
+class TestFixedServiceInstanceListSupplier:
+    def setup_class(self):
+        self.supplier = FixedServiceInstanceListSupplier(SERVICE_ID, INSTANCES)
+
+    def test_get_service_id(self):
+        assert SERVICE_ID == self.supplier.service_id
+
+    def test_get_instances(self):
+        assert INSTANCES == self.supplier.get()
 
 
-def test_get_service_id():
-    assert SERVICE_ID == supplier.service_id
+class TestDiscoveryClientServiceInstanceListSupplier:
+    def setup_class(self):
+        self.discovery_client = Mock()
+        self.discovery_client.get_instances = Mock(return_value=INSTANCES)
+        self.supplier = DiscoveryClientServiceInstanceListSupplier(SERVICE_ID, self.discovery_client)
 
+    def test_get_service_id(self):
+        assert SERVICE_ID == self.supplier.service_id
 
-def test_get_instances():
-    assert instances == supplier.get()
+    def test_get_instances(self):
+        assert INSTANCES == self.supplier.get()
diff --git a/tests/client/loadbalancer/supplier/stubs.py b/tests/client/loadbalancer/supplier/stubs.py
new file mode 100644
index 0000000..3269dab
--- /dev/null
+++ b/tests/client/loadbalancer/supplier/stubs.py
@@ -0,0 +1,10 @@
+# -*- coding: utf-8 -*-
+# scip plugin
+from commons.client.service_instance import StaticServiceInstance
+
+__author__ = "Waterball (johnny850807@gmail.com)"
+__license__ = "Apache 2.0"
+
+
+SERVICE_ID = "serviceId"
+INSTANCES = [StaticServiceInstance("uri", SERVICE_ID, instance_id) for instance_id in ["1", "2", "3"]]

From 72d9118577d41acd60d6c774088f0372ffd5107b Mon Sep 17 00:00:00 2001
From: johnny850807 <johnny850807@gmail.com>
Date: Fri, 6 Nov 2020 20:24:25 +0800
Subject: [PATCH 12/24] :art: Add type checking syntax

---
 .../client/loadbalancer/supplier/decorator.py | 10 ++++++++--
 .../service_instance_list_supplier.py         | 20 ++++++++++---------
 external/cache/cache_manager.py               |  3 ++-
 3 files changed, 21 insertions(+), 12 deletions(-)

diff --git a/commons/client/loadbalancer/supplier/decorator.py b/commons/client/loadbalancer/supplier/decorator.py
index 1f84079..c6873da 100644
--- a/commons/client/loadbalancer/supplier/decorator.py
+++ b/commons/client/loadbalancer/supplier/decorator.py
@@ -1,4 +1,10 @@
 # -*- coding: utf-8 -*-
+# standard library
+from typing import List
+
+# scip plugin
+from commons.client.service_instance import ServiceInstance
+
 __author__ = "Waterball (johnny850807@gmail.com)"
 __license__ = "Apache 2.0"
 # standard library
@@ -20,7 +26,7 @@ def __init__(self, delegate: ServiceInstanceListSupplier):
         self.delegate = delegate
 
     @property
-    def service_id(self):
+    def service_id(self) -> str:
         return self.delegate.service_id
 
 
@@ -31,5 +37,5 @@ def __init__(self, cache_manager: CacheManager, delegate: ServiceInstanceListSup
         super().__init__(delegate)
         self.__cache_manager = cache_manager
 
-    def get(self, request=None):
+    def get(self, request=None) -> List[ServiceInstance]:
         return self.__cache_manager.get(self.CACHE_NAME).on_cache_miss(self.delegate.get)
diff --git a/commons/client/loadbalancer/supplier/service_instance_list_supplier.py b/commons/client/loadbalancer/supplier/service_instance_list_supplier.py
index 8b9aa12..6c046e7 100644
--- a/commons/client/loadbalancer/supplier/service_instance_list_supplier.py
+++ b/commons/client/loadbalancer/supplier/service_instance_list_supplier.py
@@ -6,9 +6,11 @@
 """
 # standard library
 from abc import ABC, abstractmethod
+from typing import List
 
 # scip plugin
 from commons.client.discovery.discovery_client import DiscoveryClient
+from commons.client.service_instance import ServiceInstance
 
 __author__ = "Waterball (johnny850807@gmail.com)"
 __license__ = "Apache 2.0"
@@ -25,14 +27,14 @@ class ServiceInstanceListSupplier(ABC):
 
     @property
     @abstractmethod
-    def service_id(self):
+    def service_id(self) -> str:
         """
         :return: (str) the service's id
         """
         pass
 
     @abstractmethod
-    def get(self, request=None):
+    def get(self, request=None) -> List[ServiceInstance]:
         """
         :param request (opt) TODO not sure will we need this,
                         this extension was designed by spring-cloud.
@@ -46,7 +48,7 @@ class FixedServiceInstanceListSupplier(ServiceInstanceListSupplier):
     A supplier that is initialized with fixed instances. (i.e. they won't be changed)
     """
 
-    def __init__(self, service_id, instances):
+    def __init__(self, service_id: str, instances: List[ServiceInstance]):
         """
         :param service_id: (str)
         :param instances: (*ServiceInstance)
@@ -54,11 +56,11 @@ def __init__(self, service_id, instances):
         self._service_id = service_id
         self._instances = instances
 
-    def get(self, request=None):
+    def get(self, request=None) -> List[ServiceInstance]:
         return self._instances
 
     @property
-    def service_id(self):
+    def service_id(self) -> str:
         return self._service_id
 
 
@@ -67,7 +69,7 @@ class DiscoveryClientServiceInstanceListSupplier(ServiceInstanceListSupplier):
     The adapter delegating to discovery client for querying instances
     """
 
-    def __init__(self, service_id, discovery_client):
+    def __init__(self, service_id: str, discovery_client: DiscoveryClient):
         """
         :param service_id: (str)
         :param discovery_client: (DiscoveryClient)
@@ -76,8 +78,8 @@ def __init__(self, service_id, discovery_client):
         self.__delegate = discovery_client
 
     @property
-    def service_id(self):
+    def service_id(self) -> str:
         return self.__service_id
 
-    def get(self, request=None):
-        return self.__delegate.get_instances()
+    def get(self, request=None) -> List[ServiceInstance]:
+        return self.__delegate.get_instances(self.service_id)
diff --git a/external/cache/cache_manager.py b/external/cache/cache_manager.py
index 595826b..65a5862 100644
--- a/external/cache/cache_manager.py
+++ b/external/cache/cache_manager.py
@@ -38,6 +38,7 @@ class CacheManager(ABC):
     """
     Service Provider Interface (SPI) for basic caching.
     We might want to extend this class with many features in the future.
+    (e.g. timeout, evict-and-replacement)
     """
 
     def get(self, key) -> OnCacheMiss:
@@ -55,7 +56,7 @@ def put(self, key, value):
 
 class NaiveCacheManager(CacheManager):
     """
-    A very simple cache implementation that is not concerned about evict-and-replacement.
+    A very simple cache implementation.
     """
 
     def __init__(self):

From 464b9dd832247c2b11528a1658f93aac647e798d Mon Sep 17 00:00:00 2001
From: johnny850807 <johnny850807@gmail.com>
Date: Fri, 6 Nov 2020 21:13:41 +0800
Subject: [PATCH 13/24] :art: Add NoneTypeError and some type checking

---
 .../client/loadbalancer/supplier/decorator.py   |  3 ++-
 commons/client/service_instance.py              |  2 +-
 commons/exceptions/__init__.py                  |  7 +++++++
 commons/exceptions/primitive.py                 | 17 +++++++++++++++++
 4 files changed, 27 insertions(+), 2 deletions(-)
 create mode 100644 commons/exceptions/__init__.py
 create mode 100644 commons/exceptions/primitive.py

diff --git a/commons/client/loadbalancer/supplier/decorator.py b/commons/client/loadbalancer/supplier/decorator.py
index c6873da..6fe9566 100644
--- a/commons/client/loadbalancer/supplier/decorator.py
+++ b/commons/client/loadbalancer/supplier/decorator.py
@@ -4,6 +4,7 @@
 
 # scip plugin
 from commons.client.service_instance import ServiceInstance
+from commons.exceptions.primitive import NoneTypeError
 
 __author__ = "Waterball (johnny850807@gmail.com)"
 __license__ = "Apache 2.0"
@@ -22,7 +23,7 @@ class DelegatingServiceInstanceListSupplier(ServiceInstanceListSupplier, ABC):
     """
 
     def __init__(self, delegate: ServiceInstanceListSupplier):
-        assert delegate is not None
+        NoneTypeError.raise_if_none(delegate)
         self.delegate = delegate
 
     @property
diff --git a/commons/client/service_instance.py b/commons/client/service_instance.py
index 4951070..ad7acfa 100644
--- a/commons/client/service_instance.py
+++ b/commons/client/service_instance.py
@@ -95,7 +95,7 @@ def uri(self) -> str:
     def scheme(self) -> str:
         return self._scheme
 
-    def __eq__(self, o):
+    def __eq__(self, o: object) -> bool:
         if isinstance(o, ServiceInstance):
             return (
                 self.uri == o.uri
diff --git a/commons/exceptions/__init__.py b/commons/exceptions/__init__.py
new file mode 100644
index 0000000..e59d4cf
--- /dev/null
+++ b/commons/exceptions/__init__.py
@@ -0,0 +1,7 @@
+# -*- coding: utf-8 -*-
+"""
+
+"""
+
+__author__ = "Waterball (johnny850807@gmail.com)"
+__license__ = "Apache 2.0"
diff --git a/commons/exceptions/primitive.py b/commons/exceptions/primitive.py
new file mode 100644
index 0000000..f2efcd5
--- /dev/null
+++ b/commons/exceptions/primitive.py
@@ -0,0 +1,17 @@
+# -*- coding: utf-8 -*-
+"""
+All primitive exceptions are included here
+"""
+
+__author__ = "Waterball (johnny850807@gmail.com)"
+__license__ = "Apache 2.0"
+
+
+class NoneTypeError(Exception):
+    @staticmethod
+    def raise_if_none(obj):
+        if obj is None:
+            raise NoneTypeError
+
+    def __init__(self, message):
+        self.message = message

From 0b5401773f6fbabd248b6f1eecf44d6b4a8b9a8c Mon Sep 17 00:00:00 2001
From: johnny850807 <johnny850807@gmail.com>
Date: Fri, 6 Nov 2020 00:33:22 +0800
Subject: [PATCH 14/24] :beer: implement DiscoveryClient design

---
 commons/__init__.py                          |  0
 commons/client/ServiceInstance.py            | 90 ++++++++++++++++++++
 commons/client/__init__.py                   |  0
 commons/client/discovery/__init__.py         |  0
 commons/client/discovery/composite.py        | 23 +++++
 commons/client/discovery/discovery_client.py | 49 +++++++++++
 commons/client/loadbalancer/__init__.py      |  0
 commons/client/service_registry/__init__.py  |  0
 commons/utils/__init__.py                    |  0
 commons/utils/functional_operators.py        | 13 +++
 commons/utils/list_utils.py                  |  6 ++
 context/__init__.py                          |  0
 pyproject.toml                               |  2 +
 src/__init__.py                              | 13 ---
 src/stub.py                                  | 17 ----
 tests/__init__.py                            | 13 ---
 tests/client/__init__.py                     |  0
 tests/client/discovery/__init__.py           |  0
 tests/client/discovery/composite_test.py     | 30 +++++++
 tests/stub_test.py                           | 20 -----
 20 files changed, 213 insertions(+), 63 deletions(-)
 create mode 100644 commons/__init__.py
 create mode 100644 commons/client/ServiceInstance.py
 create mode 100644 commons/client/__init__.py
 create mode 100644 commons/client/discovery/__init__.py
 create mode 100644 commons/client/discovery/composite.py
 create mode 100644 commons/client/discovery/discovery_client.py
 create mode 100644 commons/client/loadbalancer/__init__.py
 create mode 100644 commons/client/service_registry/__init__.py
 create mode 100644 commons/utils/__init__.py
 create mode 100644 commons/utils/functional_operators.py
 create mode 100644 commons/utils/list_utils.py
 create mode 100644 context/__init__.py
 delete mode 100644 src/__init__.py
 delete mode 100644 src/stub.py
 create mode 100644 tests/client/__init__.py
 create mode 100644 tests/client/discovery/__init__.py
 create mode 100644 tests/client/discovery/composite_test.py
 delete mode 100644 tests/stub_test.py

diff --git a/commons/__init__.py b/commons/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/commons/client/ServiceInstance.py b/commons/client/ServiceInstance.py
new file mode 100644
index 0000000..c716605
--- /dev/null
+++ b/commons/client/ServiceInstance.py
@@ -0,0 +1,90 @@
+# standard library
+from abc import ABC, abstractmethod
+from urllib.parse import urlparse
+
+__author__ = "Waterball (johnny850807@gmail.com)"
+__license__ = "Apache 2.0"
+
+
+class ServiceInstance(ABC):
+    @property
+    @abstractmethod
+    def instance_id(self):
+        pass
+
+    @property
+    @abstractmethod
+    def service_id(self):
+        pass
+
+    @property
+    @abstractmethod
+    def host(self):
+        pass
+
+    @property
+    @abstractmethod
+    def port(self):
+        pass
+
+    @property
+    @abstractmethod
+    def secure(self):
+        pass
+
+    @property
+    @abstractmethod
+    def uri(self):
+        pass
+
+    @property
+    @abstractmethod
+    def scheme(self):
+        pass
+
+
+class StaticServiceInstance(ServiceInstance):
+    """
+    A service instance that is initialized with its basic properties
+    """
+
+    def __init__(self, uri, service_id, instance_id):
+        """
+        :param uri: the url in the string type
+        """
+        url_obj = urlparse(uri)
+        self._uri = uri
+        self._scheme = url_obj.scheme
+        self._secure = self._scheme == "https"
+        self._host = url_obj.netloc
+        self._port = url_obj.port
+        self._service_id = service_id
+        self._instance_id = instance_id
+
+    @property
+    def service_id(self):
+        return self._service_id
+
+    @property
+    def instance_id(self):
+        return self._instance_id
+
+    @property
+    def host(self):
+        return self._host
+
+    @property
+    def port(self):
+        return self._port
+
+    @property
+    def secure(self):
+        return self._secure
+
+    @property
+    def uri(self):
+        return self._uri
+
+    @property
+    def scheme(self):
+        return self._scheme
diff --git a/commons/client/__init__.py b/commons/client/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/commons/client/discovery/__init__.py b/commons/client/discovery/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/commons/client/discovery/composite.py b/commons/client/discovery/composite.py
new file mode 100644
index 0000000..cb39845
--- /dev/null
+++ b/commons/client/discovery/composite.py
@@ -0,0 +1,23 @@
+# scip plugin
+from commons.client.discovery.discovery_client import DiscoveryClient
+from commons.utils.functional_operators import flat_map
+from commons.utils.list_utils import not_null_nor_empty
+
+__author__ = "Waterball (johnny850807@gmail.com)"
+__license__ = "Apache 2.0"
+
+
+class CompositeDiscoveryClient(DiscoveryClient):
+    def __init__(self, *discovery_clients):
+        self.discovery_clients = discovery_clients
+
+    def get_instances(self, service_id):
+        for client in self.discovery_clients:
+            services = client.get_instances(service_id)
+            if not_null_nor_empty(services):
+                return services
+        return []
+
+    @property
+    def services(self):
+        return flat_map(lambda d: d.services, self.discovery_clients)
diff --git a/commons/client/discovery/discovery_client.py b/commons/client/discovery/discovery_client.py
new file mode 100644
index 0000000..a3bca2e
--- /dev/null
+++ b/commons/client/discovery/discovery_client.py
@@ -0,0 +1,49 @@
+# standard library
+from abc import ABC, abstractmethod
+
+# scip plugin
+from commons.client.ServiceInstance import StaticServiceInstance
+from commons.utils.functional_operators import filter_get_first
+
+__author__ = "Waterball (johnny850807@gmail.com)"
+__license__ = "Apache 2.0"
+
+
+class DiscoveryClient(ABC):
+    @abstractmethod
+    def get_instances(self, service_id):
+        """
+        Gets all ServiceInstances associated with a particular serviceId.
+        :param service_id: The serviceId to query.
+        :return: A List of ServiceInstance.
+        """
+        pass
+
+    @property
+    @abstractmethod
+    def services(self):
+        """
+        :return: All known service IDs.
+        """
+        pass
+
+
+class StaticDiscoveryClient(DiscoveryClient):
+    """
+    A DiscoveryClient initialized with the services
+    """
+
+    def __init__(self, services):
+        self._services = services
+
+    def get_instances(self, service_id):
+        return list(filter(lambda s: s.service_id == service_id, self.services))
+
+    @property
+    def services(self):
+        return self._services
+
+
+def static_discovery_client(uri, service_id, instance_ids):
+    services = [StaticServiceInstance(uri, service_id, instance_id) for instance_id in instance_ids]
+    return StaticDiscoveryClient(services)
diff --git a/commons/client/loadbalancer/__init__.py b/commons/client/loadbalancer/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/commons/client/service_registry/__init__.py b/commons/client/service_registry/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/commons/utils/__init__.py b/commons/utils/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/commons/utils/functional_operators.py b/commons/utils/functional_operators.py
new file mode 100644
index 0000000..013b851
--- /dev/null
+++ b/commons/utils/functional_operators.py
@@ -0,0 +1,13 @@
+# standard library
+from functools import reduce
+
+__author__ = "Waterball (johnny850807@gmail.com)"
+__license__ = "Apache 2.0"
+
+
+def flat_map(f, xs):
+    return reduce(lambda a, b: a + b, map(f, xs))
+
+
+def filter_get_first(f, the_list):
+    return next(filter(f, the_list), None)
diff --git a/commons/utils/list_utils.py b/commons/utils/list_utils.py
new file mode 100644
index 0000000..6d403ff
--- /dev/null
+++ b/commons/utils/list_utils.py
@@ -0,0 +1,6 @@
+__author__ = "Waterball (johnny850807@gmail.com)"
+__license__ = "Apache 2.0"
+
+
+def not_null_nor_empty(the_list):
+    return the_list is not None and len(the_list) != 0
diff --git a/context/__init__.py b/context/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/pyproject.toml b/pyproject.toml
index de09a11..f3222f3 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -7,6 +7,8 @@ license = "Apache-2.0"
 
 [tool.poetry.dependencies]
 python = "^3.7"
+pytest = "^6.1.2"
+pytest-cov = "^2.10.1"
 
 [tool.poetry.dev-dependencies]
 pre-commit = "^2.7.1"
diff --git a/src/__init__.py b/src/__init__.py
deleted file mode 100644
index b9b3c0c..0000000
--- a/src/__init__.py
+++ /dev/null
@@ -1,13 +0,0 @@
-#  Copyright 2020 Waterball (johnny850807@gmail.com)
-#
-#     Licensed under the Apache License, Version 2.0 (the "License");
-#     you may not use this file except in compliance with the License.
-#     You may obtain a copy of the License at
-#
-#         http://www.apache.org/licenses/LICENSE-2.0
-#
-#     Unless required by applicable law or agreed to in writing, software
-#     distributed under the License is distributed on an "AS IS" BASIS,
-#     WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-#     See the License for the specific language governing permissions and
-#     limitations under the License.
diff --git a/src/stub.py b/src/stub.py
deleted file mode 100644
index 56d99ca..0000000
--- a/src/stub.py
+++ /dev/null
@@ -1,17 +0,0 @@
-#  Copyright 2020 Waterball (johnny850807@gmail.com)
-#
-#     Licensed under the Apache License, Version 2.0 (the "License");
-#     you may not use this file except in compliance with the License.
-#     You may obtain a copy of the License at
-#
-#         http://www.apache.org/licenses/LICENSE-2.0
-#
-#     Unless required by applicable law or agreed to in writing, software
-#     distributed under the License is distributed on an "AS IS" BASIS,
-#     WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-#     See the License for the specific language governing permissions and
-#     limitations under the License.
-
-
-def add(a, b):
-    return a + b
diff --git a/tests/__init__.py b/tests/__init__.py
index b9b3c0c..e69de29 100644
--- a/tests/__init__.py
+++ b/tests/__init__.py
@@ -1,13 +0,0 @@
-#  Copyright 2020 Waterball (johnny850807@gmail.com)
-#
-#     Licensed under the Apache License, Version 2.0 (the "License");
-#     you may not use this file except in compliance with the License.
-#     You may obtain a copy of the License at
-#
-#         http://www.apache.org/licenses/LICENSE-2.0
-#
-#     Unless required by applicable law or agreed to in writing, software
-#     distributed under the License is distributed on an "AS IS" BASIS,
-#     WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-#     See the License for the specific language governing permissions and
-#     limitations under the License.
diff --git a/tests/client/__init__.py b/tests/client/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/tests/client/discovery/__init__.py b/tests/client/discovery/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/tests/client/discovery/composite_test.py b/tests/client/discovery/composite_test.py
new file mode 100644
index 0000000..39a4990
--- /dev/null
+++ b/tests/client/discovery/composite_test.py
@@ -0,0 +1,30 @@
+# scip plugin
+from commons.client.discovery.composite import CompositeDiscoveryClient
+from commons.client.discovery.discovery_client import static_discovery_client
+
+__author__ = "Waterball (johnny850807@gmail.com)"
+__license__ = "Apache"
+
+client = CompositeDiscoveryClient(
+    static_discovery_client("url-1", "service-1", ["1-1", "1-2", "1-3"]),
+    static_discovery_client("url-2", "service-2", ["2-1", "2-2", "2-3"]),
+    static_discovery_client("url-3", "service-3", ["3-1", "3-2", "3-3"]),
+)
+
+
+def test_get_instances():
+    for service_id in range(1, 4):
+        instances = client.get_instances("service-{}".format(service_id))
+        for instance_id in range(1, 4):
+            instance = instances[instance_id - 1]
+            assert instance.instance_id == "{}-{}".format(service_id, instance_id)
+
+
+def test_get_services():
+    instances = client.services
+    i = 0
+    for service_id in range(1, 4):
+        for instance_id in range(1, 4):
+            assert instances[i].service_id == "service-{}".format(service_id)
+            assert instances[i].instance_id == "{}-{}".format(service_id, instance_id)
+            i += 1
diff --git a/tests/stub_test.py b/tests/stub_test.py
deleted file mode 100644
index de90136..0000000
--- a/tests/stub_test.py
+++ /dev/null
@@ -1,20 +0,0 @@
-#  Copyright 2020 Waterball (johnny850807@gmail.com)
-#
-#     Licensed under the Apache License, Version 2.0 (the "License");
-#     you may not use this file except in compliance with the License.
-#     You may obtain a copy of the License at
-#
-#         http://www.apache.org/licenses/LICENSE-2.0
-#
-#     Unless required by applicable law or agreed to in writing, software
-#     distributed under the License is distributed on an "AS IS" BASIS,
-#     WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-#     See the License for the specific language governing permissions and
-#     limitations under the License.
-
-# scip plugin
-from src import stub
-
-
-def test():
-    assert 10 == stub.add(4, 6)

From 83a45fe1c6d16fb63a7bc1716aa6e7a9a3d33638 Mon Sep 17 00:00:00 2001
From: johnny850807 <johnny850807@gmail.com>
Date: Fri, 6 Nov 2020 01:06:38 +0800
Subject: [PATCH 15/24] Add more comments

---
 commons/client/discovery/composite.py        | 4 ++++
 commons/client/discovery/discovery_client.py | 4 ++++
 2 files changed, 8 insertions(+)

diff --git a/commons/client/discovery/composite.py b/commons/client/discovery/composite.py
index cb39845..ddc3432 100644
--- a/commons/client/discovery/composite.py
+++ b/commons/client/discovery/composite.py
@@ -8,6 +8,10 @@
 
 
 class CompositeDiscoveryClient(DiscoveryClient):
+    """
+    Composite pattern application: aggregate the service sources from a list of discovery client
+    """
+
     def __init__(self, *discovery_clients):
         self.discovery_clients = discovery_clients
 
diff --git a/commons/client/discovery/discovery_client.py b/commons/client/discovery/discovery_client.py
index a3bca2e..c5fb607 100644
--- a/commons/client/discovery/discovery_client.py
+++ b/commons/client/discovery/discovery_client.py
@@ -45,5 +45,9 @@ def services(self):
 
 
 def static_discovery_client(uri, service_id, instance_ids):
+    """
+    Usage:
+        static_discovery_client("url-1", "service-1", ["id-1", "id-2", "id-3"])
+    """
     services = [StaticServiceInstance(uri, service_id, instance_id) for instance_id in instance_ids]
     return StaticDiscoveryClient(services)

From d65d60d10a3c2012814934d9b7553f2cb3261614 Mon Sep 17 00:00:00 2001
From: johnny850807 <johnny850807@gmail.com>
Date: Fri, 6 Nov 2020 11:32:19 +0800
Subject: [PATCH 16/24] Modify some functional operation concerning performance

---
 README.md                                |  6 ++++++
 commons/client/ServiceInstance.py        |  4 ++++
 commons/client/discovery/composite.py    |  4 ++--
 commons/utils/functional_operators.py    |  7 +++++--
 commons/utils/list_utils.py              |  7 ++++++-
 pyproject.toml                           |  2 --
 tests/client/discovery/composite_test.py |  6 +++---
 tests/utils/__init__.py                  |  2 ++
 tests/utils/functional_operators_test.py | 17 +++++++++++++++++
 9 files changed, 45 insertions(+), 10 deletions(-)
 create mode 100644 tests/utils/__init__.py
 create mode 100644 tests/utils/functional_operators_test.py

diff --git a/README.md b/README.md
index 36e8225..bf1e281 100644
--- a/README.md
+++ b/README.md
@@ -49,3 +49,9 @@ poetry run pre-commit install
 ```
 
 6. Start to commit :)
+
+## Test
+
+1. Show test coverage report on terminal
+
+`poetry run pytest --cov-report term --cov ${package} tests/`
diff --git a/commons/client/ServiceInstance.py b/commons/client/ServiceInstance.py
index c716605..0edc3ba 100644
--- a/commons/client/ServiceInstance.py
+++ b/commons/client/ServiceInstance.py
@@ -7,6 +7,10 @@
 
 
 class ServiceInstance(ABC):
+    """
+
+    """
+
     @property
     @abstractmethod
     def instance_id(self):
diff --git a/commons/client/discovery/composite.py b/commons/client/discovery/composite.py
index ddc3432..c3be2fa 100644
--- a/commons/client/discovery/composite.py
+++ b/commons/client/discovery/composite.py
@@ -1,7 +1,7 @@
 # scip plugin
 from commons.client.discovery.discovery_client import DiscoveryClient
 from commons.utils.functional_operators import flat_map
-from commons.utils.list_utils import not_null_nor_empty
+from commons.utils.list_utils import not_none_nor_empty
 
 __author__ = "Waterball (johnny850807@gmail.com)"
 __license__ = "Apache 2.0"
@@ -18,7 +18,7 @@ def __init__(self, *discovery_clients):
     def get_instances(self, service_id):
         for client in self.discovery_clients:
             services = client.get_instances(service_id)
-            if not_null_nor_empty(services):
+            if not_none_nor_empty(services):
                 return services
         return []
 
diff --git a/commons/utils/functional_operators.py b/commons/utils/functional_operators.py
index 013b851..54463f6 100644
--- a/commons/utils/functional_operators.py
+++ b/commons/utils/functional_operators.py
@@ -6,8 +6,11 @@
 
 
 def flat_map(f, xs):
-    return reduce(lambda a, b: a + b, map(f, xs))
+    results = []
+    for element in xs:
+        results += f(element)
+    return results
 
 
 def filter_get_first(f, the_list):
-    return next(filter(f, the_list), None)
+    return [x for x in the_list if f(x)][0]
diff --git a/commons/utils/list_utils.py b/commons/utils/list_utils.py
index 6d403ff..8e2c8e3 100644
--- a/commons/utils/list_utils.py
+++ b/commons/utils/list_utils.py
@@ -2,5 +2,10 @@
 __license__ = "Apache 2.0"
 
 
-def not_null_nor_empty(the_list):
+def not_none_nor_empty(the_list):
+    """
+    :param the_list: a list
+    :return: true if the list is neither none nor empty
+    """
+    assert not the_list or isinstance(the_list, list)
     return the_list is not None and len(the_list) != 0
diff --git a/pyproject.toml b/pyproject.toml
index f3222f3..de09a11 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -7,8 +7,6 @@ license = "Apache-2.0"
 
 [tool.poetry.dependencies]
 python = "^3.7"
-pytest = "^6.1.2"
-pytest-cov = "^2.10.1"
 
 [tool.poetry.dev-dependencies]
 pre-commit = "^2.7.1"
diff --git a/tests/client/discovery/composite_test.py b/tests/client/discovery/composite_test.py
index 39a4990..758c4e3 100644
--- a/tests/client/discovery/composite_test.py
+++ b/tests/client/discovery/composite_test.py
@@ -17,7 +17,7 @@ def test_get_instances():
         instances = client.get_instances("service-{}".format(service_id))
         for instance_id in range(1, 4):
             instance = instances[instance_id - 1]
-            assert instance.instance_id == "{}-{}".format(service_id, instance_id)
+            assert "{}-{}".format(service_id, instance_id) == instance.instance_id
 
 
 def test_get_services():
@@ -25,6 +25,6 @@ def test_get_services():
     i = 0
     for service_id in range(1, 4):
         for instance_id in range(1, 4):
-            assert instances[i].service_id == "service-{}".format(service_id)
-            assert instances[i].instance_id == "{}-{}".format(service_id, instance_id)
+            assert "service-{}".format(service_id) == instances[i].service_id
+            assert "{}-{}".format(service_id, instance_id) == instances[i].instance_id
             i += 1
diff --git a/tests/utils/__init__.py b/tests/utils/__init__.py
new file mode 100644
index 0000000..d7b1237
--- /dev/null
+++ b/tests/utils/__init__.py
@@ -0,0 +1,2 @@
+__author__ = "Waterball (johnny850807@gmail.com)"
+__license__ = "Apache 2.0"
diff --git a/tests/utils/functional_operators_test.py b/tests/utils/functional_operators_test.py
new file mode 100644
index 0000000..cf40349
--- /dev/null
+++ b/tests/utils/functional_operators_test.py
@@ -0,0 +1,17 @@
+# scip plugin
+from commons.utils.functional_operators import filter_get_first, flat_map
+
+__author__ = "Waterball (johnny850807@gmail.com)"
+__license__ = "Apache 2.0"
+
+
+def test_flat_map():
+    the_list = ["a,b,c", "1,2,3", ""]
+    results = flat_map(lambda e: e.split(","), the_list)
+    assert ["a", "b", "c", "1", "2", "3", ""] == results
+
+
+def test_filter_get_first():
+    the_list = [1, 2, 3, 4, 5, 6, 7, 8]
+    result = filter_get_first(lambda e: e > 4, the_list)
+    assert 5 == result

From 37e1afee3e007ec33aa9f8582f27df51557f6de4 Mon Sep 17 00:00:00 2001
From: johnny850807 <johnny850807@gmail.com>
Date: Fri, 6 Nov 2020 11:54:22 +0800
Subject: [PATCH 17/24] :memo: add typing and docstrings

---
 commons/__init__.py                          |  1 +
 commons/client/ServiceInstance.py            | 11 +++++++----
 commons/client/__init__.py                   |  1 +
 commons/client/discovery/__init__.py         |  1 +
 commons/client/discovery/composite.py        | 14 ++++++++++----
 commons/client/discovery/discovery_client.py | 18 +++++++++++++++---
 commons/client/loadbalancer/__init__.py      |  1 +
 commons/client/service_registry/__init__.py  |  1 +
 commons/utils/__init__.py                    |  1 +
 commons/utils/functional_operators.py        | 17 +++++++++++++----
 commons/utils/list_utils.py                  |  2 ++
 context/__init__.py                          |  1 +
 tests/__init__.py                            |  1 +
 tests/client/__init__.py                     |  1 +
 tests/client/discovery/__init__.py           |  1 +
 tests/client/discovery/composite_test.py     |  8 +++++---
 16 files changed, 62 insertions(+), 18 deletions(-)

diff --git a/commons/__init__.py b/commons/__init__.py
index e69de29..40a96af 100644
--- a/commons/__init__.py
+++ b/commons/__init__.py
@@ -0,0 +1 @@
+# -*- coding: utf-8 -*-
diff --git a/commons/client/ServiceInstance.py b/commons/client/ServiceInstance.py
index 0edc3ba..1d2f701 100644
--- a/commons/client/ServiceInstance.py
+++ b/commons/client/ServiceInstance.py
@@ -1,14 +1,17 @@
-# standard library
-from abc import ABC, abstractmethod
-from urllib.parse import urlparse
+# -*- coding: utf-8 -*-
 
 __author__ = "Waterball (johnny850807@gmail.com)"
 __license__ = "Apache 2.0"
 
 
+# standard library
+from abc import ABC, abstractmethod
+from urllib.parse import urlparse
+
+
 class ServiceInstance(ABC):
     """
-
+    A service's instance that owns basic HTTP info.
     """
 
     @property
diff --git a/commons/client/__init__.py b/commons/client/__init__.py
index e69de29..40a96af 100644
--- a/commons/client/__init__.py
+++ b/commons/client/__init__.py
@@ -0,0 +1 @@
+# -*- coding: utf-8 -*-
diff --git a/commons/client/discovery/__init__.py b/commons/client/discovery/__init__.py
index e69de29..40a96af 100644
--- a/commons/client/discovery/__init__.py
+++ b/commons/client/discovery/__init__.py
@@ -0,0 +1 @@
+# -*- coding: utf-8 -*-
diff --git a/commons/client/discovery/composite.py b/commons/client/discovery/composite.py
index c3be2fa..7bc38b0 100644
--- a/commons/client/discovery/composite.py
+++ b/commons/client/discovery/composite.py
@@ -1,18 +1,24 @@
+# -*- coding: utf-8 -*-
+
+__author__ = "Waterball (johnny850807@gmail.com)"
+__license__ = "Apache 2.0"
+
 # scip plugin
 from commons.client.discovery.discovery_client import DiscoveryClient
 from commons.utils.functional_operators import flat_map
 from commons.utils.list_utils import not_none_nor_empty
 
-__author__ = "Waterball (johnny850807@gmail.com)"
-__license__ = "Apache 2.0"
-
 
 class CompositeDiscoveryClient(DiscoveryClient):
     """
-    Composite pattern application: aggregate the service sources from a list of discovery client
+    Composite pattern application:
+        aggregate the service sources from a list of discovery client
     """
 
     def __init__(self, *discovery_clients):
+        """
+        :param discovery_clients: a list of DiscoveryClient
+        """
         self.discovery_clients = discovery_clients
 
     def get_instances(self, service_id):
diff --git a/commons/client/discovery/discovery_client.py b/commons/client/discovery/discovery_client.py
index c5fb607..6606b9e 100644
--- a/commons/client/discovery/discovery_client.py
+++ b/commons/client/discovery/discovery_client.py
@@ -1,3 +1,11 @@
+# -*- coding: utf-8 -*-
+"""
+DiscoveryClient is responsible for providing a list of service's instances.
+There may be many services, for example: 'user-service', 'order-service', 'product-service', and so on.
+Where each service may have several instances registered (e.g. user-service-1, user-service-2, ...),
+and the registered instances are called ServiceInstances.
+"""
+
 # standard library
 from abc import ABC, abstractmethod
 
@@ -14,7 +22,7 @@ class DiscoveryClient(ABC):
     def get_instances(self, service_id):
         """
         Gets all ServiceInstances associated with a particular serviceId.
-        :param service_id: The serviceId to query.
+        :param service_id: (str) The serviceId to query.
         :return: A List of ServiceInstance.
         """
         pass
@@ -23,14 +31,14 @@ def get_instances(self, service_id):
     @abstractmethod
     def services(self):
         """
-        :return: All known service IDs.
+        :return: All known service IDs (*str).
         """
         pass
 
 
 class StaticDiscoveryClient(DiscoveryClient):
     """
-    A DiscoveryClient initialized with the services
+    A DiscoveryClient initialized with the services.
     """
 
     def __init__(self, services):
@@ -46,6 +54,10 @@ def services(self):
 
 def static_discovery_client(uri, service_id, instance_ids):
     """
+    A helper method that helps create a list of instances from the same service.
+    :param uri the uri of every service's
+    :param service_id the service_id of the service
+    :param instance_ids a list of ids (*str) of the instances'
     Usage:
         static_discovery_client("url-1", "service-1", ["id-1", "id-2", "id-3"])
     """
diff --git a/commons/client/loadbalancer/__init__.py b/commons/client/loadbalancer/__init__.py
index e69de29..40a96af 100644
--- a/commons/client/loadbalancer/__init__.py
+++ b/commons/client/loadbalancer/__init__.py
@@ -0,0 +1 @@
+# -*- coding: utf-8 -*-
diff --git a/commons/client/service_registry/__init__.py b/commons/client/service_registry/__init__.py
index e69de29..40a96af 100644
--- a/commons/client/service_registry/__init__.py
+++ b/commons/client/service_registry/__init__.py
@@ -0,0 +1 @@
+# -*- coding: utf-8 -*-
diff --git a/commons/utils/__init__.py b/commons/utils/__init__.py
index e69de29..40a96af 100644
--- a/commons/utils/__init__.py
+++ b/commons/utils/__init__.py
@@ -0,0 +1 @@
+# -*- coding: utf-8 -*-
diff --git a/commons/utils/functional_operators.py b/commons/utils/functional_operators.py
index 54463f6..084fb45 100644
--- a/commons/utils/functional_operators.py
+++ b/commons/utils/functional_operators.py
@@ -1,16 +1,25 @@
-# standard library
-from functools import reduce
+# -*- coding: utf-8 -*-
 
 __author__ = "Waterball (johnny850807@gmail.com)"
 __license__ = "Apache 2.0"
 
 
 def flat_map(f, xs):
+    """
+    :param f: a mapping function
+    :param xs: (Iterable)
+    :return: list
+    """
     results = []
     for element in xs:
         results += f(element)
     return results
 
 
-def filter_get_first(f, the_list):
-    return [x for x in the_list if f(x)][0]
+def filter_get_first(f, xs):
+    """
+    :param f: a mapping function
+    :param xs: (Iterable)
+    :return: an element
+    """
+    return [x for x in xs if f(x)][0]
diff --git a/commons/utils/list_utils.py b/commons/utils/list_utils.py
index 8e2c8e3..5185483 100644
--- a/commons/utils/list_utils.py
+++ b/commons/utils/list_utils.py
@@ -1,3 +1,5 @@
+# -*- coding: utf-8 -*-
+
 __author__ = "Waterball (johnny850807@gmail.com)"
 __license__ = "Apache 2.0"
 
diff --git a/context/__init__.py b/context/__init__.py
index e69de29..40a96af 100644
--- a/context/__init__.py
+++ b/context/__init__.py
@@ -0,0 +1 @@
+# -*- coding: utf-8 -*-
diff --git a/tests/__init__.py b/tests/__init__.py
index e69de29..40a96af 100644
--- a/tests/__init__.py
+++ b/tests/__init__.py
@@ -0,0 +1 @@
+# -*- coding: utf-8 -*-
diff --git a/tests/client/__init__.py b/tests/client/__init__.py
index e69de29..40a96af 100644
--- a/tests/client/__init__.py
+++ b/tests/client/__init__.py
@@ -0,0 +1 @@
+# -*- coding: utf-8 -*-
diff --git a/tests/client/discovery/__init__.py b/tests/client/discovery/__init__.py
index e69de29..40a96af 100644
--- a/tests/client/discovery/__init__.py
+++ b/tests/client/discovery/__init__.py
@@ -0,0 +1 @@
+# -*- coding: utf-8 -*-
diff --git a/tests/client/discovery/composite_test.py b/tests/client/discovery/composite_test.py
index 758c4e3..f9eb46d 100644
--- a/tests/client/discovery/composite_test.py
+++ b/tests/client/discovery/composite_test.py
@@ -1,10 +1,12 @@
+# -*- coding: utf-8 -*-
+
+__author__ = "Waterball (johnny850807@gmail.com)"
+__license__ = "Apache 2.0"
+
 # scip plugin
 from commons.client.discovery.composite import CompositeDiscoveryClient
 from commons.client.discovery.discovery_client import static_discovery_client
 
-__author__ = "Waterball (johnny850807@gmail.com)"
-__license__ = "Apache"
-
 client = CompositeDiscoveryClient(
     static_discovery_client("url-1", "service-1", ["1-1", "1-2", "1-3"]),
     static_discovery_client("url-2", "service-2", ["2-1", "2-2", "2-3"]),

From b2bb27b9105334af946645434d88c49557b9d7bb Mon Sep 17 00:00:00 2001
From: johnny850807 <johnny850807@gmail.com>
Date: Fri, 6 Nov 2020 12:31:09 +0800
Subject: [PATCH 18/24] Make some fields private

---
 commons/client/discovery/composite.py        | 6 +++---
 commons/client/discovery/discovery_client.py | 4 ++--
 2 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/commons/client/discovery/composite.py b/commons/client/discovery/composite.py
index 7bc38b0..bd4ecb2 100644
--- a/commons/client/discovery/composite.py
+++ b/commons/client/discovery/composite.py
@@ -19,10 +19,10 @@ def __init__(self, *discovery_clients):
         """
         :param discovery_clients: a list of DiscoveryClient
         """
-        self.discovery_clients = discovery_clients
+        self.__discovery_clients = discovery_clients
 
     def get_instances(self, service_id):
-        for client in self.discovery_clients:
+        for client in self.__discovery_clients:
             services = client.get_instances(service_id)
             if not_none_nor_empty(services):
                 return services
@@ -30,4 +30,4 @@ def get_instances(self, service_id):
 
     @property
     def services(self):
-        return flat_map(lambda d: d.services, self.discovery_clients)
+        return flat_map(lambda d: d.services, self.__discovery_clients)
diff --git a/commons/client/discovery/discovery_client.py b/commons/client/discovery/discovery_client.py
index 6606b9e..f013fc7 100644
--- a/commons/client/discovery/discovery_client.py
+++ b/commons/client/discovery/discovery_client.py
@@ -42,14 +42,14 @@ class StaticDiscoveryClient(DiscoveryClient):
     """
 
     def __init__(self, services):
-        self._services = services
+        self.__services = services
 
     def get_instances(self, service_id):
         return list(filter(lambda s: s.service_id == service_id, self.services))
 
     @property
     def services(self):
-        return self._services
+        return self.__services
 
 
 def static_discovery_client(uri, service_id, instance_ids):

From 3c1a01457b7a9cb5170cacc5e3e1611c1544d8d6 Mon Sep 17 00:00:00 2001
From: johnny850807 <johnny850807@gmail.com>
Date: Fri, 6 Nov 2020 12:32:27 +0800
Subject: [PATCH 19/24] Fix naming

---
 commons/client/discovery/discovery_client.py               | 2 +-
 commons/client/{ServiceInstance.py => service_instance.py} | 0
 2 files changed, 1 insertion(+), 1 deletion(-)
 rename commons/client/{ServiceInstance.py => service_instance.py} (100%)

diff --git a/commons/client/discovery/discovery_client.py b/commons/client/discovery/discovery_client.py
index f013fc7..dc94e94 100644
--- a/commons/client/discovery/discovery_client.py
+++ b/commons/client/discovery/discovery_client.py
@@ -10,7 +10,7 @@
 from abc import ABC, abstractmethod
 
 # scip plugin
-from commons.client.ServiceInstance import StaticServiceInstance
+from commons.client.service_instance import StaticServiceInstance
 from commons.utils.functional_operators import filter_get_first
 
 __author__ = "Waterball (johnny850807@gmail.com)"
diff --git a/commons/client/ServiceInstance.py b/commons/client/service_instance.py
similarity index 100%
rename from commons/client/ServiceInstance.py
rename to commons/client/service_instance.py

From fbc9a100de4ed049f204e306fbdc7c29da09e34b Mon Sep 17 00:00:00 2001
From: johnny850807 <johnny850807@gmail.com>
Date: Fri, 6 Nov 2020 14:57:32 +0800
Subject: [PATCH 20/24] pending discussion: next+filter vs list comprehension

---
 commons/utils/functional_operators.py | 6 +++---
 commons/utils/list_utils.py           | 1 -
 2 files changed, 3 insertions(+), 4 deletions(-)

diff --git a/commons/utils/functional_operators.py b/commons/utils/functional_operators.py
index 084fb45..386d3da 100644
--- a/commons/utils/functional_operators.py
+++ b/commons/utils/functional_operators.py
@@ -18,8 +18,8 @@ def flat_map(f, xs):
 
 def filter_get_first(f, xs):
     """
-    :param f: a mapping function
-    :param xs: (Iterable)
+    :param f: a predicate function that returns a boolean
+    :param xs: (Iterable
     :return: an element
     """
-    return [x for x in xs if f(x)][0]
+    return next(filter(f, xs), None)
diff --git a/commons/utils/list_utils.py b/commons/utils/list_utils.py
index 5185483..044008d 100644
--- a/commons/utils/list_utils.py
+++ b/commons/utils/list_utils.py
@@ -9,5 +9,4 @@ def not_none_nor_empty(the_list):
     :param the_list: a list
     :return: true if the list is neither none nor empty
     """
-    assert not the_list or isinstance(the_list, list)
     return the_list is not None and len(the_list) != 0

From 5510384774a9d44ce9025cbd7a248816f7802779 Mon Sep 17 00:00:00 2001
From: johnny850807 <johnny850807@gmail.com>
Date: Fri, 6 Nov 2020 20:22:58 +0800
Subject: [PATCH 21/24] :art: Add type checking syntax

---
 commons/client/discovery/composite.py        | 16 ++++++-----
 commons/client/discovery/discovery_client.py | 21 +++++++-------
 commons/client/service_instance.py           | 30 ++++++++++----------
 commons/utils/functional_operators.py        | 17 ++++++-----
 commons/utils/list_utils.py                  |  3 +-
 tests/client/discovery/composite_test.py     | 13 +++------
 6 files changed, 48 insertions(+), 52 deletions(-)

diff --git a/commons/client/discovery/composite.py b/commons/client/discovery/composite.py
index bd4ecb2..5541ec0 100644
--- a/commons/client/discovery/composite.py
+++ b/commons/client/discovery/composite.py
@@ -1,4 +1,9 @@
 # -*- coding: utf-8 -*-
+# standard library
+from typing import List, Set
+
+# scip plugin
+from commons.client.service_instance import ServiceInstance
 
 __author__ = "Waterball (johnny850807@gmail.com)"
 __license__ = "Apache 2.0"
@@ -15,13 +20,10 @@ class CompositeDiscoveryClient(DiscoveryClient):
         aggregate the service sources from a list of discovery client
     """
 
-    def __init__(self, *discovery_clients):
-        """
-        :param discovery_clients: a list of DiscoveryClient
-        """
+    def __init__(self, *discovery_clients: DiscoveryClient):
         self.__discovery_clients = discovery_clients
 
-    def get_instances(self, service_id):
+    def get_instances(self, service_id: str) -> List[ServiceInstance]:
         for client in self.__discovery_clients:
             services = client.get_instances(service_id)
             if not_none_nor_empty(services):
@@ -29,5 +31,5 @@ def get_instances(self, service_id):
         return []
 
     @property
-    def services(self):
-        return flat_map(lambda d: d.services, self.__discovery_clients)
+    def services(self) -> Set[str]:
+        return set(flat_map(lambda d: d.services, self.__discovery_clients))
diff --git a/commons/client/discovery/discovery_client.py b/commons/client/discovery/discovery_client.py
index dc94e94..d65fa78 100644
--- a/commons/client/discovery/discovery_client.py
+++ b/commons/client/discovery/discovery_client.py
@@ -8,9 +8,10 @@
 
 # standard library
 from abc import ABC, abstractmethod
+from typing import List, Set
 
 # scip plugin
-from commons.client.service_instance import StaticServiceInstance
+from commons.client.service_instance import ServiceInstance, StaticServiceInstance
 from commons.utils.functional_operators import filter_get_first
 
 __author__ = "Waterball (johnny850807@gmail.com)"
@@ -19,7 +20,7 @@
 
 class DiscoveryClient(ABC):
     @abstractmethod
-    def get_instances(self, service_id):
+    def get_instances(self, service_id: str) -> List[ServiceInstance]:
         """
         Gets all ServiceInstances associated with a particular serviceId.
         :param service_id: (str) The serviceId to query.
@@ -29,7 +30,7 @@ def get_instances(self, service_id):
 
     @property
     @abstractmethod
-    def services(self):
+    def services(self) -> List[str]:
         """
         :return: All known service IDs (*str).
         """
@@ -41,18 +42,18 @@ class StaticDiscoveryClient(DiscoveryClient):
     A DiscoveryClient initialized with the services.
     """
 
-    def __init__(self, services):
-        self.__services = services
+    def __init__(self, instances: List[ServiceInstance]):
+        self.__instances = instances
 
-    def get_instances(self, service_id):
-        return list(filter(lambda s: s.service_id == service_id, self.services))
+    def get_instances(self, service_id: str) -> List[ServiceInstance]:
+        return list(filter(lambda s: s.service_id == service_id, self.__instances))
 
     @property
-    def services(self):
-        return self.__services
+    def services(self) -> Set[str]:
+        return {s.service_id for s in self.__instances}
 
 
-def static_discovery_client(uri, service_id, instance_ids):
+def static_discovery_client(uri: str, service_id: str, instance_ids: List[str]) -> StaticDiscoveryClient:
     """
     A helper method that helps create a list of instances from the same service.
     :param uri the uri of every service's
diff --git a/commons/client/service_instance.py b/commons/client/service_instance.py
index 1d2f701..a7103e6 100644
--- a/commons/client/service_instance.py
+++ b/commons/client/service_instance.py
@@ -16,37 +16,37 @@ class ServiceInstance(ABC):
 
     @property
     @abstractmethod
-    def instance_id(self):
+    def instance_id(self) -> str:
         pass
 
     @property
     @abstractmethod
-    def service_id(self):
+    def service_id(self) -> str:
         pass
 
     @property
     @abstractmethod
-    def host(self):
+    def host(self) -> str:
         pass
 
     @property
     @abstractmethod
-    def port(self):
+    def port(self) -> int:
         pass
 
     @property
     @abstractmethod
-    def secure(self):
+    def secure(self) -> bool:
         pass
 
     @property
     @abstractmethod
-    def uri(self):
+    def uri(self) -> str:
         pass
 
     @property
     @abstractmethod
-    def scheme(self):
+    def scheme(self) -> str:
         pass
 
 
@@ -55,7 +55,7 @@ class StaticServiceInstance(ServiceInstance):
     A service instance that is initialized with its basic properties
     """
 
-    def __init__(self, uri, service_id, instance_id):
+    def __init__(self, uri: str, service_id: str, instance_id: str):
         """
         :param uri: the url in the string type
         """
@@ -69,29 +69,29 @@ def __init__(self, uri, service_id, instance_id):
         self._instance_id = instance_id
 
     @property
-    def service_id(self):
+    def service_id(self) -> str:
         return self._service_id
 
     @property
-    def instance_id(self):
+    def instance_id(self) -> str:
         return self._instance_id
 
     @property
-    def host(self):
+    def host(self) -> str:
         return self._host
 
     @property
-    def port(self):
+    def port(self) -> int:
         return self._port
 
     @property
-    def secure(self):
+    def secure(self) -> bool:
         return self._secure
 
     @property
-    def uri(self):
+    def uri(self) -> str:
         return self._uri
 
     @property
-    def scheme(self):
+    def scheme(self) -> str:
         return self._scheme
diff --git a/commons/utils/functional_operators.py b/commons/utils/functional_operators.py
index 386d3da..78ac357 100644
--- a/commons/utils/functional_operators.py
+++ b/commons/utils/functional_operators.py
@@ -1,25 +1,24 @@
 # -*- coding: utf-8 -*-
+# standard library
+from typing import Iterable
 
 __author__ = "Waterball (johnny850807@gmail.com)"
 __license__ = "Apache 2.0"
 
 
-def flat_map(f, xs):
+def flat_map(f, xs: Iterable):
     """
     :param f: a mapping function
     :param xs: (Iterable)
     :return: list
     """
-    results = []
-    for element in xs:
-        results += f(element)
-    return results
+    return [item for element in xs for item in f(element)]
 
 
-def filter_get_first(f, xs):
+def filter_get_first(f, xs: Iterable):
     """
     :param f: a predicate function that returns a boolean
-    :param xs: (Iterable
-    :return: an element
+    :param xs: (Iterable)
+    :return: the first element matches the predicate
     """
-    return next(filter(f, xs), None)
+    return [x for x in xs if f(x)][0]
diff --git a/commons/utils/list_utils.py b/commons/utils/list_utils.py
index 044008d..f9433f3 100644
--- a/commons/utils/list_utils.py
+++ b/commons/utils/list_utils.py
@@ -4,9 +4,8 @@
 __license__ = "Apache 2.0"
 
 
-def not_none_nor_empty(the_list):
+def not_none_nor_empty(the_list: list):
     """
-    :param the_list: a list
     :return: true if the list is neither none nor empty
     """
     return the_list is not None and len(the_list) != 0
diff --git a/tests/client/discovery/composite_test.py b/tests/client/discovery/composite_test.py
index f9eb46d..c1fbaed 100644
--- a/tests/client/discovery/composite_test.py
+++ b/tests/client/discovery/composite_test.py
@@ -16,17 +16,12 @@
 
 def test_get_instances():
     for service_id in range(1, 4):
-        instances = client.get_instances("service-{}".format(service_id))
+        instances = client.get_instances(f"service-{service_id}")
         for instance_id in range(1, 4):
             instance = instances[instance_id - 1]
-            assert "{}-{}".format(service_id, instance_id) == instance.instance_id
+            assert f"{service_id}-{instance_id}" == instance.instance_id
 
 
 def test_get_services():
-    instances = client.services
-    i = 0
-    for service_id in range(1, 4):
-        for instance_id in range(1, 4):
-            assert "service-{}".format(service_id) == instances[i].service_id
-            assert "{}-{}".format(service_id, instance_id) == instances[i].instance_id
-            i += 1
+    services = client.services
+    assert {"service-1", "service-2", "service-3"} == services

From cdd9e197c88f61b8f138ab2097f7a0a36bf25715 Mon Sep 17 00:00:00 2001
From: johnny850807 <johnny850807@gmail.com>
Date: Fri, 6 Nov 2020 20:41:38 +0800
Subject: [PATCH 22/24] improve filter_get_first utils

---
 commons/utils/functional_operators.py    | 2 +-
 tests/utils/functional_operators_test.py | 8 +++++++-
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/commons/utils/functional_operators.py b/commons/utils/functional_operators.py
index 78ac357..9c04da7 100644
--- a/commons/utils/functional_operators.py
+++ b/commons/utils/functional_operators.py
@@ -21,4 +21,4 @@ def filter_get_first(f, xs: Iterable):
     :param xs: (Iterable)
     :return: the first element matches the predicate
     """
-    return [x for x in xs if f(x)][0]
+    return next((x for x in xs if f(x)), None)
diff --git a/tests/utils/functional_operators_test.py b/tests/utils/functional_operators_test.py
index cf40349..8aa999f 100644
--- a/tests/utils/functional_operators_test.py
+++ b/tests/utils/functional_operators_test.py
@@ -11,7 +11,13 @@ def test_flat_map():
     assert ["a", "b", "c", "1", "2", "3", ""] == results
 
 
-def test_filter_get_first():
+def test_filter_get_first_Given_numbers():
     the_list = [1, 2, 3, 4, 5, 6, 7, 8]
     result = filter_get_first(lambda e: e > 4, the_list)
     assert 5 == result
+
+
+def test_filter_get_first_Given_empty_list():
+    the_list = []
+    result = filter_get_first(lambda e: e > 4, the_list)
+    assert result is None

From bae830a74908752db1808b9a1a674e76f2be4612 Mon Sep 17 00:00:00 2001
From: johnny850807 <johnny850807@gmail.com>
Date: Mon, 9 Nov 2020 11:43:10 +0800
Subject: [PATCH 23/24] :truck: Modify the package layout, rooted from
 spring_cloud

---
 context/__init__.py                                   |  1 -
 {commons => spring_cloud}/__init__.py                 |  0
 {commons/client => spring_cloud/commons}/__init__.py  |  0
 spring_cloud/commons/client/__init__.py               |  3 +++
 spring_cloud/commons/client/discovery/__init__.py     |  4 ++++
 .../commons}/client/discovery/composite.py            |  8 ++++----
 .../commons}/client/discovery/discovery_client.py     |  3 +--
 .../commons/client/loadbalancer}/__init__.py          |  0
 .../commons}/client/service_instance.py               |  0
 .../commons/client/service_registry}/__init__.py      |  0
 .../commons/utils}/__init__.py                        |  0
 .../commons}/utils/functional_operators.py            |  0
 {commons => spring_cloud/commons}/utils/list_utils.py |  2 +-
 {commons/utils => spring_cloud/context}/__init__.py   |  0
 tests/__init__.py                                     |  1 -
 tests/client/__init__.py                              |  1 -
 tests/client/discovery/__init__.py                    |  1 -
 .../{ => commons}/client/discovery/composite_test.py  |  3 +--
 .../{ => commons}/utils/functional_operators_test.py  |  2 +-
 tests/commons/utils/list_utils_test.py                | 11 +++++++++++
 tests/utils/__init__.py                               |  2 --
 21 files changed, 26 insertions(+), 16 deletions(-)
 delete mode 100644 context/__init__.py
 rename {commons => spring_cloud}/__init__.py (100%)
 rename {commons/client => spring_cloud/commons}/__init__.py (100%)
 create mode 100644 spring_cloud/commons/client/__init__.py
 create mode 100644 spring_cloud/commons/client/discovery/__init__.py
 rename {commons => spring_cloud/commons}/client/discovery/composite.py (75%)
 rename {commons => spring_cloud/commons}/client/discovery/discovery_client.py (93%)
 rename {commons/client/discovery => spring_cloud/commons/client/loadbalancer}/__init__.py (100%)
 rename {commons => spring_cloud/commons}/client/service_instance.py (100%)
 rename {commons/client/loadbalancer => spring_cloud/commons/client/service_registry}/__init__.py (100%)
 rename {commons/client/service_registry => spring_cloud/commons/utils}/__init__.py (100%)
 rename {commons => spring_cloud/commons}/utils/functional_operators.py (100%)
 rename {commons => spring_cloud/commons}/utils/list_utils.py (79%)
 rename {commons/utils => spring_cloud/context}/__init__.py (100%)
 delete mode 100644 tests/__init__.py
 delete mode 100644 tests/client/__init__.py
 delete mode 100644 tests/client/discovery/__init__.py
 rename tests/{ => commons}/client/discovery/composite_test.py (84%)
 rename tests/{ => commons}/utils/functional_operators_test.py (86%)
 create mode 100644 tests/commons/utils/list_utils_test.py
 delete mode 100644 tests/utils/__init__.py

diff --git a/context/__init__.py b/context/__init__.py
deleted file mode 100644
index 40a96af..0000000
--- a/context/__init__.py
+++ /dev/null
@@ -1 +0,0 @@
-# -*- coding: utf-8 -*-
diff --git a/commons/__init__.py b/spring_cloud/__init__.py
similarity index 100%
rename from commons/__init__.py
rename to spring_cloud/__init__.py
diff --git a/commons/client/__init__.py b/spring_cloud/commons/__init__.py
similarity index 100%
rename from commons/client/__init__.py
rename to spring_cloud/commons/__init__.py
diff --git a/spring_cloud/commons/client/__init__.py b/spring_cloud/commons/client/__init__.py
new file mode 100644
index 0000000..cdeb409
--- /dev/null
+++ b/spring_cloud/commons/client/__init__.py
@@ -0,0 +1,3 @@
+# -*- coding: utf-8 -*-
+
+from .service_instance import *
diff --git a/spring_cloud/commons/client/discovery/__init__.py b/spring_cloud/commons/client/discovery/__init__.py
new file mode 100644
index 0000000..5d5a70e
--- /dev/null
+++ b/spring_cloud/commons/client/discovery/__init__.py
@@ -0,0 +1,4 @@
+# -*- coding: utf-8 -*-
+
+from .composite import *
+from .discovery_client import *
diff --git a/commons/client/discovery/composite.py b/spring_cloud/commons/client/discovery/composite.py
similarity index 75%
rename from commons/client/discovery/composite.py
rename to spring_cloud/commons/client/discovery/composite.py
index 5541ec0..39ad2f4 100644
--- a/commons/client/discovery/composite.py
+++ b/spring_cloud/commons/client/discovery/composite.py
@@ -3,15 +3,15 @@
 from typing import List, Set
 
 # scip plugin
-from commons.client.service_instance import ServiceInstance
+from spring_cloud.commons.client import ServiceInstance
 
 __author__ = "Waterball (johnny850807@gmail.com)"
 __license__ = "Apache 2.0"
 
 # scip plugin
-from commons.client.discovery.discovery_client import DiscoveryClient
-from commons.utils.functional_operators import flat_map
-from commons.utils.list_utils import not_none_nor_empty
+from spring_cloud.commons.client.discovery.discovery_client import DiscoveryClient
+from spring_cloud.commons.utils.functional_operators import flat_map
+from spring_cloud.commons.utils.list_utils import not_none_nor_empty
 
 
 class CompositeDiscoveryClient(DiscoveryClient):
diff --git a/commons/client/discovery/discovery_client.py b/spring_cloud/commons/client/discovery/discovery_client.py
similarity index 93%
rename from commons/client/discovery/discovery_client.py
rename to spring_cloud/commons/client/discovery/discovery_client.py
index d65fa78..a335b71 100644
--- a/commons/client/discovery/discovery_client.py
+++ b/spring_cloud/commons/client/discovery/discovery_client.py
@@ -11,8 +11,7 @@
 from typing import List, Set
 
 # scip plugin
-from commons.client.service_instance import ServiceInstance, StaticServiceInstance
-from commons.utils.functional_operators import filter_get_first
+from spring_cloud.commons.client import ServiceInstance, StaticServiceInstance
 
 __author__ = "Waterball (johnny850807@gmail.com)"
 __license__ = "Apache 2.0"
diff --git a/commons/client/discovery/__init__.py b/spring_cloud/commons/client/loadbalancer/__init__.py
similarity index 100%
rename from commons/client/discovery/__init__.py
rename to spring_cloud/commons/client/loadbalancer/__init__.py
diff --git a/commons/client/service_instance.py b/spring_cloud/commons/client/service_instance.py
similarity index 100%
rename from commons/client/service_instance.py
rename to spring_cloud/commons/client/service_instance.py
diff --git a/commons/client/loadbalancer/__init__.py b/spring_cloud/commons/client/service_registry/__init__.py
similarity index 100%
rename from commons/client/loadbalancer/__init__.py
rename to spring_cloud/commons/client/service_registry/__init__.py
diff --git a/commons/client/service_registry/__init__.py b/spring_cloud/commons/utils/__init__.py
similarity index 100%
rename from commons/client/service_registry/__init__.py
rename to spring_cloud/commons/utils/__init__.py
diff --git a/commons/utils/functional_operators.py b/spring_cloud/commons/utils/functional_operators.py
similarity index 100%
rename from commons/utils/functional_operators.py
rename to spring_cloud/commons/utils/functional_operators.py
diff --git a/commons/utils/list_utils.py b/spring_cloud/commons/utils/list_utils.py
similarity index 79%
rename from commons/utils/list_utils.py
rename to spring_cloud/commons/utils/list_utils.py
index f9433f3..507af58 100644
--- a/commons/utils/list_utils.py
+++ b/spring_cloud/commons/utils/list_utils.py
@@ -8,4 +8,4 @@ def not_none_nor_empty(the_list: list):
     """
     :return: true if the list is neither none nor empty
     """
-    return the_list is not None and len(the_list) != 0
+    return isinstance(the_list, list) and the_list
diff --git a/commons/utils/__init__.py b/spring_cloud/context/__init__.py
similarity index 100%
rename from commons/utils/__init__.py
rename to spring_cloud/context/__init__.py
diff --git a/tests/__init__.py b/tests/__init__.py
deleted file mode 100644
index 40a96af..0000000
--- a/tests/__init__.py
+++ /dev/null
@@ -1 +0,0 @@
-# -*- coding: utf-8 -*-
diff --git a/tests/client/__init__.py b/tests/client/__init__.py
deleted file mode 100644
index 40a96af..0000000
--- a/tests/client/__init__.py
+++ /dev/null
@@ -1 +0,0 @@
-# -*- coding: utf-8 -*-
diff --git a/tests/client/discovery/__init__.py b/tests/client/discovery/__init__.py
deleted file mode 100644
index 40a96af..0000000
--- a/tests/client/discovery/__init__.py
+++ /dev/null
@@ -1 +0,0 @@
-# -*- coding: utf-8 -*-
diff --git a/tests/client/discovery/composite_test.py b/tests/commons/client/discovery/composite_test.py
similarity index 84%
rename from tests/client/discovery/composite_test.py
rename to tests/commons/client/discovery/composite_test.py
index c1fbaed..1dd5f24 100644
--- a/tests/client/discovery/composite_test.py
+++ b/tests/commons/client/discovery/composite_test.py
@@ -4,8 +4,7 @@
 __license__ = "Apache 2.0"
 
 # scip plugin
-from commons.client.discovery.composite import CompositeDiscoveryClient
-from commons.client.discovery.discovery_client import static_discovery_client
+from spring_cloud.commons.client.discovery import CompositeDiscoveryClient, static_discovery_client
 
 client = CompositeDiscoveryClient(
     static_discovery_client("url-1", "service-1", ["1-1", "1-2", "1-3"]),
diff --git a/tests/utils/functional_operators_test.py b/tests/commons/utils/functional_operators_test.py
similarity index 86%
rename from tests/utils/functional_operators_test.py
rename to tests/commons/utils/functional_operators_test.py
index 8aa999f..199ac13 100644
--- a/tests/utils/functional_operators_test.py
+++ b/tests/commons/utils/functional_operators_test.py
@@ -1,5 +1,5 @@
 # scip plugin
-from commons.utils.functional_operators import filter_get_first, flat_map
+from spring_cloud.commons.utils.functional_operators import filter_get_first, flat_map
 
 __author__ = "Waterball (johnny850807@gmail.com)"
 __license__ = "Apache 2.0"
diff --git a/tests/commons/utils/list_utils_test.py b/tests/commons/utils/list_utils_test.py
new file mode 100644
index 0000000..ef7c209
--- /dev/null
+++ b/tests/commons/utils/list_utils_test.py
@@ -0,0 +1,11 @@
+# -*- coding: utf-8 -*-
+# scip plugin
+from spring_cloud.commons.utils.list_utils import not_none_nor_empty
+
+__author__ = "Waterball (johnny850807@gmail.com)"
+__license__ = "Apache 2.0"
+
+
+def test_not_none_nor_empty():
+    assert not_none_nor_empty([1])
+    assert not not_none_nor_empty([])
diff --git a/tests/utils/__init__.py b/tests/utils/__init__.py
deleted file mode 100644
index d7b1237..0000000
--- a/tests/utils/__init__.py
+++ /dev/null
@@ -1,2 +0,0 @@
-__author__ = "Waterball (johnny850807@gmail.com)"
-__license__ = "Apache 2.0"

From 2718ef0794f23a51c28bd7a9f00e03c35439c7a9 Mon Sep 17 00:00:00 2001
From: johnny850807 <johnny850807@gmail.com>
Date: Fri, 13 Nov 2020 00:58:43 +0800
Subject: [PATCH 24/24] add utils/validate

---
 .../client/loadbalancer/supplier/decorator.py  |  4 ++--
 spring_cloud/commons/utils/validate.py         | 18 ++++++++++++++++++
 2 files changed, 20 insertions(+), 2 deletions(-)
 create mode 100644 spring_cloud/commons/utils/validate.py

diff --git a/spring_cloud/commons/client/loadbalancer/supplier/decorator.py b/spring_cloud/commons/client/loadbalancer/supplier/decorator.py
index 8b6da37..8d2e564 100644
--- a/spring_cloud/commons/client/loadbalancer/supplier/decorator.py
+++ b/spring_cloud/commons/client/loadbalancer/supplier/decorator.py
@@ -5,6 +5,7 @@
 # scip plugin
 from spring_cloud.commons.client import ServiceInstance
 from spring_cloud.commons.exceptions import NoneTypeError
+from spring_cloud.commons.utils import validate
 
 __author__ = "Waterball (johnny850807@gmail.com)"
 __license__ = "Apache 2.0"
@@ -24,8 +25,7 @@ class DelegatingServiceInstanceListSupplier(ServiceInstanceListSupplier, ABC):
     """
 
     def __init__(self, delegate: ServiceInstanceListSupplier):
-        NoneTypeError.raise_if_none(delegate)
-        self.delegate = delegate
+        self.delegate = validate.not_none(delegate)
 
     @property
     def service_id(self) -> str:
diff --git a/spring_cloud/commons/utils/validate.py b/spring_cloud/commons/utils/validate.py
new file mode 100644
index 0000000..e1e0a8d
--- /dev/null
+++ b/spring_cloud/commons/utils/validate.py
@@ -0,0 +1,18 @@
+# -*- coding: utf-8 -*-
+# scip plugin
+from spring_cloud.commons.exceptions.primitive import NoneTypeError
+
+__author__ = "Waterball (johnny850807@gmail.com)"
+__license__ = "Apache 2.0"
+
+
+def not_none(obj):
+    if obj:
+        return obj
+    raise NoneTypeError
+
+
+def is_instance_of(obj, the_type):
+    if isinstance(obj, the_type):
+        return obj
+    raise TypeError()