watcha-synapse/docs/workers.md

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.

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 documenting it here to help admins needing a highly scalable Synapse instance similar to the one running matrix.org.

All processes continue to share the same database instance, and as such, workers only work with PostgreSQL-based Synapse deployments. SQLite should only be used for demo purposes and any admin considering workers should already be running PostgreSQL.

Master/worker communication

The workers communicate with the master process via a Synapse-specific protocol called 'replication' (analogous to MySQL- or Postgres-style database replication) which feeds a stream of relevant data from the master to the workers so they can be kept in sync with the master process and database state.

Additionally, workers may make HTTP requests to the master, to send information in the other direction. Typically this is used for operations which need to wait for a reply - such as sending an event.

Configuration

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. Note that this includes requests made to the federation port. See reverse_proxy.md for information on setting up a reverse proxy.

To enable workers, you need to add two replication listeners to the main Synapse configuration file (homeserver.yaml). For example:

listeners:
  # The TCP replication port
  - port: 9092
    bind_address: '127.0.0.1'
    type: replication

  # The HTTP replication port
  - port: 9093
    bind_address: '127.0.0.1'
    type: http
    resources:
     - names: [replication]

Under no circumstances should these replication API listeners be exposed to the public internet; they have no authentication and are unencrypted.

You should then create a set of configs for the various worker processes. Each worker configuration file inherits the configuration of the main homeserver configuration file. You can then override configuration specific to 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.

In the config file for each worker, you must specify the type of worker application (worker_app). The currently available worker applications are listed below. You must also specify the replication endpoints that it should talk to on the main synapse process. worker_replication_host should specify the host of the main synapse, worker_replication_port should point to the TCP replication listener port and worker_replication_http_port should point to the HTTP replication port.

For example:

worker_app: synapse.app.synchrotron

# The replication listener on the synapse to talk to.
worker_replication_host: 127.0.0.1
worker_replication_port: 9092
worker_replication_http_port: 9093

worker_listeners:
 - type: http
   port: 8083
   resources:
     - names:
       - client

worker_log_config: /home/matrix/synapse/config/synchrotron_log_config.yaml

...is a full configuration for a synchrotron worker instance, which will expose a plain HTTP /sync endpoint on port 8083 separately from the /sync endpoint provided by the main synapse.

Obviously you should configure your reverse-proxy to route the relevant endpoints to the worker (localhost:8083 in the above example).

Finally, you need to start your worker processes. This can be done with either synctl or your distribution's preferred service manager such as systemd. We recommend the use of systemd where available: for information on setting up systemd to start synapse workers, see systemd-with-workers. To use synctl, see below.

Experimental support for replication over redis

As of Synapse v1.13.0, it is possible to configure Synapse to send replication via a Redis pub/sub channel. This 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 upcoming performance improvements.

Note that this support is currently experimental; you may experience lost messages and similar problems! It is strongly recommended that admins setting up workers for the first time use direct TCP replication as above.

To configure Synapse to use Redis:

1. Install Redis following the normal procedure for your distribution - for example, on Debian, apt install redis-server. (It is safe to use an existing Redis deployment if you have one: we use a pub/sub stream named according to the server_name of your synapse server.)
2. Check Redis is running and accessible: you should be able to echo PING | nc -q1 localhost 6379 and get a response of +PONG.
3. Install the python prerequisites. If you installed synapse into a virtualenv, this can be done with:

   pip install matrix-synapse[redis]
   

   The debian packages from matrix.org already include the required dependencies.
4. Add config to the shared configuration (homeserver.yaml):

   redis:
     enabled: true
   

   Optional parameters which can go alongside enabled are host, port, password. Normally none of these are required.
5. Restart master and all workers.

Once redis replication is in use, worker_replication_port is redundant and can be removed from the worker configuration files. Similarly, the configuration for the listener for the TCP replication port can be removed from the main configuration file. Note that the HTTP replication port is still required.

Using synctl

If you want to use synctl to manage your synapse processes, you will need to create an an additional configuration file for the master synapse process. That configuration should look like this:

worker_app: synapse.app.homeserver

Additionally, each worker app must be configured with the name of a "pid file", to which it will write its process ID when it starts. For example, for a synchrotron, you might write:

worker_pid_file: /home/matrix/synapse/synchrotron.pid

Finally, to actually run your worker-based synapse, you must pass synctl the -a commandline option to tell it to operate on all the worker configurations found in the given directory, e.g.:

synctl -a $CONFIG/workers start

Currently one should always restart all workers when restarting or upgrading synapse, unless you explicitly know it's safe not to. For instance, restarting synapse without restarting all the synchrotrons may result in broken typing notifications.

To manipulate a specific worker, you pass the -w option to synctl:

synctl -w $CONFIG/workers/synchrotron.yaml restart

Available worker applications

synapse.app.pusher

Handles sending push notifications to sygnal and email. Doesn't handle any REST endpoints itself, but you should set start_pushers: False in the shared configuration file to stop the main synapse sending these notifications.

Note this worker cannot be load-balanced: only one instance should be active.

synapse.app.synchrotron

The synchrotron handles sync requests from clients. In particular, it can handle REST endpoints matching the following regular expressions:

^/_matrix/client/(v2_alpha|r0)/sync$
^/_matrix/client/(api/v1|v2_alpha|r0)/events$
^/_matrix/client/(api/v1|r0)/initialSync$
^/_matrix/client/(api/v1|r0)/rooms/[^/]+/initialSync$

The above endpoints should all be routed to the synchrotron worker by the reverse-proxy configuration.

It is possible to run multiple instances of the synchrotron to scale horizontally. In this case the reverse-proxy should be configured to load-balance across the instances, though it will be more efficient if all requests from a particular user are routed to a single instance. Extracting a userid from the access token is currently left as an exercise for the reader.

synapse.app.appservice

Handles sending output traffic to Application Services. Doesn't handle any REST endpoints itself, but you should set notify_appservices: False in the shared configuration file to stop the main synapse sending these notifications.

Note this worker cannot be load-balanced: only one instance should be active.

synapse.app.federation_reader

Handles a subset of federation endpoints. In particular, it can handle REST endpoints matching the following regular expressions:

^/_matrix/federation/v1/event/
^/_matrix/federation/v1/state/
^/_matrix/federation/v1/state_ids/
^/_matrix/federation/v1/backfill/
^/_matrix/federation/v1/get_missing_events/
^/_matrix/federation/v1/publicRooms
^/_matrix/federation/v1/query/
^/_matrix/federation/v1/make_join/
^/_matrix/federation/v1/make_leave/
^/_matrix/federation/v1/send_join/
^/_matrix/federation/v2/send_join/
^/_matrix/federation/v1/send_leave/
^/_matrix/federation/v2/send_leave/
^/_matrix/federation/v1/invite/
^/_matrix/federation/v2/invite/
^/_matrix/federation/v1/query_auth/
^/_matrix/federation/v1/event_auth/
^/_matrix/federation/v1/exchange_third_party_invite/
^/_matrix/federation/v1/user/devices/
^/_matrix/federation/v1/send/
^/_matrix/federation/v1/get_groups_publicised$
^/_matrix/key/v2/query

Additionally, the following REST endpoints can be handled for GET requests:

^/_matrix/federation/v1/groups/

The above endpoints should all be routed to the federation_reader worker by the reverse-proxy configuration.

The ^/_matrix/federation/v1/send/ endpoint must only be handled by a single instance.

Note that federation must be added to the listener resources in the worker config:

worker_app: synapse.app.federation_reader
...
worker_listeners:
 - type: http
   port: <port>
   resources:
     - names:
       - federation

synapse.app.federation_sender

Handles sending federation traffic to other servers. Doesn't handle any REST endpoints itself, but you should set send_federation: False in the shared configuration file to stop the main synapse sending this traffic.

Note this worker cannot be load-balanced: only one instance should be active.

synapse.app.media_repository

Handles the media repository. It can handle all endpoints starting with:

/_matrix/media/

... and the following regular expressions matching media-specific administration APIs:

^/_synapse/admin/v1/purge_media_cache$
^/_synapse/admin/v1/room/.*/media.*$
^/_synapse/admin/v1/user/.*/media.*$
^/_synapse/admin/v1/media/.*$
^/_synapse/admin/v1/quarantine_media/.*$

You should also set enable_media_repo: False in the shared configuration file to stop the main synapse running background jobs related to managing the media repository.

In the media_repository worker configuration file, configure the http listener to expose the media resource. For example:

    worker_listeners:
     - type: http
       port: 8085
       resources:
         - names:
           - media

Note this worker cannot be load-balanced: only one instance should be active.

synapse.app.client_reader

Handles client API endpoints. It can handle REST endpoints matching the following regular expressions:

^/_matrix/client/(api/v1|r0|unstable)/publicRooms$
^/_matrix/client/(api/v1|r0|unstable)/rooms/.*/joined_members$
^/_matrix/client/(api/v1|r0|unstable)/rooms/.*/context/.*$
^/_matrix/client/(api/v1|r0|unstable)/rooms/.*/members$
^/_matrix/client/(api/v1|r0|unstable)/rooms/.*/state$
^/_matrix/client/(api/v1|r0|unstable)/login$
^/_matrix/client/(api/v1|r0|unstable)/account/3pid$
^/_matrix/client/(api/v1|r0|unstable)/keys/query$
^/_matrix/client/(api/v1|r0|unstable)/keys/changes$
^/_matrix/client/versions$
^/_matrix/client/(api/v1|r0|unstable)/voip/turnServer$
^/_matrix/client/(api/v1|r0|unstable)/joined_groups$
^/_matrix/client/(api/v1|r0|unstable)/publicised_groups$
^/_matrix/client/(api/v1|r0|unstable)/publicised_groups/

Additionally, the following REST endpoints can be handled for GET requests:

^/_matrix/client/(api/v1|r0|unstable)/pushrules/.*$
^/_matrix/client/(api/v1|r0|unstable)/groups/.*$
^/_matrix/client/(api/v1|r0|unstable)/user/[^/]*/account_data/
^/_matrix/client/(api/v1|r0|unstable)/user/[^/]*/rooms/[^/]*/account_data/

Additionally, the following REST endpoints can be handled, but all requests must be routed to the same instance:

^/_matrix/client/(r0|unstable)/register$
^/_matrix/client/(r0|unstable)/auth/.*/fallback/web$

Pagination requests can also be handled, but all requests with the same path room must be routed to the same instance. Additionally, care must be taken to ensure that the purge history admin API is not used while pagination requests for the room are in flight:

^/_matrix/client/(api/v1|r0|unstable)/rooms/.*/messages$

synapse.app.user_dir

Handles searches in the user directory. It can handle REST endpoints matching the following regular expressions:

^/_matrix/client/(api/v1|r0|unstable)/user_directory/search$

When using this worker you must also set update_user_directory: False in the shared configuration file to stop the main synapse running background jobs related to updating the user directory.

synapse.app.frontend_proxy

Proxies some frequently-requested client endpoints to add caching and remove load from the main synapse. It can handle REST endpoints matching the following regular expressions:

^/_matrix/client/(api/v1|r0|unstable)/keys/upload

If use_presence is False in the homeserver config, it can also handle REST endpoints matching the following regular expressions:

^/_matrix/client/(api/v1|r0|unstable)/presence/[^/]+/status

This "stub" presence handler will pass through GET request but make the PUT effectively a no-op.

It will proxy any requests it cannot handle to the main synapse instance. It must therefore be configured with the location of the main instance, via the worker_main_http_uri setting in the frontend_proxy worker configuration file. For example:

worker_main_http_uri: http://127.0.0.1:8008

synapse.app.event_creator

Handles some event creation. It can handle REST endpoints matching:

^/_matrix/client/(api/v1|r0|unstable)/rooms/.*/send
^/_matrix/client/(api/v1|r0|unstable)/rooms/.*/state/
^/_matrix/client/(api/v1|r0|unstable)/rooms/.*/(join|invite|leave|ban|unban|kick)$
^/_matrix/client/(api/v1|r0|unstable)/join/
^/_matrix/client/(api/v1|r0|unstable)/profile/

It will create events locally and then send them on to the main synapse instance to be persisted and handled.