# Scaling synapse via workers

For small instances it is 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 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.

See also Matrix.org blog post for a higher level overview.

## Main process/worker communication

The processes communicate with each other via a Synapse-specific protocol called 'replication' (analogous to MySQL- or Postgres-style database replication) which feeds streams of newly written data between processes so they can be kept in sync with the database state.

When configured to do so, Synapse uses a Redis pub/sub channel 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.

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.

If Redis support is enabled Synapse will use it as a shared cache, as well as a pub/sub mechanism.

See the 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 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 a response of `+PONG`.

The appropriate dependencies must also be installed for Synapse. If using a virtualenv, these can be installed with:

    pip install "matrix-synapse[redis]"

Note that these dependencies are included when synapse is installed with `pip install matrix-synapse[all]`. They are also included in the debian packages from `matrix.org` and in the docker images at 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 the reverse proxy documentation for information on setting up a reverse proxy.

When using workers, each worker process has its own configuration file which contains settings specific to that worker, such as the HTTP listener that it provides (if any), logging configuration, etc.

Normally, the worker processes are configured to read from a shared configuration file as well as the worker-specific configuration files. This makes it easier to keep common configuration settings synchronised across all the processes.

The main process is somewhat special in this respect: it does not normally need its own configuration file and can take all of its configuration from the shared configuration file.

### Shared configuration

Normally, only a couple of changes are needed to make an existing configuration file suitable for use with workers. First, you need to enable an "HTTP replication listener" for the main process; and secondly, you need to enable redis-based replication. Optionally, a shared secret can be used to authenticate HTTP traffic between workers. For example:

    # extend the existing `listeners` section. This defines the ports that the
    # main process will listen on.
    listeners:
      # The HTTP replication port
      - port: 9093
        bind_address: '127.0.0.1'
        type: http
        resources:
         - names: [replication]

    # Add a random shared secret to authenticate traffic.
    worker_replication_secret: ""

    redis:
        enabled: true

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 unique name for the worker (`worker_name`). The currently available worker applications are listed below. You must also specify the HTTP replication endpoint that it should talk to on the main synapse process. `worker_replication_host` should specify the host of the main synapse and `worker_replication_http_port` should point to the HTTP replication port. If the worker will handle HTTP requests then the `worker_listeners` option should be set with a `http` listener, in the same way as the `listeners` option in the shared config.

For example:

    worker_app: synapse.app.generic_worker
    worker_name: generic_worker1

    # The replication listener on the main synapse process.
    worker_replication_host: 127.0.0.1
    worker_replication_http_port: 9093

    worker_listeners:
      - type: http
        port: 8083
        resources:
          - names: [client, federation]

    worker_log_config: /etc/matrix-synapse/generic-worker-log.yaml

...is a full configuration for a generic worker instance, which will expose a plain HTTP endpoint on port 8083 separately serving various endpoints, e.g. `/sync`, which are listed below.

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

### Running Synapse with workers

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 Using synctl with Workers.

## Available worker applications

### `synapse.app.generic_worker`

This worker can handle API requests matching the following regular expressions. These endpoints can be routed to any worker. If a worker is set up to handle a stream then, for maximum efficiency, additional endpoints should be routed to that worker: refer to the stream writers section below for further information.

    # Sync requests
    ^/_matrix/client/(r0|v3)/sync$
    ^/_matrix/client/(api/v1|r0|v3)/events$
    ^/_matrix/client/(api/v1|r0|v3)/initialSync$
    ^/_matrix/client/(api/v1|r0|v3)/rooms/[^/]+/initialSync$

    # Federation requests
    ^/_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|v2)/send_join/
    ^/_matrix/federation/(v1|v2)/send_leave/
    ^/_matrix/federation/(v1|v2)/invite/
    ^/_matrix/federation/v1/event_auth/
    ^/_matrix/federation/v1/exchange_third_party_invite/
    ^/_matrix/federation/v1/user/devices/
    ^/_matrix/key/v2/query
    ^/_matrix/federation/v1/hierarchy/

    # Inbound federation transaction request
    ^/_matrix/federation/v1/send/

    # Client API requests
    ^/_matrix/client/(api/v1|r0|v3|unstable)/createRoom$
    ^/_matrix/client/(api/v1|r0|v3|unstable)/publicRooms$
    ^/_matrix/client/(api/v1|r0|v3|unstable)/rooms/.*/joined_members$
    ^/_matrix/client/(api/v1|r0|v3|unstable)/rooms/.*/context/.*$
    ^/_matrix/client/(api/v1|r0|v3|unstable)/rooms/.*/members$
    ^/_matrix/client/(api/v1|r0|v3|unstable)/rooms/.*/state$
    ^/_matrix/client/v1/rooms/.*/hierarchy$
    ^/_matrix/client/unstable/org.matrix.msc2716/rooms/.*/batch_send$
    ^/_matrix/client/unstable/im.nheko.summary/rooms/.*/summary$
    ^/_matrix/client/(r0|v3|unstable)/account/3pid$
    ^/_matrix/client/(r0|v3|unstable)/account/whoami$
    ^/_matrix/client/(r0|v3|unstable)/devices$
    ^/_matrix/client/versions$
    ^/_matrix/client/(api/v1|r0|v3|unstable)/voip/turnServer$
    ^/_matrix/client/(api/v1|r0|v3|unstable)/rooms/.*/event/
    ^/_matrix/client/(api/v1|r0|v3|unstable)/joined_rooms$
    ^/_matrix/client/(api/v1|r0|v3|unstable)/search$

    # Encryption requests
    ^/_matrix/client/(r0|v3|unstable)/keys/query$
    ^/_matrix/client/(r0|v3|unstable)/keys/changes$
    ^/_matrix/client/(r0|v3|unstable)/keys/claim$
    ^/_matrix/client/(r0|v3|unstable)/room_keys/

    # Registration/login requests
    ^/_matrix/client/(api/v1|r0|v3|unstable)/login$
    ^/_matrix/client/(r0|v3|unstable)/register$
    ^/_matrix/client/v1/register/m.login.registration_token/validity$

    # Event sending requests
    ^/_matrix/client/(api/v1|r0|v3|unstable)/rooms/.*/redact
    ^/_matrix/client/(api/v1|r0|v3|unstable)/rooms/.*/send
    ^/_matrix/client/(api/v1|r0|v3|unstable)/rooms/.*/state/
    ^/_matrix/client/(api/v1|r0|v3|unstable)/rooms/.*/(join|invite|leave|ban|unban|kick)$
    ^/_matrix/client/(api/v1|r0|v3|unstable)/join/
    ^/_matrix/client/(api/v1|r0|v3|unstable)/profile/

    # Account data requests
    ^/_matrix/client/(r0|v3|unstable)/.*/tags
    ^/_matrix/client/(r0|v3|unstable)/.*/account_data

    # Receipts requests
    ^/_matrix/client/(r0|v3|unstable)/rooms/.*/receipt
    ^/_matrix/client/(r0|v3|unstable)/rooms/.*/read_markers

    # Presence requests
    ^/_matrix/client/(api/v1|r0|v3|unstable)/presence/

    # User directory search requests
    ^/_matrix/client/(r0|v3|unstable)/user_directory/search$

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

    ^/_matrix/client/(api/v1|r0|v3|unstable)/pushrules/

Pagination requests can also be handled, but all requests for a given 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|v3|unstable)/rooms/.*/messages$

Additionally, the following endpoints should be included if Synapse is configured to use SSO (you only need to include the ones for whichever SSO provider you're using):

    # for all SSO providers
    ^/_matrix/client/(api/v1|r0|v3|unstable)/login/sso/redirect
    ^/_synapse/client/pick_idp$
    ^/_synapse/client/pick_username
    ^/_synapse/client/new_user_consent$
    ^/_synapse/client/sso_register$

    # OpenID Connect requests.
    ^/_synapse/client/oidc/callback$

    # SAML requests.
    ^/_synapse/client/saml2/authn_response$

    # CAS requests.
    ^/_matrix/client/(api/v1|r0|v3|unstable)/login/cas/ticket$

Ensure that all SSO logins go to a single process. For multiple workers not handling the SSO endpoints properly, see #7530 and #9427.

Note that a HTTP listener with `client` and `federation` resources must be configured in the `worker_listeners` option in the worker config.

#### Load balancing

It is possible to run multiple instances of this worker app, with incoming requests being load-balanced between them by the reverse-proxy. However, different endpoints have different characteristics and so admins may wish to run multiple groups of workers handling different endpoints so that load balancing can be done in different ways.

For `/sync` and `/initialSync` requests it will be more efficient if all requests from a particular user are routed to a single instance. Extracting a user ID from the access token or `Authorization` header is currently left as an exercise for the reader. Admins may additionally wish to separate out `/sync` requests that have a `since` query parameter from those that don't (and `/initialSync`), as requests that don't are known as "initial sync" that happens when a user logs in on a new device and can be very resource intensive, so isolating these requests will stop them from interfering with other users ongoing syncs.

Federation and client requests can be balanced via simple round robin.

The inbound federation transaction request `^/_matrix/federation/v1/send/` should be balanced by source IP so that transactions from the same remote server go to the same process.

Registration/login requests can be handled separately purely to help ensure that unexpected load doesn't affect new logins and sign ups.

Finally, event sending requests can be balanced by the room ID in the URI (or the full URI, or even just round robin), the room ID is the path component after `/rooms/`. If there is a large bridge connected that is sending or may send lots of events, then a dedicated set of workers can be provisioned to limit the effects of bursts of events from that bridge on events sent by normal users.

#### Stream writers

Additionally, the writing of specific streams (such as events) can be moved off of the main process to a particular worker. (This is only supported with Redis-based replication.)

To enable this, the worker must have a HTTP replication listener configured, have a `worker_name` and be listed in the `instance_map` config. The same worker can handle multiple streams, but unless otherwise documented, each stream can only have a single writer.

For example, to move event persistence off to a dedicated worker, the shared configuration would include:

    instance_map:
        event_persister1:
            host: localhost
            port: 8034

    stream_writers:
        events: event_persister1

An example for a stream writer instance:

    worker_app: synapse.app.generic_worker
    worker_name: event_persister1

    # The replication listener on the main synapse process.
    worker_replication_host: 127.0.0.1
    worker_replication_http_port: 9093

    worker_listeners:
      - type: http
        port: 8034
        resources:
          - names: [replication]

      # Enable listener if this stream writer handles endpoints for the `typing` or
      # `to_device` streams. Uses a different port to the `replication` listener to
      # avoid exposing the `replication` listener publicly.
      #
      #- type: http
      #  port: 8035
      #  resources:
      #    - names: [client]

    worker_log_config: /etc/matrix-synapse/event-persister-log.yaml

Some of the streams have associated endpoints which, for maximum efficiency, should be routed to the workers handling that stream. See below for the currently supported streams and the endpoints associated with them:

##### The `events` stream

The `events` stream experimentally supports having multiple writers, where work is sharded between them by room ID. Note that you must restart all worker instances when adding or removing event persisters. An example `stream_writers` configuration with multiple writers:

    stream_writers:
        events:
            - event_persister1
            - event_persister2

##### The `typing` stream

The following endpoints should be routed directly to the worker configured as the stream writer for the `typing` stream:

    ^/_matrix/client/(api/v1|r0|v3|unstable)/rooms/.*/typing

##### The `to_device` stream

The following endpoints should be routed directly to the worker configured as the stream writer for the `to_device` stream:

    ^/_matrix/client/(r0|v3|unstable)/sendToDevice/

##### The `account_data` stream

The following endpoints should be routed directly to the worker configured as the stream writer for the `account_data` stream:

    ^/_matrix/client/(r0|v3|unstable)/.*/tags
    ^/_matrix/client/(r0|v3|unstable)/.*/account_data

##### The `receipts` stream

The following endpoints should be routed directly to the worker configured as the stream writer for the `receipts` stream:

    ^/_matrix/client/(r0|v3|unstable)/rooms/.*/receipt
    ^/_matrix/client/(r0|v3|unstable)/rooms/.*/read_markers

##### The `presence` stream

The following endpoints should be routed directly to the worker configured as the stream writer for the `presence` stream:

    ^/_matrix/client/(api/v1|r0|v3|unstable)/presence/

#### Background tasks

There is also support for moving background tasks to a separate worker. Background tasks are run periodically or started via replication. Exactly which tasks are configured to run depends on your Synapse configuration (e.g. if stats is enabled).

To enable this, the worker must have a `worker_name` and can be configured to run background tasks. For example, to move background tasks to a dedicated worker, the shared configuration would include:

    run_background_tasks_on: background_worker

You might also wish to investigate the `update_user_directory_from_worker` and `media_instance_running_background_jobs` settings.

An example for a dedicated background worker instance:

    worker_app: synapse.app.generic_worker
    worker_name: background_worker

    # The replication listener on the main synapse process.
    worker_replication_host: 127.0.0.1
    worker_replication_http_port: 9093

    worker_log_config: /etc/matrix-synapse/background-worker-log.yaml

#### Updating the User Directory

You can designate one generic worker to update the user directory.

Specify its name in the shared configuration as follows:

    update_user_directory_from_worker: worker_name

This work cannot be load-balanced; please ensure the main process is restarted after setting this option in the shared configuration!

User directory updates allow REST endpoints matching the following regular expressions to work:

    ^/_matrix/client/(r0|v3|unstable)/user_directory/search$

The above endpoints can be routed to any worker, though you may choose to route it to the chosen user directory worker.

This style of configuration supersedes the legacy `synapse.app.user_dir` worker application type.

#### Notifying Application Services

You can designate one generic worker to send output traffic to Application Services.

Specify its name in the shared configuration as follows:

    notify_appservices_from_worker: worker_name

This work cannot be load-balanced; please ensure the main process is restarted after setting this option in the shared configuration!

This style of configuration supersedes the legacy `synapse.app.appservice` worker application type.

### `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 push notifications.

To run multiple instances at once the `pusher_instances` option should list all pusher instances by their worker name, e.g.:

    pusher_instances:
        - pusher_worker1
        - pusher_worker2

### `synapse.app.appservice`

Deprecated as of Synapse v1.59. Use `synapse.app.generic_worker` with the `notify_appservices_from_worker` option instead.

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 appservice notifications.

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

### `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.

If running multiple federation senders then you must list each instance in the `federation_sender_instances` option by their `worker_name`. All instances must be stopped and started when adding or removing instances. For example:

    federation_sender_instances:
        - federation_sender1
        - federation_sender2

### `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/.*$
    ^/_synapse/admin/v1/users/.*/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. Note that doing so will prevent the main process from being able to handle the above endpoints.

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 that if running multiple media repositories they must be on the same server and you must configure a single instance to run the background tasks, e.g.:

    media_instance_running_background_jobs: "media-repository-1"

Note that if a reverse proxy is used , then `/_matrix/media/` must be routed for both inbound client and federation requests (if they are handled separately).

### `synapse.app.user_dir`

Deprecated as of Synapse v1.59. Use `synapse.app.generic_worker` with the `update_user_directory_from_worker` option instead.

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

    ^/_matrix/client/(r0|v3|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.

Above endpoint is not required to be routed to this worker. By default, `update_user_directory` is set to `true`, which means the main process will handle updates. All workers configured with `client` can handle the above endpoint as long as either this worker or the main process are configured to handle it, and are online.

If `update_user_directory` is set to `false`, and this worker is not running, the above endpoint may give outdated results.

### `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/(r0|v3|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|v3|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

### Historical apps

Note: Historically there used to be more apps, however they have been amalgamated into a single `synapse.app.generic_worker` app. The remaining apps are ones that do specific processing unrelated to requests, e.g. the `pusher` that handles sending out push notifications for new events. The intention is for all these to be folded into the `generic_worker` app and to use config to define which processes handle the various proccessing such as push notifications.

## Migration from old config

There are two main independent changes that have been made: introducing Redis support and merging apps into `synapse.app.generic_worker`. Both these changes are backwards compatible and so no changes to the config are required, however server admins are encouraged to plan to migrate to Redis as the old style direct TCP replication config is deprecated.

To migrate to Redis add the `redis` config as above, and optionally remove the TCP `replication` listener from master and `worker_replication_port` from worker config.

To migrate apps to use `synapse.app.generic_worker` simply update the `worker_app` option in the worker configs, and where worker are started (e.g. in systemd service files, but not required for synctl).

## Architectural diagram

The following shows an example setup using Redis and a reverse proxy:

                         Clients & Federation
                                  |
                                  v
                            +-----------+
                            |           |
                            |  Reverse  |
                            |  Proxy    |
                            |           |
                            +-----------+
                                | | |
                                | | | HTTP requests
            +-------------------+ | +-----------+
            |                 +---+             |
            |                 |                 |
            v                 v                 v
    +--------------+  +--------------+  +--------------+  +--------------+
    |   Main       |  |   Generic    |  |   Generic    |  |  Event       |
    |   Process    |  |   Worker 1   |  |   Worker 2   |  |  Persister   |
    +--------------+  +--------------+  +--------------+  +--------------+
          ^    ^          |   ^   |         |   ^   |         |   ^   ^
          |    |          |   |   |         |   |   |         |   |   |
          |    |          |   |   |  HTTP   |   |   |         |   |   |
          |    +----------+<--|---|---------+<--|---|---------+   |   |
          |                   |   +-------------|-->+-------------+   |
          |                   |                 |                     |
          |                   |                 |                     |
          v                   v                 v                     v
    ======================================================================
                                                             Redis pub/sub channel