Add experimental support for sharding event persister. (#8170)

This is *not* ready for production yet. Caveats:

1. We should write some tests...
2. The stream token that we use for events can get stalled at the minimum position of all writers. This means that new events may not be processed and e.g. sent down sync streams if a writer isn't writing or is slow.

synapse/handlers/message.py

@@ -376,9 +376,8 @@ class EventCreationHandler(object):
         self.notifier = hs.get_notifier()
         self.config = hs.config
         self.require_membership_for_aliases = hs.config.require_membership_for_aliases
-        self._is_event_writer = (
-            self.config.worker.writers.events == hs.get_instance_name()
-        )
+        self._events_shard_config = self.config.worker.events_shard_config
+        self._instance_name = hs.get_instance_name()
 
         self.room_invite_state_types = self.hs.config.room_invite_state_types
 
@@ -904,9 +903,10 @@ class EventCreationHandler(object):
 
         try:
             # If we're a worker we need to hit out to the master.
-            if not self._is_event_writer:
+            writer_instance = self._events_shard_config.get_instance(event.room_id)
+            if writer_instance != self._instance_name:
                 result = await self.send_event(
-                    instance_name=self.config.worker.writers.events,
+                    instance_name=writer_instance,
                     event_id=event.event_id,
                     store=self.store,
                     requester=requester,
@@ -974,7 +974,9 @@ class EventCreationHandler(object):
 
         This should only be run on the instance in charge of persisting events.
         """
-        assert self._is_event_writer
+        assert self._events_shard_config.should_handle(
+            self._instance_name, event.room_id
+        )
 
         if ratelimit:
             # We check if this is a room admin redacting an event so that we