Add PouchDB Context store plugin

.gitignore

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+/node_modules
+coverage
+.nyc_output

README.md

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-# node-red-context-pouchdb
-A Node-RED Context store plugin backed by PouchDB
+# PouchDB Context store plugin
+
+The PouchDB Context store plugin holds context data in the PouchDB.
+
+## Install
+
+1. Run the following command in your Node-RED user directory - typically `~/.node-red`
+
+    npm install git+https://github.com/node-red/node-red-context-pouchdb
+
+2. Add a configuration in settings.js:
+
+```javascript
+contextStorage: {
+    pouchdb: {
+        module: require("node-red-context-pouchdb"),
+        config: {
+            // see below options
+        }
+    }
+}
+```
+
+## Options
+
+| Options  | Description                                                                   |
+| -------- | ----------------------------------------------------------------------------- |
+| name     | Specifies the `name` argument for creating the PouchDB databse. You can specify the following.<br>- SQLite : Database storage file path<br>- LevelDB: Database storage folder path<br>- CouchDB: Database URL<br>`default(SQLite): settings.userDir/context/context.db` |
+| options  | Specifies the PouchDB database `options`. <br>Example<br>- SQLite : {adapter: 'websql'}<br>- LevelDB: {adapter: 'leveldb'} or {}<br>- CouchDB: {} <br>`default(SQLite): {adapter: 'websql'}`|
+
+Reference: [PouchDB Create a database options](https://pouchdb.com/api.html#create_database)
+
+## Data Model
+
+- This plugin uses a PouchDB database for all context scope.
+- The NodeSQLite adapter is added to the PouchDB adapter. You can save the data in SQLite3.
+- You can also specify saving to a database (LevelDB, CouchDB) that PouchDB supports as standard.
+- This plugin saves a JSON object of keys and values in a document for each scope.
+  - The keys of `global context` will be id with `global` .
+  - The keys of `flow context` will be id with `<id of the flow>` .
+  - The keys of `node context` will be id with `<id of the node>` .
+  - Context data is stored in `doc.data` as a document for each scope.
+
+Structure of data stored in PouchDB:
+```json
+{
+    "total_rows": 3,
+    "offset": 0,
+    "rows": [
+        {
+            "id": "2052fca8.312154:a77d79a4.d1a908",
+            "key": "2052fca8.312154:a77d79a4.d1a908",
+            "value": {
+                "rev": "6-55e0513ffba64a8b8efec1ba8e43c90f"
+            },
+            "doc": {
+                "data": {
+                    "NODE-KEY-1": "NODE-DATA-1",
+                    "NODE-KEY-2": "NODE-DATA-2"
+                },
+                "_id": "2052fca8.312154:a77d79a4.d1a908",
+                "_rev": "6-55e0513ffba64a8b8efec1ba8e43c90f"
+            }
+        },
+        {
+            "id": "a77d79a4.d1a908",
+            "key": "a77d79a4.d1a908",
+            "value": {
+                "rev": "61-2c2a457388db1c3859b79e4bb62e9375"
+            },
+            "doc": {
+                "data": {
+                    "FLOW-KEY-1": "FLOW-DATA-1",
+                    "FLOW-KEY-2": "FLOW-DATA-2",
+                },
+                "_id": "a77d79a4.d1a908",
+                "_rev": "61-2c2a457388db1c3859b79e4bb62e9375"
+            }
+        },
+        {
+            "id": "global",
+            "key": "global",
+            "value": {
+                "rev": "73-ee260387e51ae20132076ccc83957600"
+            },
+            "doc": {
+                "data": {
+                    "GLOBAL-KEY-1": "GLOBAL-DATA-1",
+                    "GLOBAL-KEY-2": "GLOBAL-DATA-2",
+                },
+                "_id": "global",
+                "_rev": "73-ee260387e51ae20132076ccc83957600"
+            }
+        }
+    ]
+}
+```
+
+## Data Structure
+
+- Data is saved in the JSON object format supported by PouchDB. The plugin does not convert JSON data to a string for storage.
+
+Code example that references database data :
+```javascript
+var pd = require('pouchdb');
+pd.plugin(require('pouchdb-adapter-node-websql'));
+var db;
+
+db = new pd( "/home/user/.node-red/context/context.db", { adapter: 'websql' });
+db.allDocs({include_docs: true}, function(err, doc) {
+   if (err) {
+        return console.log(err);
+   } else {
+        var data = JSON.stringify(doc,null,4);
+        console.log(data);
+   }
+});
+```
+
+## Database replication
+
+- The data in the context store can be replicated to other DBs using the PouchDB feature.
+This allows you to back up the data stored in your local SQLite to a remote CouchDB.This allows you to back up context data stored in your local SQLite to a remote CouchDB.
+
+- In an environment with multiple context stores, only contexts using the PouchDB plugin will be backed up.
+
+Code example of replication to remote database(CouchDB) :
+```javascript
+var pd = require('pouchdb');
+pd.plugin(require('pouchdb-adapter-node-websql'));
+
+var source = "/home/user/.node-red/context/context.db";
+var target = "http://localhost:5984/couchdb_mycouchdb_1";
+
+var db_source = new pd(source, { adapter: 'websql' });
+var db_target = new pd(target);
+
+db_source.replicate.to(db_target)
+.on('complete', function () {
+        console.log ("Database replicated.");
+}).on('error', function (err) {
+        console.log(err);
+});
+```
+Reference: [PouchDB replication](https://pouchdb.com/api.html#replication)
+
+- Replication can also be filtered.You can also consider replicating a partial database (for example, only the global context part).
+
+Code example of replication filtering in global context :
+```javascript
+db_source.replicate.to(db_target, {
+  filter: function (doc) {
+    return doc._id === 'global';
+}})
+```
+Reference: [PouchDB filtered replication](https://pouchdb.com/api.html#filtered-replication)