Send `USER_IP` commands on a different Redis channel, in order to reduce...

Send `USER_IP` commands on a different Redis channel, in order to reduce traffic to workers that do not process these commands. (#12809)

changelog.d/12672.feature

+Send `USER_IP` commands on a different Redis channel, in order to reduce traffic to workers that do not process these commands.
\ No newline at end of file

changelog.d/12672.misc

-Lay some foundation work to allow workers to only subscribe to some kinds of messages, reducing replication traffic.
\ No newline at end of file

changelog.d/12809.feature

+Send `USER_IP` commands on a different Redis channel, in order to reduce traffic to workers that do not process these commands.
\ No newline at end of file

synapse/replication/tcp/commands.py

@@ -58,6 +58,15 @@ class Command(metaclass=abc.ABCMeta):
         # by default, we just use the command name.
         return self.NAME
 
+    def redis_channel_name(self, prefix: str) -> str:
+        """
+        Returns the Redis channel name upon which to publish this command.
+
+        Args:
+            prefix: The prefix for the channel.
+        """
+        return prefix
+
 
 SC = TypeVar("SC", bound="_SimpleCommand")
 
@@ -395,6 +404,9 @@ class UserIpCommand(Command):
             f"{self.user_agent!r}, {self.device_id!r}, {self.last_seen})"
         )
 
+    def redis_channel_name(self, prefix: str) -> str:
+        return f"{prefix}/USER_IP"
+
 
 class RemoteServerUpCommand(_SimpleCommand):
     """Sent when a worker has detected that a remote server is no longer

synapse/replication/tcp/redis.py

@@ -221,10 +221,10 @@ class RedisSubscriber(txredisapi.SubscriberProtocol):
         # remote instances.
         tcp_outbound_commands_counter.labels(cmd.NAME, "redis").inc()
 
+        channel_name = cmd.redis_channel_name(self.synapse_stream_prefix)
+
         await make_deferred_yieldable(
-            self.synapse_outbound_redis_connection.publish(
-                self.synapse_stream_prefix, encoded_string
-            )
+            self.synapse_outbound_redis_connection.publish(channel_name, encoded_string)
         )