Lay some foundation work to allow workers to only subscribe to some kinds of messages, reducing replication traffic. (#12672)

5 files changed, 173 insertions, 24 deletions

diff --git a/changelog.d/12672.misc b/changelog.d/12672.misc
new file mode 100644
index 0000000000..265e0a801f
--- /dev/null
+++ b/changelog.d/12672.misc
@@ -0,0 +1 @@
+Lay some foundation work to allow workers to only subscribe to some kinds of messages, reducing replication traffic.
\ No newline at end of file
diff --git a/synapse/replication/tcp/handler.py b/synapse/replication/tcp/handler.py
index 9aba1cd451..e1cbfa50eb 100644
--- a/synapse/replication/tcp/handler.py
+++ b/synapse/replication/tcp/handler.py
@@ -1,5 +1,5 @@
 # Copyright 2017 Vector Creations Ltd
-# Copyright 2020 The Matrix.org Foundation C.I.C.
+# Copyright 2020, 2022 The Matrix.org Foundation C.I.C.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -101,6 +101,9 @@ class ReplicationCommandHandler:
         self._instance_id = hs.get_instance_id()
         self._instance_name = hs.get_instance_name()
 
+        # Additional Redis channel suffixes to subscribe to.
+        self._channels_to_subscribe_to: List[str] = []
+
         self._is_presence_writer = (
             hs.get_instance_name() in hs.config.worker.writers.presence
         )
@@ -243,6 +246,31 @@ class ReplicationCommandHandler:
             # If we're NOT using Redis, this must be handled by the master
             self._should_insert_client_ips = hs.get_instance_name() == "master"
 
+        if self._is_master or self._should_insert_client_ips:
+            self.subscribe_to_channel("USER_IP")
+
+    def subscribe_to_channel(self, channel_name: str) -> None:
+        """
+        Indicates that we wish to subscribe to a Redis channel by name.
+
+        (The name will later be prefixed with the server name; i.e. subscribing
+        to the 'ABC' channel actually subscribes to 'example.com/ABC' Redis-side.)
+
+        Raises:
+          - If replication has already started, then it's too late to subscribe
+            to new channels.
+        """
+
+        if self._factory is not None:
+            # We don't allow subscribing after the fact to avoid the chance
+            # of missing an important message because we didn't subscribe in time.
+            raise RuntimeError(
+                "Cannot subscribe to more channels after replication started."
+            )
+
+        if channel_name not in self._channels_to_subscribe_to:
+            self._channels_to_subscribe_to.append(channel_name)
+
     def _add_command_to_stream_queue(
         self, conn: IReplicationConnection, cmd: Union[RdataCommand, PositionCommand]
     ) -> None:
@@ -321,7 +349,9 @@ class ReplicationCommandHandler:
 
             # Now create the factory/connection for the subscription stream.
             self._factory = RedisDirectTcpReplicationClientFactory(
-                hs, outbound_redis_connection
+                hs,
+                outbound_redis_connection,
+                channel_names=self._channels_to_subscribe_to,
             )
             hs.get_reactor().connectTCP(
                 hs.config.redis.redis_host,
diff --git a/synapse/replication/tcp/redis.py b/synapse/replication/tcp/redis.py
index 989c5be032..73294654ef 100644
--- a/synapse/replication/tcp/redis.py
+++ b/synapse/replication/tcp/redis.py
@@ -14,7 +14,7 @@
 
 import logging
 from inspect import isawaitable
-from typing import TYPE_CHECKING, Any, Generic, Optional, Type, TypeVar, cast
+from typing import TYPE_CHECKING, Any, Generic, List, Optional, Type, TypeVar, cast
 
 import attr
 import txredisapi
@@ -85,14 +85,15 @@ class RedisSubscriber(txredisapi.SubscriberProtocol):
 
     Attributes:
         synapse_handler: The command handler to handle incoming commands.
-        synapse_stream_name: The *redis* stream name to subscribe to and publish
+        synapse_stream_prefix: The *redis* stream name to subscribe to and publish
             from (not anything to do with Synapse replication streams).
         synapse_outbound_redis_connection: The connection to redis to use to send
             commands.
     """
 
     synapse_handler: "ReplicationCommandHandler"
-    synapse_stream_name: str
+    synapse_stream_prefix: str
+    synapse_channel_names: List[str]
     synapse_outbound_redis_connection: txredisapi.ConnectionHandler
 
     def __init__(self, *args: Any, **kwargs: Any):
@@ -117,8 +118,13 @@ class RedisSubscriber(txredisapi.SubscriberProtocol):
         # it's important to make sure that we only send the REPLICATE command once we
         # have successfully subscribed to the stream - otherwise we might miss the
         # POSITION response sent back by the other end.
-        logger.info("Sending redis SUBSCRIBE for %s", self.synapse_stream_name)
-        await make_deferred_yieldable(self.subscribe(self.synapse_stream_name))
+        fully_qualified_stream_names = [
+            f"{self.synapse_stream_prefix}/{stream_suffix}"
+            for stream_suffix in self.synapse_channel_names
+        ] + [self.synapse_stream_prefix]
+        logger.info("Sending redis SUBSCRIBE for %r", fully_qualified_stream_names)
+        await make_deferred_yieldable(self.subscribe(fully_qualified_stream_names))
+
         logger.info(
             "Successfully subscribed to redis stream, sending REPLICATE command"
         )
@@ -217,7 +223,7 @@ class RedisSubscriber(txredisapi.SubscriberProtocol):
 
         await make_deferred_yieldable(
             self.synapse_outbound_redis_connection.publish(
-                self.synapse_stream_name, encoded_string
+                self.synapse_stream_prefix, encoded_string
             )
         )
 
@@ -300,20 +306,27 @@ def format_address(address: IAddress) -> str:
 
 class RedisDirectTcpReplicationClientFactory(SynapseRedisFactory):
     """This is a reconnecting factory that connects to redis and immediately
-    subscribes to a stream.
+    subscribes to some streams.
 
     Args:
         hs
         outbound_redis_connection: A connection to redis that will be used to
             send outbound commands (this is separate to the redis connection
             used to subscribe).
+        channel_names: A list of channel names to append to the base channel name
+            to additionally subscribe to.
+            e.g. if ['ABC', 'DEF'] is specified then we'll listen to:
+            example.com; example.com/ABC; and example.com/DEF.
     """
 
     maxDelay = 5
     protocol = RedisSubscriber
 
     def __init__(
-        self, hs: "HomeServer", outbound_redis_connection: txredisapi.ConnectionHandler
+        self,
+        hs: "HomeServer",
+        outbound_redis_connection: txredisapi.ConnectionHandler,
+        channel_names: List[str],
     ):
 
         super().__init__(
@@ -326,7 +339,8 @@ class RedisDirectTcpReplicationClientFactory(SynapseRedisFactory):
         )
 
         self.synapse_handler = hs.get_replication_command_handler()
-        self.synapse_stream_name = hs.hostname
+        self.synapse_stream_prefix = hs.hostname
+        self.synapse_channel_names = channel_names
 
         self.synapse_outbound_redis_connection = outbound_redis_connection
 
@@ -340,7 +354,8 @@ class RedisDirectTcpReplicationClientFactory(SynapseRedisFactory):
         # protocol.
         p.synapse_handler = self.synapse_handler
         p.synapse_outbound_redis_connection = self.synapse_outbound_redis_connection
-        p.synapse_stream_name = self.synapse_stream_name
+        p.synapse_stream_prefix = self.synapse_stream_prefix
+        p.synapse_channel_names = self.synapse_channel_names
 
         return p
 
diff --git a/tests/replication/_base.py b/tests/replication/_base.py
index a7602b4c96..970d5e533b 100644
--- a/tests/replication/_base.py
+++ b/tests/replication/_base.py
@@ -12,7 +12,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import logging
-from typing import Any, Dict, List, Optional, Tuple
+from collections import defaultdict
+from typing import Any, Dict, List, Optional, Set, Tuple
 
 from twisted.internet.address import IPv4Address
 from twisted.internet.protocol import Protocol
@@ -32,6 +33,7 @@ from synapse.server import HomeServer
 
 from tests import unittest
 from tests.server import FakeTransport
+from tests.utils import USE_POSTGRES_FOR_TESTS
 
 try:
     import hiredis
@@ -475,22 +477,25 @@ class FakeRedisPubSubServer:
     """A fake Redis server for pub/sub."""
 
     def __init__(self):
-        self._subscribers = set()
+        self._subscribers_by_channel: Dict[
+            bytes, Set["FakeRedisPubSubProtocol"]
+        ] = defaultdict(set)
 
-    def add_subscriber(self, conn):
+    def add_subscriber(self, conn, channel: bytes):
         """A connection has called SUBSCRIBE"""
-        self._subscribers.add(conn)
+        self._subscribers_by_channel[channel].add(conn)
 
     def remove_subscriber(self, conn):
-        """A connection has called UNSUBSCRIBE"""
-        self._subscribers.discard(conn)
+        """A connection has lost connection"""
+        for subscribers in self._subscribers_by_channel.values():
+            subscribers.discard(conn)
 
-    def publish(self, conn, channel, msg) -> int:
+    def publish(self, conn, channel: bytes, msg) -> int:
         """A connection want to publish a message to subscribers."""
-        for sub in self._subscribers:
+        for sub in self._subscribers_by_channel[channel]:
             sub.send(["message", channel, msg])
 
-        return len(self._subscribers)
+        return len(self._subscribers_by_channel)
 
     def buildProtocol(self, addr):
         return FakeRedisPubSubProtocol(self)
@@ -531,9 +536,10 @@ class FakeRedisPubSubProtocol(Protocol):
             num_subscribers = self._server.publish(self, channel, message)
             self.send(num_subscribers)
         elif command == b"SUBSCRIBE":
-            (channel,) = args
-            self._server.add_subscriber(self)
-            self.send(["subscribe", channel, 1])
+            for idx, channel in enumerate(args):
+                num_channels = idx + 1
+                self._server.add_subscriber(self, channel)
+                self.send(["subscribe", channel, num_channels])
 
         # Since we use SET/GET to cache things we can safely no-op them.
         elif command == b"SET":
@@ -576,3 +582,27 @@ class FakeRedisPubSubProtocol(Protocol):
 
     def connectionLost(self, reason):
         self._server.remove_subscriber(self)
+
+
+class RedisMultiWorkerStreamTestCase(BaseMultiWorkerStreamTestCase):
+    """
+    A test case that enables Redis, providing a fake Redis server.
+    """
+
+    if not hiredis:
+        skip = "Requires hiredis"
+
+    if not USE_POSTGRES_FOR_TESTS:
+        # Redis replication only takes place on Postgres
+        skip = "Requires Postgres"
+
+    def default_config(self) -> Dict[str, Any]:
+        """
+        Overrides the default config to enable Redis.
+        Even if the test only uses make_worker_hs, the main process needs Redis
+        enabled otherwise it won't create a Fake Redis server to listen on the
+        Redis port and accept fake TCP connections.
+        """
+        base = super().default_config()
+        base["redis"] = {"enabled": True}
+        return base
diff --git a/tests/replication/tcp/test_handler.py b/tests/replication/tcp/test_handler.py
new file mode 100644
index 0000000000..e6a19eafd5
--- /dev/null
+++ b/tests/replication/tcp/test_handler.py
@@ -0,0 +1,73 @@
+# Copyright 2022 The Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from tests.replication._base import RedisMultiWorkerStreamTestCase
+
+
+class ChannelsTestCase(RedisMultiWorkerStreamTestCase):
+    def test_subscribed_to_enough_redis_channels(self) -> None:
+        # The default main process is subscribed to the USER_IP channel.
+        self.assertCountEqual(
+            self.hs.get_replication_command_handler()._channels_to_subscribe_to,
+            ["USER_IP"],
+        )
+
+    def test_background_worker_subscribed_to_user_ip(self) -> None:
+        # The default main process is subscribed to the USER_IP channel.
+        worker1 = self.make_worker_hs(
+            "synapse.app.generic_worker",
+            extra_config={
+                "worker_name": "worker1",
+                "run_background_tasks_on": "worker1",
+                "redis": {"enabled": True},
+            },
+        )
+        self.assertIn(
+            "USER_IP",
+            worker1.get_replication_command_handler()._channels_to_subscribe_to,
+        )
+
+        # Advance so the Redis subscription gets processed
+        self.pump(0.1)
+
+        # The counts are 2 because both the main process and the worker are subscribed.
+        self.assertEqual(len(self._redis_server._subscribers_by_channel[b"test"]), 2)
+        self.assertEqual(
+            len(self._redis_server._subscribers_by_channel[b"test/USER_IP"]), 2
+        )
+
+    def test_non_background_worker_not_subscribed_to_user_ip(self) -> None:
+        # The default main process is subscribed to the USER_IP channel.
+        worker2 = self.make_worker_hs(
+            "synapse.app.generic_worker",
+            extra_config={
+                "worker_name": "worker2",
+                "run_background_tasks_on": "worker1",
+                "redis": {"enabled": True},
+            },
+        )
+        self.assertNotIn(
+            "USER_IP",
+            worker2.get_replication_command_handler()._channels_to_subscribe_to,
+        )
+
+        # Advance so the Redis subscription gets processed
+        self.pump(0.1)
+
+        # The count is 2 because both the main process and the worker are subscribed.
+        self.assertEqual(len(self._redis_server._subscribers_by_channel[b"test"]), 2)
+        # For USER_IP, the count is 1 because only the main process is subscribed.
+        self.assertEqual(
+            len(self._redis_server._subscribers_by_channel[b"test/USER_IP"]), 1
+        )