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Gitaly development guidelines

Gitaly is a high-level Git RPC service used by GitLab Rails, Workhorse and GitLab Shell.

Deep Dive

In May 2019, Bob Van Landuyt hosted a Deep Dive (GitLab team members only: https://gitlab.com/gitlab-org/create-stage/-/issues/1) on the Gitaly project. It included how to contribute to it as a Ruby developer, and shared domain-specific knowledge with anyone who may work in this part of the codebase in the future.

You can find the recording on YouTube, and the slides on Google Slides and in PDF.

Everything covered in this deep dive was accurate as of GitLab 11.11, and while specific details may have changed, it should still serve as a good introduction.

Beginner's guide

Start by reading the Gitaly repository's Beginner's guide to Gitaly contributions. It describes how to set up Gitaly, the various components of Gitaly and what they do, and how to run its test suites.

Developing new Git features

To read or write Git data, a request has to be made to Gitaly. This means that if you're developing a new feature where you need data that's not yet available in lib/gitlab/git changes have to be made to Gitaly.

There should be no new code that touches Git repositories via disk access (for example, Rugged, git, rm -rf) anywhere in the gitlab repository. Anything that needs direct access to the Git repository must be implemented in Gitaly, and exposed via an RPC.

It's often easier to develop a new feature in Gitaly if you make the changes to GitLab that intends to use the new feature in a separate merge request, to be merged immediately after the Gitaly one. This allows you to test your changes before they are merged.

See below for instructions on running GitLab tests with a modified version of Gitaly.
In GDK run gdk install and restart GDK using gdk restart to use a locally modified Gitaly version for development

Gitaly-Related Test Failures

If your test-suite is failing with Gitaly issues, as a first step, try running:

rm -rf tmp/tests/gitaly

During RSpec tests, the Gitaly instance writes logs to gitlab/log/gitaly-test.log.

Legacy Rugged code

While Gitaly can handle all Git access, many of GitLab customers still run Gitaly atop NFS. The legacy Rugged implementation for Git calls may be faster than the Gitaly RPC due to N+1 Gitaly calls and other reasons. See the issue for more details.

Until GitLab has eliminated most of these inefficiencies or the use of NFS is discontinued for Git data, Rugged implementations of some of the most commonly-used RPCs can be enabled via feature flags:

rugged_find_commit
rugged_get_tree_entries
rugged_tree_entry
rugged_commit_is_ancestor
rugged_commit_tree_entry
rugged_list_commits_by_oid

A convenience Rake task can be used to enable or disable these flags all together. To enable:

bundle exec rake gitlab:features:enable_rugged

To disable:

bundle exec rake gitlab:features:disable_rugged

Most of this code exists in the lib/gitlab/git/rugged_impl directory.

NOTE: You should not have to add or modify code related to Rugged unless explicitly discussed with the Gitaly Team. This code does not work on GitLab.com or other GitLab instances that do not use NFS.

`TooManyInvocationsError` errors

During development and testing, you may experience Gitlab::GitalyClient::TooManyInvocationsError failures. The GitalyClient attempts to block against potential n+1 issues by raising this error when Gitaly is called more than 30 times in a single Rails request or Sidekiq execution.

As a temporary measure, export GITALY_DISABLE_REQUEST_LIMITS=1 to suppress the error. This disables the n+1 detection in your development environment.

Please raise an issue in the GitLab CE or EE repositories to report the issue. Include the labels ~Gitaly ~performance ~"technical debt". Please ensure that the issue contains the full stack trace and error message of the TooManyInvocationsError. Also include any known failing tests if possible.

Isolate the source of the n+1 problem. This is usually a loop that results in Gitaly being called for each element in an array. If you are unable to isolate the problem, please contact a member of the Gitaly Team for assistance.

After the source has been found, wrap it in an allow_n_plus_1_calls block, as follows:

# n+1: link to n+1 issue
Gitlab::GitalyClient.allow_n_plus_1_calls do
  # original code
  commits.each { |commit| ... }
end

After the code is wrapped in this block, this code path is excluded from n+1 detection.

Request counts

Commits and other Git data, is now fetched through Gitaly. These fetches can, much like with a database, be batched. This improves performance for the client and for Gitaly itself and therefore for the users too. To keep performance stable and guard performance regressions, Gitaly calls can be counted and the call count can be tested against. This requires the :request_store flag to be set.

describe 'Gitaly Request count tests' do
  context 'when the request store is activated', :request_store do
    it 'correctly counts the gitaly requests made' do
      expect { subject }.to change { Gitlab::GitalyClient.get_request_count }.by(10)
    end
  end
end

Running tests with a locally modified version of Gitaly

Usually, GitLab CE/EE tests use a local clone of Gitaly in tmp/tests/gitaly pinned at the version specified in GITALY_SERVER_VERSION. The GITALY_SERVER_VERSION file supports also branches and SHA to use a custom commit in the repository.

NOTE: With the introduction of auto-deploy for Gitaly, the format of GITALY_SERVER_VERSION was aligned with Omnibus syntax. It no longer supports =revision, it evaluates the file content as a Git reference (branch or SHA). Only if it matches a semantic version does it prepend a v.

If you want to run tests locally against a modified version of Gitaly you can replace tmp/tests/gitaly with a symlink. This is much faster because it avoids a Gitaly re-install each time you run rspec.

Make sure this directory contains the files config.toml and praefect.config.toml. You can copy config.toml from config.toml.example, and praefect.config.toml from config.praefect.toml.example. After copying, make sure to edit them so everything points to the correct paths.

rm -rf tmp/tests/gitaly
ln -s /path/to/gitaly tmp/tests/gitaly

Make sure you run make in your local Gitaly directory before running tests. Otherwise, Gitaly fails to boot.

If you make changes to your local Gitaly in between test runs you need to manually run make again.

Note that CI tests do not use your locally modified version of Gitaly. To use a custom Gitaly version in CI, you must update GITALY_SERVER_VERSION as described at the beginning of this section.

To use a different Gitaly repository, such as if your changes are present on a fork, you can specify a GITALY_REPO_URL environment variable when running tests:

GITALY_REPO_URL=https://gitlab.com/nick.thomas/gitaly bundle exec rspec spec/lib/gitlab/git/repository_spec.rb

If your fork of Gitaly is private, you can generate a Deploy Token and specify it in the URL:

GITALY_REPO_URL=https://gitlab+deploy-token-1000:token-here@gitlab.com/nick.thomas/gitaly bundle exec rspec spec/lib/gitlab/git/repository_spec.rb

To use a custom Gitaly repository in CI/CD, for instance if you want your GitLab fork to always use your own Gitaly fork, set GITALY_REPO_URL as a CI/CD variable.

Use a locally modified version of Gitaly RPC client

If you are making changes to the RPC client, such as adding a new endpoint or adding a new parameter to an existing endpoint, follow the guide for Gitaly protobuf specifications. Then:

Run bundle install in the tools/protogem directory of Gitaly.
Build the RPC client gem from the root directory of Gitaly:
```
BUILD_GEM_OPTIONS=--skip-verify-tag make build-proto-gem
```
In the _build directory of Gitaly, unpack the newly created .gem file and create a gemspec:
```
gem unpack gitaly.gem &&
gem spec gitaly.gem > gitaly/gitaly.gemspec
```
Change the gitaly line in the Rails' Gemfile to:
```
gem 'gitaly', path: '../gitaly/_build'
```
Run bundle install to use the modified RPC client.

Re-run steps 2-5 each time you want to try out new changes.

Return to Development documentation

Wrapping RPCs in feature flags

Here are the steps to gate a new feature in Gitaly behind a feature flag.

Gitaly

Create a package scoped flag name:

var findAllTagsFeatureFlag = "go-find-all-tags"

Create a switch in the code using the featureflag package:

if featureflag.IsEnabled(ctx, findAllTagsFeatureFlag) {
  // go implementation
} else {
  // ruby implementation
}

Create Prometheus metrics:

var findAllTagsRequests = prometheus.NewCounterVec(
  prometheus.CounterOpts{
    Name: "gitaly_find_all_tags_requests_total",
    Help: "Counter of go vs ruby implementation of FindAllTags",
  },
  []string{"implementation"},
)

func init() {
  prometheus.Register(findAllTagsRequests)
}

if featureflag.IsEnabled(ctx, findAllTagsFeatureFlag) {
  findAllTagsRequests.WithLabelValues("go").Inc()
  // go implementation
} else {
  findAllTagsRequests.WithLabelValues("ruby").Inc()
  // ruby implementation
}

Set headers in tests:

import (
  "google.golang.org/grpc/metadata"

  "gitlab.com/gitlab-org/gitaly/internal/featureflag"
)

//...

md := metadata.New(map[string]string{featureflag.HeaderKey(findAllTagsFeatureFlag): "true"})
ctx = metadata.NewOutgoingContext(context.Background(), md)

c, err = client.FindAllTags(ctx, rpcRequest)
require.NoError(t, err)

GitLab Rails

Test in a Rails console by setting the feature flag:

Feature.enable('gitaly_go_find_all_tags')

Pay attention to the name of the flag and the one used in the Rails console. There is a difference between them (dashes replaced by underscores and name prefix is changed). Make sure to prefix all flags with gitaly_.

NOTE: If not set in GitLab, feature flags are read as false from the console and Gitaly uses their default value. The default value depends on the GitLab version.

Testing with GDK

To be sure that the flag is set correctly and it goes into Gitaly, you can check the integration by using GDK:

The state of the flag must be observable. To check it, you must enable it by fetching the Prometheus metrics:
1. Navigate to the GDK root directory.
2. Make sure you have the proper branch checked out for Gitaly.
3. Recompile it with make gitaly-setup and restart the service with gdk restart gitaly.
4. Make sure your setup is running: gdk status | grep praefect.
5. Check what configuration file is used: cat ./services/praefect/run | grep praefect value of the -config flag
6. Uncomment prometheus_listen_addr in the configuration file and run gdk restart gitaly.
Make sure that the flag is not enabled yet:
1. Perform whatever action is required to trigger your changes, such as project creation, submitting commit, or observing history.
2. Check that the list of current metrics has the new counter for the feature flag:
```
curl --silent "http://localhost:9236/metrics" | grep go_find_all_tags
```
After you observe the metrics for the new feature flag and it increments, you can enable the new feature:
1. Navigate to the GDK root directory.
2. Start a Rails console:
```
bundle install && bundle exec rails console
```
3. Check the list of feature flags:
```
Feature::Gitaly.server_feature_flags
```
  It should be disabled "gitaly-feature-go-find-all-tags"=>"false".
4. Enable it:
```
Feature.enable('gitaly_go_find_all_tags')
```
5. Exit the Rails console and perform whatever action is required to trigger your changes, such as project creation, submitting commit, or observing history.
6. Verify the feature is on by observing the metrics for it:
```
curl --silent "http://localhost:9236/metrics" | grep go_find_all_tags
```

Using Praefect in test

By default Praefect in test uses an in-memory election strategy. This strategy is deprecated and no longer used in production. It mainly is kept for unit-testing purposes.

A more modern election strategy requires a connection with a PostgreSQL database. This behavior is disabled by default when running tests, but you can enable it by setting GITALY_PRAEFECT_WITH_DB=1 in your environment.

This requires you have PostgreSQL running, and you have the database created. When you are using GDK, you can set it up with:

Start the database: gdk start db
Load the environment from GDK: eval $(cd ../gitaly && gdk env)
Create the database: createdb --encoding=UTF8 --locale=C --echo praefect_test

Git references used by Gitaly

Gitaly uses many Git references (refs) to provide Git services to GitLab.

Standard Git references

These standard Git references are used by GitLab (through Gitaly) in any Git repository:

refs/heads/. Used for branches. See the git branch documentation.
refs/tags/. Used for tags. See the git tag documentation.

GitLab-specific references

These GitLab-specific references are used exclusively by GitLab (through Gitaly):

refs/keep-around/<object-id>. References to commits that have pipeline jobs or merge requests. The object-id points to the commit the pipeline was run on.
refs/merge-requests/<merge-request-iid>/. Merges merge two histories together. This ref namespace tracks information about a merge using the following refs under it:
- head. Current HEAD of the merge request.
- merge. Commit for the merge request. Every merge request creates a commit object under refs/keep-around.
- If merge trains are enabled: train. Commit for the merge train.
refs/pipelines/<pipeline-iid>. References to pipelines. Temporarily used to store the pipeline commit object ID.
refs/environments/<environment-slug>. References to commits where deployments to environments were performed.
refs/heads/revert-<source-commit-short-object-id>. References to the commit's object ID created when reverting changes.

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Gitaly development guidelines

Deep Dive

Beginner's guide

Developing new Git features

Gitaly-Related Test Failures

Legacy Rugged code

`TooManyInvocationsError` errors

Request counts

Running tests with a locally modified version of Gitaly

Use a locally modified version of Gitaly RPC client

Wrapping RPCs in feature flags

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Using Praefect in test

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Gitaly development guidelines

Deep Dive

Beginner's guide

Developing new Git features

Gitaly-Related Test Failures

Legacy Rugged code

TooManyInvocationsError errors

Request counts

Running tests with a locally modified version of Gitaly

Use a locally modified version of Gitaly RPC client

Wrapping RPCs in feature flags

Gitaly

GitLab Rails

Testing with GDK

Using Praefect in test

Git references used by Gitaly

Standard Git references

GitLab-specific references

`TooManyInvocationsError` errors