Merge branch 'release-v0.17.1' of github.com:matrix-org/synapse v0.17.1

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+Scaling synapse via workers
+---------------------------
+
+Synapse has experimental support for splitting out functionality into
+multiple separate python processes, helping greatly with scalability.  These
+processes are called 'workers', and are (eventually) intended to scale
+horizontally independently.
+
+All processes continue to share the same database instance, and as such, workers
+only work with postgres based synapse deployments (sharing a single sqlite
+across multiple processes is a recipe for disaster, plus you should be using
+postgres anyway if you care about scalability).
+
+The workers communicate with the master synapse process via a synapse-specific
+HTTP protocol called 'replication' - analogous to MySQL or Postgres style
+database replication; feeding a stream of relevant data to the workers so they
+can be kept in sync with the main synapse process and database state.
+
+To enable workers, you need to add a replication listener to the master synapse, e.g.::
+
+    listeners:
+      - port: 9092
+        bind_address: '127.0.0.1'
+        type: http
+        tls: false
+        x_forwarded: false
+        resources:
+          - names: [replication]
+            compress: false
+
+Under **no circumstances** should this replication API listener be exposed to the
+public internet; it currently implements no authentication whatsoever and is
+unencrypted HTTP.
+
+You then create a set of configs for the various worker processes.  These should be
+worker configuration files should be stored in a dedicated subdirectory, to allow
+synctl to manipulate them.
+
+The current available worker applications are:
+ * synapse.app.pusher - handles sending push notifications to sygnal and email
+ * synapse.app.synchrotron - handles /sync endpoints.  can scales horizontally through multiple instances.
+ * synapse.app.appservice - handles output traffic to Application Services
+ * synapse.app.federation_reader - handles receiving federation traffic (including public_rooms API)
+ * synapse.app.media_repository - handles the media repository.
+
+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.
+
+You must specify the type of worker application (worker_app) and the replication
+endpoint that it's talking to on the main synapse process (worker_replication_url).
+
+For instance::
+
+    worker_app: synapse.app.synchrotron
+
+    # The replication listener on the synapse to talk to.
+    worker_replication_url: http://127.0.0.1:9092/_synapse/replication
+
+    worker_listeners:
+     - type: http
+       port: 8083
+       resources:
+         - names:
+           - client
+
+    worker_daemonize: True
+    worker_pid_file: /home/matrix/synapse/synchrotron.pid
+    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 loadbalancer to route the /sync endpoint to
+the synchrotron instance(s) in this instance.
+
+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
+
+All of the above is highly experimental and subject to change as Synapse evolves,
+but documenting it here to help folks needing highly scalable Synapses similar
+to the one running matrix.org!
+