Add Redis plugin

[HirokiUchikawa]

This makes it possible to store context data to Redis.

The design note is https://github.com/node-red/node-red/wiki/Design%3A-Persistable-Context#redis-plugin.

Install prototype

1. Run the following command

cd ~/.node-red
npm install git+https://github.com/node-red-hitachi/node-red-context-redis

2. Add a configuration in settings.js

contextStorage: {
    redis: {
        module: require("node-red-context-redis"),
        config: {
            // see Readme.md
        }
    }
}

I would appreciate if you could give me feedback on design, codes, following concerns, etc.

Concerns

• How to clean unnecessary context ?
  clean function detects and removes unnecessary contexts based on activeNodes argument and data in the store.
  But when there are some data stored by others in the store, the plugin cannot determine if the data is not used.

  Options:

  1. Clean the data without caring others.
     The plugin cleans the data according to its activeNodes only. But others data are also deleted.
     memory and localfilesystem do this because their store are not shared with others.

  2. clean function receives deactiveNodes list instead of activeNodes.
     The plugin can know what is not used according to it, but we have to change codes. e.g. other plugin clean.

  3. Don't clean.
     User has to clean manually if needed.
     This prototype does not clean the data.

[cmerchant]

Any reason not to reconnect? If this is set to undefined and the connection is lost to the db, then no attempts to reconnect will happen until the plugin is re-initialized which is at node-red container startup. I would suggest removing this line of code unless there is a reason to not reconnect.

Here is more context regarding this issue,

#2

[travisghansen]

I also would like to start using this. Has anyone done anything with the reconnect issue mentioned above?

[mannharleen]

Is this usable? Is it “ratified” by the NR core team?

[knolleary]

No it is no "ratified". It is an unmerged pr that has been waiting to be reviewed for a long time. Anyone using it does so at their own risk without any support from the core team.

We still really want to move forwards with this, but it has fallen down the to-do list and not had the attention it needs.

[KazuhiroItoh]

Updated Redis plugin.

The following has been changed.

1. Enable clean function
   Clean was disabled because of concerns about references from multiple instances.
   Currently, this usage is not expected, so I enabled the function like other plugins.

2. Add Redis retry_strategy option
   In response to Issue # 2, I have added an option to change retry_strategy.

[KazuhiroItoh]

@knolleary
I would appreciate it if you could check on updating the Redis plugin.
I am making modifications to match the specifications of existing plug-ins.

[knolleary]

@KazuhiroItoh I'm happy to merge this as-is without doing an in depth review at this time.

I think there will be some follow-up actions to take we can handle via individual issues and PRs.

Thanks for your work (and thanks to @HirokiUchikawa for doing the original work so long ago!)

[mannharleen]

The README.md still says "This is a work in progress." . Is it still so?

[KazuhiroItoh]

Thank you for your advice.
I will correct README.md as it is not appropriate.