Update worker docs to update preferred settings for pusher and federation_sender (#14493)

* Fix one typo on line 3700(and apparently do something to other lines, no idea)

* Update config_documentation.md with more information about how federation_senders and pushers settings can be handled.

Specifically, that the instance map style of config does not require the special other variables that enable and disable functionality and that a single worker CAN be added to the map not only just two or more.

* Extra line here for consistency and appearance.

* Add link to sygnal repo.

* Add deprecation notice to workers.md and point to the newer alternative method of defining this functionality.

* Changelog

* Correct version number of Synapse the deprecation is happening in.

* Update quiet deprecation with simple notice and suggestion.

3 files changed, 53 insertions, 31 deletions

diff --git a/changelog.d/14493.doc b/changelog.d/14493.doc
new file mode 100644
index 0000000000..e26c68ffc2
--- /dev/null
+++ b/changelog.d/14493.doc
@@ -0,0 +1 @@
+Update worker settings for `pusher` and `federation_sender` functionality.
diff --git a/docs/usage/configuration/config_documentation.md b/docs/usage/configuration/config_documentation.md
index b9bde8f47e..0f47684f8c 100644
--- a/docs/usage/configuration/config_documentation.md
+++ b/docs/usage/configuration/config_documentation.md
@@ -858,7 +858,7 @@ which are older than the room's maximum retention period. Synapse will also
 filter events received over federation so that events that should have been
 purged are ignored and not stored again.
 
-The message retention policies feature is disabled by default. Please be advised 
+The message retention policies feature is disabled by default. Please be advised
 that enabling this feature carries some risk. There are known bugs with the implementation
 which can cause database corruption. Setting retention to delete older history
 is less risky than deleting newer history but in general caution is advised when enabling this
@@ -3003,7 +3003,7 @@ Options for each entry include:
      which is set to the claims returned by the UserInfo Endpoint and/or
      in the ID Token.
 
-* `backchannel_logout_enabled`: set to `true` to process OIDC Back-Channel Logout notifications. 
+* `backchannel_logout_enabled`: set to `true` to process OIDC Back-Channel Logout notifications.
   Those notifications are expected to be received on `/_synapse/client/oidc/backchannel_logout`.
   Defaults to `false`.
 
@@ -3425,7 +3425,7 @@ This option has the following sub-options:
     NB. If you set this to true, and the last time the user_directory search
     indexes were (re)built was before Synapse 1.44, you'll have to
     rebuild the indexes in order to search through all known users.
-    
+
     These indexes are built the first time Synapse starts; admins can
     manually trigger a rebuild via the API following the instructions
     [for running background updates](../administration/admin_api/background_updates.md#run),
@@ -3684,7 +3684,7 @@ As a result, the worker configuration is divided into two parts.
 
 1. The first part (in this section of the manual) defines which shardable tasks
    are delegated to privileged workers. This allows unprivileged workers to make
-   request a privileged worker to act on their behalf.
+   requests to a privileged worker to act on their behalf.
 1. [The second part](#individual-worker-configuration)
    controls the behaviour of individual workers in isolation.
 
@@ -3696,7 +3696,7 @@ For guidance on setting up workers, see the [worker documentation](../../workers
 A shared secret used by the replication APIs on the main process to authenticate
 HTTP requests from workers.
 
-The default, this value is omitted (equivalently `null`), which means that 
+The default, this value is omitted (equivalently `null`), which means that
 traffic between the workers and the main process is not authenticated.
 
 Example configuration:
@@ -3706,6 +3706,8 @@ worker_replication_secret: "secret_secret"
 ---
 ### `start_pushers`
 
+Unnecessary to set if using [`pusher_instances`](#pusher_instances) with [`generic_workers`](../../workers.md#synapseappgeneric_worker).
+
 Controls sending of push notifications on the main process. Set to `false`
 if using a [pusher worker](../../workers.md#synapseapppusher). Defaults to `true`.
 
@@ -3716,25 +3718,30 @@ start_pushers: false
 ---
 ### `pusher_instances`
 
-It is possible to run multiple [pusher workers](../../workers.md#synapseapppusher),
-in which case the work is balanced across them. Use this setting to list the pushers by
-[`worker_name`](#worker_name). Ensure the main process and all pusher workers are
-restarted after changing this option.
-
-If no or only one pusher worker is configured, this setting is not necessary.
-The main process will send out push notifications by default if you do not disable
-it by setting [`start_pushers: false`](#start_pushers).
+It is possible to scale the processes that handle sending push notifications to [sygnal](https://github.com/matrix-org/sygnal)
+and email by running a [`generic_worker`](../../workers.md#synapseappgeneric_worker) and adding it's [`worker_name`](#worker_name) to
+a `pusher_instances` map. Doing so will remove handling of this function from the main
+process. 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.
 
-Example configuration:
+Example configuration for a single worker:
+```yaml
+pusher_instances:
+  - pusher_worker1
+```
+And for multiple workers:
 ```yaml
-start_pushers: false
 pusher_instances:
   - pusher_worker1
   - pusher_worker2
 ```
+
 ---
 ### `send_federation`
 
+Unnecessary to set if using [`federation_sender_instances`](#federation_sender_instances) with [`generic_workers`](../../workers.md#synapseappgeneric_worker).
+
 Controls sending of outbound federation transactions on the main process.
 Set to `false` if using a [federation sender worker](../../workers.md#synapseappfederation_sender).
 Defaults to `true`.
@@ -3746,29 +3753,36 @@ send_federation: false
 ---
 ### `federation_sender_instances`
 
-It is possible to run multiple
-[federation sender worker](../../workers.md#synapseappfederation_sender), in which
-case the work is balanced across them. Use this setting to list the senders.
+It is possible to scale the processes that handle sending outbound federation requests
+by running a [`generic_worker`](../../workers.md#synapseappgeneric_worker) and adding it's [`worker_name`](#worker_name) to
+a `federation_sender_instances` map. Doing so will remove handling of this function from
+the main process. Multiple workers can be added to this map, in which case the work is
+balanced across them.
 
-This configuration setting must be shared between all federation sender workers, and if
-changed all federation sender workers must be stopped at the same time and then
-started, to ensure that all instances are running with the same config (otherwise
+This configuration setting must be shared between all workers handling federation
+sending, and if changed all federation sender workers must be stopped at the same time
+and then started, to ensure that all instances are running with the same config (otherwise
 events may be dropped).
 
-Example configuration:
+Example configuration for a single worker:
 ```yaml
-send_federation: false
 federation_sender_instances:
   - federation_sender1
 ```
+And for multiple workers:
+```yaml
+federation_sender_instances:
+  - federation_sender1
+  - federation_sender2
+```
 ---
 ### `instance_map`
 
 When using workers this should be a map from [`worker_name`](#worker_name) to the
 HTTP replication listener of the worker, if configured.
-Each worker declared under [`stream_writers`](../../workers.md#stream-writers) needs 
+Each worker declared under [`stream_writers`](../../workers.md#stream-writers) needs
 a HTTP replication listener, and that listener should be included in the `instance_map`.
-(The main process also needs an HTTP replication listener, but it should not be 
+(The main process also needs an HTTP replication listener, but it should not be
 listed in the `instance_map`.)
 
 Example configuration:
@@ -3902,8 +3916,8 @@ worker_replication_http_tls: true
 ---
 ### `worker_listeners`
 
-A worker can handle HTTP requests. To do so, a `worker_listeners` option 
-must be declared, in the same way as the [`listeners` option](#listeners) 
+A worker can handle HTTP requests. To do so, a `worker_listeners` option
+must be declared, in the same way as the [`listeners` option](#listeners)
 in the shared config.
 
 Workers declared in [`stream_writers`](#stream_writers) will need to include a
@@ -3922,7 +3936,7 @@ worker_listeners:
 ### `worker_daemonize`
 
 Specifies whether the worker should be started as a daemon process.
-If Synapse is being managed by [systemd](../../systemd-with-workers/README.md), this option 
+If Synapse is being managed by [systemd](../../systemd-with-workers/README.md), this option
 must be omitted or set to `false`.
 
 Defaults to `false`.
@@ -3934,11 +3948,11 @@ worker_daemonize: true
 ---
 ### `worker_pid_file`
 
-When running a worker as a daemon, we need a place to store the 
+When running a worker as a daemon, we need a place to store the
 [PID](https://en.wikipedia.org/wiki/Process_identifier) of the worker.
 This option defines the location of that "pid file".
 
-This option is required if `worker_daemonize` is `true` and ignored 
+This option is required if `worker_daemonize` is `true` and ignored
 otherwise. It has no default.
 
 See also the [`pid_file` option](#pid_file) option for the main Synapse process.
@@ -3988,4 +4002,3 @@ background_updates:
     min_batch_size: 10
     default_batch_size: 50
 ```
-
diff --git a/docs/workers.md b/docs/workers.md
index 2b65acb5ed..59a6487e0d 100644
--- a/docs/workers.md
+++ b/docs/workers.md
@@ -505,6 +505,9 @@ 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`](usage/configuration/config_documentation.md#pusher_instances).
+
 Handles sending push notifications to sygnal and email. Doesn't handle any
 REST endpoints itself, but you should set
 [`start_pushers: false`](usage/configuration/config_documentation.md#start_pushers) in the
@@ -543,6 +546,9 @@ 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`](usage/configuration/config_documentation.md#federation_sender_instances). 
+
 Handles sending federation traffic to other servers. Doesn't handle any
 REST endpoints itself, but you should set
 [`send_federation: false`](usage/configuration/config_documentation.md#send_federation)
@@ -639,7 +645,9 @@ 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`