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.

All the workers and the main process connect to Redis, which relays replication commands between processes.

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 few 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
• Secondly, you need to enable redis-based replication
• You will need to add an instance_map with the main process defined, as well as the relevant connection information from it's HTTP replication listener (defined in step 1 above).
  • Note that the host defined is the address the worker needs to look for the main process at, not necessarily the same address that is bound to.
  • If you are using Unix sockets for the replication resource, make sure to use a path to the socket file instead of a port.
• 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

instance_map:
    main:
        host: 'localhost'
        port: 9093

See the configuration manual for the full documentation of each option.

Under no circumstances should the replication listener be exposed to the public internet; replication traffic is:

• always unencrypted
• unauthenticated, unless worker_replication_secret is configured

Worker configuration

In the config file for each worker, you must specify:

• The type of worker (worker_app). The currently available worker applications are listed below.
• A unique name for the worker (worker_name).
• If handling HTTP requests, a worker_listeners option with an http listener.
• Synapse 1.72 and older: if handling the ^/_matrix/client/v3/keys/upload endpoint, the HTTP URI for the main process (worker_main_http_uri). This config option is no longer required and is ignored when running Synapse 1.73 and newer.

For example:

worker_app: synapse.app.generic_worker
worker_name: generic_worker1

worker_listeners:
  - type: http
    port: 8083
    x_forwarded: true
    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.

Start Synapse with Poetry

The following applies to Synapse installations that have been installed from source using poetry.

You can start the main Synapse process with Poetry by running the following command:

poetry run synapse_homeserver --config-file [your homeserver.yaml]

For worker setups, you can run the following command

poetry run synapse_worker --config-file [your homeserver.yaml] --config-file [your worker.yaml]

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/timestamp_to_event/
^/_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/(v1|unstable)/rooms/.*/relations/
^/_matrix/client/v1/rooms/.*/threads$
^/_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/v1/rooms/.*/timestamp_to_event$
^/_matrix/client/(api/v1|r0|v3|unstable/.*)/rooms/.*/aliases
^/_matrix/client/(api/v1|r0|v3|unstable)/search$
^/_matrix/client/(r0|v3|unstable)/user/.*/filter(/|$)
^/_matrix/client/(api/v1|r0|v3|unstable)/directory/room/.*$
^/_matrix/client/(r0|v3|unstable)/capabilities$

# 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/
^/_matrix/client/(r0|v3|unstable)/keys/upload/

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

# 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)/knock/
^/_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. This can be done in reverse proxy by extracting username part from the users access token.

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.

Example nginx configuration snippet that handles the cases above. This is just an example and probably requires some changes according to your particular setup:

# Choose sync worker based on the existence of "since" query parameter
map $arg_since $sync {
    default synapse_sync;
    '' synapse_initial_sync;
}

# Extract username from access token passed as URL parameter
map $arg_access_token $accesstoken_from_urlparam {
    # Defaults to just passing back the whole accesstoken
    default   $arg_access_token;
    # Try to extract username part from accesstoken URL parameter
    "~syt_(?<username>.*?)_.*"           $username;
}

# Extract username from access token passed as authorization header
map $http_authorization $mxid_localpart {
    # Defaults to just passing back the whole accesstoken
    default                              $http_authorization;
    # Try to extract username part from accesstoken header
    "~Bearer syt_(?<username>.*?)_.*"    $username;
    # if no authorization-header exist, try mapper for URL parameter "access_token"
    ""                                   $accesstoken_from_urlparam;
}

upstream synapse_initial_sync {
    # Use the username mapper result for hash key
    hash $mxid_localpart consistent;
    server 127.0.0.1:8016;
    server 127.0.0.1:8036;
}

upstream synapse_sync {
    # Use the username mapper result for hash key
    hash $mxid_localpart consistent;
    server 127.0.0.1:8013;
    server 127.0.0.1:8037;
    server 127.0.0.1:8038;
    server 127.0.0.1:8039;
}

# Sync initial/normal
location ~ ^/_matrix/client/(r0|v3)/sync$ {
	proxy_pass http://$sync;
}

# Normal sync
location ~ ^/_matrix/client/(api/v1|r0|v3)/events$ {
	proxy_pass http://synapse_sync;
}

# Initial_sync
location ~ ^/_matrix/client/(api/v1|r0|v3)/initialSync$ {
	proxy_pass http://synapse_initial_sync;
}
location ~ ^/_matrix/client/(api/v1|r0|v3)/rooms/[^/]+/initialSync$ {
	proxy_pass http://synapse_initial_sync;
}

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.

To enable this, the worker must have:

• An HTTP replication listener configured,
• Have a worker_name and be listed in the instance_map config.
• Have the main process declared on the instance_map as well.

Note: 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:
    main:
        host: localhost
        port: 8030
    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

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
  #  x_forwarded: true
  #  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 writer workers, where load is sharded between them by room ID. Each writer is called an event persister. They are responsible for

• receiving new events,
• linking them to those already in the room DAG,
• persisting them to the DB, and finally
• updating the events stream.

Because load is sharded in this way, you must restart all worker instances when adding or removing event persisters.

An event_persister should not be mistaken for an event_creator. An event_creator listens for requests from clients to create new events and does so. It will then pass those events over HTTP replication to any configured event persisters (or the main process if none are configured).

Note that event_creators and event_persisters are implemented using the same synapse.app.generic_worker.

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). This worker doesn't handle any REST endpoints itself.

To enable this, the worker must have a unique 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

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. Doesn't handle any REST endpoints itself, but you should 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.

Push Notifications

You can designate generic worker to sending push notifications to a push gateway such as sygnal and email.

This will stop the main process sending push notifications.

The workers responsible for sending push notifications can be defined using the pusher_instances option. For example:

pusher_instances:
  - pusher_worker1
  - pusher_worker2

Multiple workers can be added to this map, in which case the work is balanced across them. Ensure the main process and all pusher workers are restarted after changing this option.

These workers don't need to accept incoming HTTP requests to send push notifications, so no additional reverse proxy configuration is required for pusher workers.

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

synapse.app.pusher

It is likely this option will be deprecated in the future and is not recommended for new installations. Instead, use synapse.app.generic_worker with the pusher_instances.

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.:

start_pushers: false
pusher_instances:
    - pusher_worker1
    - pusher_worker2

An example for a pusher instance:

worker_app: synapse.app.pusher
worker_name: pusher_worker1

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

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

It is likely this option will be deprecated in the future and not recommended for new installations. Instead, use synapse.app.generic_worker with the federation_sender_instances.

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:

send_federation: false
federation_sender_instances:
    - federation_sender1
    - federation_sender2

An example for a federation sender instance:

worker_app: synapse.app.federation_sender
worker_name: federation_sender1

worker_log_config: /etc/matrix-synapse/federation-sender-log.yaml

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_app: synapse.app.media_repository
worker_name: media_worker

worker_listeners:
  - type: http
    port: 8085
    x_forwarded: true
    resources:
      - names: [media]

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

Note that if running multiple media repositories they must be on the same server and you must specify a single instance to run the background tasks in the shared configuration, 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.

Historical apps

The following used to be separate worker application types, but are now equivalent to synapse.app.generic_worker:

• synapse.app.client_reader
• synapse.app.event_creator
• synapse.app.federation_reader
• synapse.app.federation_sender
• synapse.app.frontend_proxy
• synapse.app.pusher
• synapse.app.synchrotron

Migration from old config

A main change that has occurred is the merging of worker apps into synapse.app.generic_worker. This change is backwards compatible and so no changes to the config are required.

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