Add Redis plugin

.gitignore

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+/node_modules
+coverage
+.nyc_output

Readme.md

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+***This is a work in progress.***
+
+# Redis plugin
+
+The Redis plugin holds context data in the Redis.
+
+## Pre-requisite
+
+To run this you need a Redis server running. For details see <a href="http://redis.io/" target="_new">the Redis site</a>.
+
+## 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-redis
+
+1. Add a configuration in settings.js:
+
+```javascript
+contextStorage: {
+    redis: {
+        module: require("node-red-context-redis"),
+        config: {
+            // see below options
+        }
+    }
+}
+```
+
+### Options
+
+This plugin exposes some options defined in [node_redis](https://github.com/NodeRedis/node_redis) as itself options.
+It needs following configuration options:
+
+| Options        | Description                                                                                                 |
+| -------------- | ----------------------------------------------------------------------------------------------------------- |
+| host           | The IP address of the Redis server.    `Default: "127.0.0.1"`                                               |
+| port           | The port of the Redis server.          `Default: 6379`                                                      |
+| db             | The Redis logical database to connect. `Default: 0`                                                         |
+| prefix         | If set, the string used to prefix all used keys.                                                            |
+| password       | If set, the plugin will run Redis AUTH command on connect. *Note: the password will be sent as plaintext.*  |
+| tls            | An object containing options to pass to tls.connect to set up a TLS connection to the server.               |
+| retry_strategy | Specifies a function to reconnect if the connection to Redis is lost.                                       |
+|                | `default: undefined (Use the default retry strategy)`                                                       |
+
+see https://github.com/NodeRedis/node_redis#options-object-properties
+
+## Data Model
+
+```text
+Node-RED                      Redis
++-------------------+         +-------------------------------+
+| global context    |         | logical database              |
+| +---------------+ |         | +---------------------------+ |
+| | +-----+-----+ | |         | | +-----------------+-----+ | |
+| | | key |value| | | <-----> | | | global:key      |value| | |
+| | +-----+-----+ | |         | | +-----------------+-----+ | |
+| +---------------+ |         | |                           | |
+|                   |         | |                           | |
+| flow context      |         | |                           | |
+| +---------------+ |         | |                           | |
+| | +-----+-----+ | |         | | +-----------------+-----+ | |
+| | | key |value| | | <-----> | | | <flow's id>:key |value| | |
+| | +-----+-----+ | |         | | +-----------------+-----+ | |
+| +---------------+ |         | |                           | |
+|                   |         | |                           | |
+| node context      |         | |                           | |
+| +---------------+ |         | |                           | |
+| | +-----+-----+ | |         | | +-----------------+-----+ | |
+| | | key |value| | | <-----> | | | <node's id>:key |value| | |
+| | +-----+-----+ | |         | | +-----------------+-----+ | |
+| +---------------+ |         | +---------------------------+ |
++-------------------+         +-------------------------------+
+```
+
+- This plugin uses a Redis logical database for all context scope.
+- This plugin prefixes all used keys with context scope in order to identify the scope of the key.
+  - The keys of `global context` will be prefixed with `global:` .
+    e.g.  Set `"foo"` to hold `"bar"` in the global context -> Set `"global:foo"` to hold `"bar"` in the Redis logical database.
+  - The keys of `flow context` will be prefixed with `<id of the flow>:` .
+    e.g.  Set `"foo"` to hold `"bar"` in the flow context whose id is `8588e4b8.784b38` -> Set `"8588e4b8.784b38:foo"` to hold `"bar"` in the Redis.
+  - The keys of `node context` will be prefixed with `<id of the node>:` .
+    e.g.  Set `"foo"` to hold `"bar"` in the node context whose id is `80d8039e.2b82:8588e4b8.784b38` -> Set `"80d8039e.2b82:8588e4b8.784b38:foo"` to hold `"bar"` in the Redis.
+
+## Data Structure
+
+- This plugin converts a value of context to JSON and stores it as string type to the Redis.
+- After getting a value from the Redis, the plugin also converts the value to an object or a primitive value.
+
+```text
+Node-RED                                 Redis
++------------------------------+         +---------------------------------------------+
+| global context               |         | logical database                            |
+| +--------------------------+ |         | +-----------------------------------------+ |
+| | +--------+-------------+ | |         | | +---------------+---------------------+ | |
+| | | str    | "foo"       | | | <-----> | | | global:str    | "\"foo\""           | | |
+| | +--------+-------------+ | |         | | +---------------+---------------------+ | |
+| | | num    | 1           | | | <-----> | | | global:num    | "1"                 | | |
+| | +--------+-------------+ | |         | | +---------------+---------------------+ | |
+| | | nstr   | "10"        | | | <-----> | | | global:nstr   | "\"10\""            | | |
+| | +--------+-------------+ | |         | | +---------------+---------------------+ | |
+| | | bool   | false       | | | <-----> | | | global:bool   | "false"             | | |
+| | +--------+-------------+ | |         | | +---------------+---------------------+ | |
+| | | arr    | ["a","b"]   | | | <-----> | | | global:arr    | "[\"a\",\"b\"]"     | | |
+| | +--------+-------------+ | |         | | +---------------+---------------------+ | |
+| | | obj    | {foo,"bar"} | | | <-----> | | | global:obj    | "{\"foo\",\"bar\"}" | | |
+| | +--------+-------------+ | |         | | +---------------+---------------------+ | |
+| +--------------------------+ |         | +-----------------------------------------+ |
++------------------------------+         +---------------------------------------------+
+```
+
+Other Redis client(e.g. redis-cli) can get the value stored by Node-RED like followings.
+
+Node-RED
+
+```javascript
+global.set("foo","bar","redis");
+global.set("obj",{key:"value"},"redis");
+```
+
+redis-cli
+
+```console
+redis> GET global:foo
+"\"var\""
+redis> GET global:obj
+"{\"key\":\"value\"}"
+redis>
+```