write some words about redis

[richvdh]

No description provided.

[richvdh]

redis is an alternative to direct tcp connections to the master: rather than all the workers connecting to the master, all the workers and the master connect to redis, which relays replication commands between processes. this can give a significant cpu saving on the master and will be a prerequisite for operation gemini work.

1. Install redis following the normal procedure for your distro - eg apt install redis-server for debian/ubuntu
2. check it's running and accessible: you should be able to echo PING | nc -q1 localhost 6379 and get a response of +PONG.
3. Add config to homeserver.yaml:

   redis:
     enabled: true

   Optional parameters which can go alongside enabled are host, port, password, dbid. If you don't know if you need the last two, you probably don't.
4. That's it. Restart master and all workers.
5. the type: replication listener documented in https://github.com/matrix-org/synapse/blob/master/docs/workers.md#configuration is now redundant. (the type: http one is still needed).

Question: should we put this in the sample config yet?

[Demi-Marie]

If we support AF_UNIX connections as well as TCP connections, a performance boost can be expected.

[richvdh]

done in #7446.

[richvdh]

@demimarie-parity txredisapi doesn't support unix-domain sockets afaict and it's not entirely obvious to me that the performance boost would be anything other than trivial

[jkhsjdhjs]

@richvdh txredisapi supports unix domain socket since at least 2013: https://github.com/IlyaSkriblovsky/txredisapi#features

[Dual-0]

Is adding redis via unix socket possible in the future? Do we need a new issue/feature_request?

[ShadowJonathan]

I'd recommend filing a new issue for that.