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Update workers docs (#7990)

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Improve workers docs.

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# Scaling synapse via workers
For small instances it recommended to run Synapse in monolith mode (the
default). For larger instances where performance is a concern it can be helpful
to split out functionality into multiple separate python processes. These
processes are called 'workers', and are (eventually) intended to scale
horizontally independently.
For small instances it recommended to run Synapse in the default monolith mode.
For larger instances where performance is a concern it can be helpful to split
out functionality into multiple separate python processes. These processes are
called 'workers', and are (eventually) intended to scale horizontally
independently.
Synapse's worker support is under active development and subject to change as
we attempt to rapidly scale ever larger Synapse instances. However we are
@ -23,29 +23,30 @@ The processes communicate with each other via a Synapse-specific protocol called
feeds streams of newly written data between processes so they can be kept in
sync with the database state.
Additionally, processes may make HTTP requests to each other. Typically this is
used for operations which need to wait for a reply - such as sending an event.
When configured to do so, Synapse uses a
[Redis pub/sub channel](https://redis.io/topics/pubsub) to send the replication
stream between all configured Synapse processes. Additionally, processes may
make HTTP requests to each other, primarily for operations which need to wait
for a reply ─ such as sending an event.
As of Synapse v1.13.0, it is possible to configure Synapse to send replication
via a [Redis pub/sub channel](https://redis.io/topics/pubsub), and is now the
recommended way of configuring replication. This is an alternative to the old
direct TCP connections to the main process: rather than all the workers
connecting to the main process, all the workers and the main process connect to
Redis, which relays replication commands between processes. This can give a
significant cpu saving on the main process and will be a prerequisite for
upcoming performance improvements.
Redis support was added in v1.13.0 with it becoming the recommended method in
v1.18.0. It replaced the old direct TCP connections (which is deprecated as of
v1.18.0) to the main process. With Redis, rather than all the workers connecting
to the main process, all the workers and the main process connect to Redis,
which relays replication commands between processes. This can give a significant
cpu saving on the main process and will be a prerequisite for upcoming
performance improvements.
(See the [Architectural diagram](#architectural-diagram) section at the end for
a visualisation of what this looks like)
See the [Architectural diagram](#architectural-diagram) section at the end for
a visualisation of what this looks like.
## Setting up workers
A Redis server is required to manage the communication between the processes.
(The older direct TCP connections are now deprecated.) The Redis server
should be installed following the normal procedure for your distribution (e.g.
`apt install redis-server` on Debian). It is safe to use an existing Redis
deployment if you have one.
The Redis server should be installed following the normal procedure for your
distribution (e.g. `apt install redis-server` on Debian). It is safe to use an
existing Redis deployment if you have one.
Once installed, check that Redis is running and accessible from the host running
Synapse, for example by executing `echo PING | nc -q1 localhost 6379` and seeing
@ -65,8 +66,9 @@ https://hub.docker.com/r/matrixdotorg/synapse/.
To make effective use of the workers, you will need to configure an HTTP
reverse-proxy such as nginx or haproxy, which will direct incoming requests to
the correct worker, or to the main synapse instance. See [reverse_proxy.md](reverse_proxy.md)
for information on setting up a reverse proxy.
the correct worker, or to the main synapse instance. See
[reverse_proxy.md](reverse_proxy.md) for information on setting up a reverse
proxy.
To enable workers you should create a configuration file for each worker
process. Each worker configuration file inherits the configuration of the shared
@ -75,8 +77,12 @@ that worker, e.g. the HTTP listener that it provides (if any); logging
configuration; etc. You should minimise the number of overrides though to
maintain a usable config.
Next you need to add both a HTTP replication listener and redis config to the
shared Synapse configuration file (`homeserver.yaml`). For example:
### Shared Configuration
Next you need to add both a HTTP replication listener, used for HTTP requests
between processes, and redis config to the shared Synapse configuration file
(`homeserver.yaml`). For example:
```yaml
# extend the existing `listeners` section. This defines the ports that the
@ -98,6 +104,9 @@ See the sample config for the full documentation of each option.
Under **no circumstances** should the replication listener be exposed to the
public internet; it has no authentication and is unencrypted.
### Worker Configuration
In the config file for each worker, you must specify the type of worker
application (`worker_app`), and you should specify a unqiue name for the worker
(`worker_name`). The currently available worker applications are listed below.