Add support for running background tasks on a worker

CHANGELOG.md

@@ -7,6 +7,7 @@ People who are interested in running a Synapse worker setup should know that **o
 - we've added support for [Stream writers](#stream-writers-support)
 - we've added support for [multiple federation sender workers](#multiple-federation-sender-workers-support)
 - we've added support for [multiple pusher workers](#multiple-pusher-workers-support)
+- we've added support for [running background tasks on a worker](#background-tasks-can-run-on-a-worker)
 - we've restored support for [`appservice` workers](#appservice-worker-support-is-back)
 - we've restored support for [`user_dir` workers](#user-directory-worker-support-is-back)
 - see the [Potential Backward Incompatibilities after these Synapse worker changes](#potential-backward-incompatibilities-after-these-synapse-worker-changes)
@@ -40,6 +41,13 @@ From now on, you can have as many as you want to help with your federation traff
 Until now, we only supported a single `pusher` worker (`matrix_synapse_workers_pusher_workers_count` could either be `0` or `1`).
 From now on, you can have as many as you want to help with pushing notifications out.
 
+### Background tasks can run on a worker
+
+From now on, you can put [background task processing on a worker](https://matrix-org.github.io/synapse/latest/workers.html#background-tasks).
+
+With `matrix_synapse_workers_preset: one-of-each`, you'll get one `background` worker automatically.
+You can also control the `background` workers count with `matrix_synapse_workers_background_workers_count`. Only  `0` or `1` workers of this type are supported by Synapse.
+
 ### Appservice worker support is back
 
 We previously had an `appservice` worker type, which [Synapse deprecated in v1.59.0](https://github.com/matrix-org/synapse/blob/v1.59.0/docs/upgrade.md#deprecation-of-the-synapseappappservice-and-synapseappuser_dir-worker-application-types). So did we, at the time.
@@ -66,7 +74,7 @@ Below we'll discuss **potential backward incompatibilities**.
 
 - **Worker names** (container names, systemd services, worker configuration files) **have changed**. Workers are now labeled sequentially (e.g. `matrix-synapse-worker_generic_worker-18111` -> `matrix-synapse-worker-generic-0`). The playbook will handle these changes automatically.
 
-- Due to increased worker types support above, people who use `matrix_synapse_workers_preset: one-of-each` should be aware that with these changes, **the playbook will deploy 8 additional workers** (6 stream writers, 1 `appservice` worker, 1 `user_dir` worker). This **may increase RAM/CPU usage**, etc. If you find your server struggling, consider disabling some workers with the appropriate `matrix_synapse_workers_*_workers_count` variables.
+- Due to increased worker types support above, people who use `matrix_synapse_workers_preset: one-of-each` should be aware that with these changes, **the playbook will deploy 9 additional workers** (6 stream writers, 1 `appservice` worker, 1 `user_dir` worker, 1 background task worker). This **may increase RAM/CPU usage**, etc. If you find your server struggling, consider disabling some workers with the appropriate `matrix_synapse_workers_*_workers_count` variables.
 
 - **Metric endpoints have also changed** (`/metrics/synapse/worker/generic_worker-18111` -> `/metrics/synapse/worker/generic-worker-0`). If you're [collecting metrics to an external Prometheus server](docs/configuring-playbook-prometheus-grafana.md#collecting-metrics-to-an-external-prometheus-server), consider revisiting our [Collecting Synapse worker metrics to an external Prometheus server](docs/configuring-playbook-prometheus-grafana.md#collecting-synapse-worker-metrics-to-an-external-prometheus-server) docs and updating your Prometheus configuration. **If you're collecting metrics to the integrated Prometheus server** (not enabled by default), **your Prometheus configuration will be updated automatically**. Old data (from before this change) may stick around though.
 

roles/matrix-synapse/defaults/main.yml

@@ -398,6 +398,7 @@ matrix_synapse_workers_presets:
     media_repository_workers_count: 0
     appservice_workers_count: 0
     user_dir_workers_count: 0
+    background_workers_count: 0
     stream_writer_events_stream_workers_count: 0
     stream_writer_typing_stream_workers_count: 0
     stream_writer_to_device_stream_workers_count: 0
@@ -411,6 +412,7 @@ matrix_synapse_workers_presets:
     media_repository_workers_count: 1
     appservice_workers_count: 1
     user_dir_workers_count: 1
+    background_workers_count: 1
     stream_writer_events_stream_workers_count: 1
     stream_writer_typing_stream_workers_count: 1
     stream_writer_to_device_stream_workers_count: 1
@@ -554,6 +556,15 @@ matrix_synapse_workers_user_dir_workers_metrics_range_start: 19661
 # `update_user_directory_from_worker` is meant to point to a worker, which is dedicated to updating the user directory and servicing some user directory URL endpoints (`matrix_synapse_workers_user_dir_worker_client_server_endpoints`).
 matrix_synapse_update_user_directory_from_worker: "{{ (matrix_synapse_workers_enabled_list | selectattr('type', 'equalto', 'user_dir') | list)[0].name if (matrix_synapse_workers_enabled and matrix_synapse_workers_enabled_list | selectattr('type', 'equalto', 'user_dir') | list | length > 0) else '' }}"
 
+# matrix_synapse_workers_background_workers_count can only be 0 or 1. More instances are not supported.
+# Our implementation uses a generic worker and assigns Synapse to perform background work on this worker using the `run_background_tasks_on` Synapse option.
+matrix_synapse_workers_background_workers_count: "{{ matrix_synapse_workers_presets[matrix_synapse_workers_preset]['background_workers_count'] }}"
+matrix_synapse_workers_background_workers_metrics_range_start: 19700
+
+# matrix_synapse_run_background_tasks_on populates the `run_background_tasks_on` Synapse configuration used when Synapse workers are in use (`matrix_synapse_workers_enabled`).
+# `run_background_tasks_on` is meant to point to a worker, which is dedicated to processing background tasks.
+matrix_synapse_run_background_tasks_on: "{{ (matrix_synapse_workers_enabled_list | selectattr('type', 'equalto', 'background') | list)[0].name if (matrix_synapse_workers_enabled and matrix_synapse_workers_enabled_list | selectattr('type', 'equalto', 'background') | list | length > 0) else '' }}"
+
 # Default list of workers to spawn.
 #
 # Unless you populate this manually, this list is dynamically generated

roles/matrix-synapse/tasks/synapse/workers/init.yml

@@ -100,6 +100,20 @@
   register: "matrix_synapse_workers_list_results_user_dir_workers"
   loop: "{{ range(0, matrix_synapse_workers_user_dir_workers_count | int) | list }}"
 
+# This type of worker can only have a count of 1, at most
+- name: Build background workers
+  ansible.builtin.set_fact:
+    worker:
+      id: "background-{{ item }}"
+      name: "matrix-synapse-worker-background-{{ item }}"
+      type: 'background'
+      app: 'generic_worker'
+      webserving: false
+      port: 0
+      metrics_port: "{{ matrix_synapse_workers_background_workers_metrics_range_start + item }}"
+  register: "matrix_synapse_workers_list_results_background_workers"
+  loop: "{{ range(0, matrix_synapse_workers_background_workers_count | int) | list }}"
+
 - name: Build media_repository workers
   ansible.builtin.set_fact:
     worker:
@@ -130,6 +144,8 @@
       matrix_synapse_workers_list_results_user_dir_workers.results
       +
       matrix_synapse_workers_list_results_media_repository_workers.results
+      +
+      matrix_synapse_workers_list_results_background_workers.results
     }}
 
 - ansible.builtin.set_fact:

roles/matrix-synapse/tasks/validate_config.yml

@@ -20,6 +20,7 @@
   with_items:
     - "matrix_synapse_workers_appservice_workers_count"
     - "matrix_synapse_workers_user_dir_workers_count"
+    - "matrix_synapse_workers_background_workers_count"
     - "matrix_synapse_workers_stream_writer_typing_stream_workers_count"
     - "matrix_synapse_workers_stream_writer_to_device_stream_workers_count"
     - "matrix_synapse_workers_stream_writer_account_data_stream_workers_count"

roles/matrix-synapse/templates/synapse/homeserver.yaml.j2

@@ -2886,6 +2886,9 @@ update_user_directory_from_worker: {{ matrix_synapse_update_user_directory_from_
 # data). If not provided this defaults to the main process.
 #
 #run_background_tasks_on: worker1
+{% if matrix_synapse_run_background_tasks_on != '' %}
+run_background_tasks_on: {{ matrix_synapse_run_background_tasks_on | to_json }}
+{% endif %}
 
 # A shared secret used by the replication APIs to authenticate HTTP requests
 # from workers.

roles/matrix-synapse/vars/main.yml

@@ -102,6 +102,8 @@ matrix_synapse_known_worker_types: |
       ['appservice']
       +
       ['user_dir']
+      +
+      ['background']
     ) | unique
   }}