Move client command handling out of TCP protocol (#7185)

The aim here is to move the command handling out of the TCP protocol classes and to also merge the client and server command handling (so that we can reuse them for redis protocol). This PR simply moves the client paths to the new `ReplicationCommandHandler`, a future PR will move the server paths too.

4 files changed, 336 insertions, 322 deletions

diff --git a/synapse/replication/tcp/__init__.py b/synapse/replication/tcp/__init__.py
index 81c2ea7ee9..523a1358d4 100644
--- a/synapse/replication/tcp/__init__.py
+++ b/synapse/replication/tcp/__init__.py
@@ -20,11 +20,31 @@ Further details can be found in docs/tcp_replication.rst
 
 
 Structure of the module:
- * client.py   - the client classes used for workers to connect to master
+ * handler.py  - the classes used to handle sending/receiving commands to
+                 replication
  * command.py  - the definitions of all the valid commands
- * protocol.py - contains bot the client and server protocol implementations,
-                 these should not be used directly
- * resource.py - the server classes that accepts and handle client connections
- * streams.py  - the definitons of all the valid streams
+ * protocol.py - the TCP protocol classes
+ * resource.py - handles streaming stream updates to replications
+ * streams/    - the definitons of all the valid streams
 
+
+The general interaction of the classes are:
+
+        +---------------------+
+        | ReplicationStreamer |
+        +---------------------+
+                    |
+                    v
+        +---------------------------+     +----------------------+
+        | ReplicationCommandHandler |---->|ReplicationDataHandler|
+        +---------------------------+     +----------------------+
+                    | ^
+                    v |
+            +-------------+
+            | Protocols   |
+            | (TCP/redis) |
+            +-------------+
+
+Where the ReplicationDataHandler (or subclasses) handles incoming stream
+updates.
 """
diff --git a/synapse/replication/tcp/client.py b/synapse/replication/tcp/client.py
index e86d9805f1..700ae79158 100644
--- a/synapse/replication/tcp/client.py
+++ b/synapse/replication/tcp/client.py
@@ -16,26 +16,16 @@
 """
 
 import logging
-from typing import Dict, List, Optional
+from typing import TYPE_CHECKING, Dict
 
-from twisted.internet import defer
 from twisted.internet.protocol import ReconnectingClientFactory
 
 from synapse.replication.slave.storage._base import BaseSlavedStore
-from synapse.replication.tcp.protocol import (
-    AbstractReplicationClientHandler,
-    ClientReplicationStreamProtocol,
-)
-
-from .commands import (
-    Command,
-    FederationAckCommand,
-    InvalidateCacheCommand,
-    RemoteServerUpCommand,
-    RemovePusherCommand,
-    UserIpCommand,
-    UserSyncCommand,
-)
+from synapse.replication.tcp.protocol import ClientReplicationStreamProtocol
+
+if TYPE_CHECKING:
+    from synapse.server import HomeServer
+    from synapse.replication.tcp.handler import ReplicationCommandHandler
 
 logger = logging.getLogger(__name__)
 
@@ -44,16 +34,20 @@ class ReplicationClientFactory(ReconnectingClientFactory):
     """Factory for building connections to the master. Will reconnect if the
     connection is lost.
 
-    Accepts a handler that will be called when new data is available or data
-    is required.
+    Accepts a handler that is passed to `ClientReplicationStreamProtocol`.
     """
 
     initialDelay = 0.1
     maxDelay = 1  # Try at least once every N seconds
 
-    def __init__(self, hs, client_name, handler: AbstractReplicationClientHandler):
+    def __init__(
+        self,
+        hs: "HomeServer",
+        client_name: str,
+        command_handler: "ReplicationCommandHandler",
+    ):
         self.client_name = client_name
-        self.handler = handler
+        self.command_handler = command_handler
         self.server_name = hs.config.server_name
         self.hs = hs
         self._clock = hs.get_clock()  # As self.clock is defined in super class
@@ -66,7 +60,11 @@ class ReplicationClientFactory(ReconnectingClientFactory):
     def buildProtocol(self, addr):
         logger.info("Connected to replication: %r", addr)
         return ClientReplicationStreamProtocol(
-            self.hs, self.client_name, self.server_name, self._clock, self.handler,
+            self.hs,
+            self.client_name,
+            self.server_name,
+            self._clock,
+            self.command_handler,
         )
 
     def clientConnectionLost(self, connector, reason):
@@ -78,41 +76,17 @@ class ReplicationClientFactory(ReconnectingClientFactory):
         ReconnectingClientFactory.clientConnectionFailed(self, connector, reason)
 
 
-class ReplicationClientHandler(AbstractReplicationClientHandler):
-    """A base handler that can be passed to the ReplicationClientFactory.
+class ReplicationDataHandler:
+    """Handles incoming stream updates from replication.
 
-    By default proxies incoming replication data to the SlaveStore.
+    This instance notifies the slave data store about updates. Can be subclassed
+    to handle updates in additional ways.
     """
 
     def __init__(self, store: BaseSlavedStore):
         self.store = store
 
-        # The current connection. None if we are currently (re)connecting
-        self.connection = None
-
-        # Any pending commands to be sent once a new connection has been
-        # established
-        self.pending_commands = []  # type: List[Command]
-
-        # Map from string -> deferred, to wake up when receiveing a SYNC with
-        # the given string.
-        # Used for tests.
-        self.awaiting_syncs = {}  # type: Dict[str, defer.Deferred]
-
-        # The factory used to create connections.
-        self.factory = None  # type: Optional[ReplicationClientFactory]
-
-    def start_replication(self, hs):
-        """Helper method to start a replication connection to the remote server
-        using TCP.
-        """
-        client_name = hs.config.worker_name
-        self.factory = ReplicationClientFactory(hs, client_name, self)
-        host = hs.config.worker_replication_host
-        port = hs.config.worker_replication_port
-        hs.get_reactor().connectTCP(host, port, self.factory)
-
-    async def on_rdata(self, stream_name, token, rows):
+    async def on_rdata(self, stream_name: str, token: int, rows: list):
         """Called to handle a batch of replication data with a given stream token.
 
         By default this just pokes the slave store. Can be overridden in subclasses to
@@ -124,30 +98,8 @@ class ReplicationClientHandler(AbstractReplicationClientHandler):
             rows (list): a list of Stream.ROW_TYPE objects as returned by
                 Stream.parse_row.
         """
-        logger.debug("Received rdata %s -> %s", stream_name, token)
         self.store.process_replication_rows(stream_name, token, rows)
 
-    async def on_position(self, stream_name, token):
-        """Called when we get new position data. By default this just pokes
-        the slave store.
-
-        Can be overriden in subclasses to handle more.
-        """
-        self.store.process_replication_rows(stream_name, token, [])
-
-    def on_sync(self, data):
-        """When we received a SYNC we wake up any deferreds that were waiting
-        for the sync with the given data.
-
-        Used by tests.
-        """
-        d = self.awaiting_syncs.pop(data, None)
-        if d:
-            d.callback(data)
-
-    def on_remote_server_up(self, server: str):
-        """Called when get a new REMOTE_SERVER_UP command."""
-
     def get_streams_to_replicate(self) -> Dict[str, int]:
         """Called when a new connection has been established and we need to
         subscribe to streams.
@@ -163,85 +115,10 @@ class ReplicationClientHandler(AbstractReplicationClientHandler):
             args["account_data"] = user_account_data
         elif room_account_data:
             args["account_data"] = room_account_data
-
         return args
 
-    def get_currently_syncing_users(self):
-        """Get the list of currently syncing users (if any). This is called
-        when a connection has been established and we need to send the
-        currently syncing users. (Overriden by the synchrotron's only)
-        """
-        return []
-
-    def send_command(self, cmd):
-        """Send a command to master (when we get establish a connection if we
-        don't have one already.)
-        """
-        if self.connection:
-            self.connection.send_command(cmd)
-        else:
-            logger.warning("Queuing command as not connected: %r", cmd.NAME)
-            self.pending_commands.append(cmd)
-
-    def send_federation_ack(self, token):
-        """Ack data for the federation stream. This allows the master to drop
-        data stored purely in memory.
-        """
-        self.send_command(FederationAckCommand(token))
-
-    def send_user_sync(self, instance_id, user_id, is_syncing, last_sync_ms):
-        """Poke the master that a user has started/stopped syncing.
-        """
-        self.send_command(
-            UserSyncCommand(instance_id, user_id, is_syncing, last_sync_ms)
-        )
-
-    def send_remove_pusher(self, app_id, push_key, user_id):
-        """Poke the master to remove a pusher for a user
-        """
-        cmd = RemovePusherCommand(app_id, push_key, user_id)
-        self.send_command(cmd)
-
-    def send_invalidate_cache(self, cache_func, keys):
-        """Poke the master to invalidate a cache.
-        """
-        cmd = InvalidateCacheCommand(cache_func.__name__, keys)
-        self.send_command(cmd)
-
-    def send_user_ip(self, user_id, access_token, ip, user_agent, device_id, last_seen):
-        """Tell the master that the user made a request.
-        """
-        cmd = UserIpCommand(user_id, access_token, ip, user_agent, device_id, last_seen)
-        self.send_command(cmd)
-
-    def send_remote_server_up(self, server: str):
-        self.send_command(RemoteServerUpCommand(server))
-
-    def await_sync(self, data):
-        """Returns a deferred that is resolved when we receive a SYNC command
-        with given data.
-
-        [Not currently] used by tests.
-        """
-        return self.awaiting_syncs.setdefault(data, defer.Deferred())
-
-    def update_connection(self, connection):
-        """Called when a connection has been established (or lost with None).
-        """
-        self.connection = connection
-        if connection:
-            for cmd in self.pending_commands:
-                connection.send_command(cmd)
-            self.pending_commands = []
-
-    def finished_connecting(self):
-        """Called when we have successfully subscribed and caught up to all
-        streams we're interested in.
-        """
-        logger.info("Finished connecting to server")
+    async def on_position(self, stream_name: str, token: int):
+        self.store.process_replication_rows(stream_name, token, [])
 
-        # We don't reset the delay any earlier as otherwise if there is a
-        # problem during start up we'll end up tight looping connecting to the
-        # server.
-        if self.factory:
-            self.factory.resetDelay()
+    def on_remote_server_up(self, server: str):
+        """Called when get a new REMOTE_SERVER_UP command."""
diff --git a/synapse/replication/tcp/handler.py b/synapse/replication/tcp/handler.py
new file mode 100644
index 0000000000..12a1cfd6d1
--- /dev/null
+++ b/synapse/replication/tcp/handler.py
@@ -0,0 +1,252 @@
+# -*- coding: utf-8 -*-
+# Copyright 2017 Vector Creations Ltd
+# Copyright 2020 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.
+
+import logging
+from typing import Any, Callable, Dict, List, Optional, Set
+
+from prometheus_client import Counter
+
+from synapse.replication.tcp.client import ReplicationClientFactory
+from synapse.replication.tcp.commands import (
+    Command,
+    FederationAckCommand,
+    InvalidateCacheCommand,
+    PositionCommand,
+    RdataCommand,
+    RemoteServerUpCommand,
+    RemovePusherCommand,
+    SyncCommand,
+    UserIpCommand,
+    UserSyncCommand,
+)
+from synapse.replication.tcp.streams import STREAMS_MAP, Stream
+from synapse.util.async_helpers import Linearizer
+
+logger = logging.getLogger(__name__)
+
+
+# number of updates received for each RDATA stream
+inbound_rdata_count = Counter(
+    "synapse_replication_tcp_protocol_inbound_rdata_count", "", ["stream_name"]
+)
+
+
+class ReplicationCommandHandler:
+    """Handles incoming commands from replication as well as sending commands
+    back out to connections.
+    """
+
+    def __init__(self, hs):
+        self._replication_data_handler = hs.get_replication_data_handler()
+        self._presence_handler = hs.get_presence_handler()
+
+        # Set of streams that we've caught up with.
+        self._streams_connected = set()  # type: Set[str]
+
+        self._streams = {
+            stream.NAME: stream(hs) for stream in STREAMS_MAP.values()
+        }  # type: Dict[str, Stream]
+
+        self._position_linearizer = Linearizer("replication_position")
+
+        # Map of stream to batched updates. See RdataCommand for info on how
+        # batching works.
+        self._pending_batches = {}  # type: Dict[str, List[Any]]
+
+        # The factory used to create connections.
+        self._factory = None  # type: Optional[ReplicationClientFactory]
+
+        # The current connection. None if we are currently (re)connecting
+        self._connection = None
+
+    def start_replication(self, hs):
+        """Helper method to start a replication connection to the remote server
+        using TCP.
+        """
+        client_name = hs.config.worker_name
+        self._factory = ReplicationClientFactory(hs, client_name, self)
+        host = hs.config.worker_replication_host
+        port = hs.config.worker_replication_port
+        hs.get_reactor().connectTCP(host, port, self._factory)
+
+    async def on_RDATA(self, cmd: RdataCommand):
+        stream_name = cmd.stream_name
+        inbound_rdata_count.labels(stream_name).inc()
+
+        try:
+            row = STREAMS_MAP[stream_name].parse_row(cmd.row)
+        except Exception:
+            logger.exception("Failed to parse RDATA: %r %r", stream_name, cmd.row)
+            raise
+
+        if cmd.token is None or stream_name not in self._streams_connected:
+            # I.e. either this is part of a batch of updates for this stream (in
+            # which case batch until we get an update for the stream with a non
+            # None token) or we're currently connecting so we queue up rows.
+            self._pending_batches.setdefault(stream_name, []).append(row)
+        else:
+            # Check if this is the last of a batch of updates
+            rows = self._pending_batches.pop(stream_name, [])
+            rows.append(row)
+            await self.on_rdata(stream_name, cmd.token, rows)
+
+    async def on_rdata(self, stream_name: str, token: int, rows: list):
+        """Called to handle a batch of replication data with a given stream token.
+
+        Args:
+            stream_name: name of the replication stream for this batch of rows
+            token: stream token for this batch of rows
+            rows: a list of Stream.ROW_TYPE objects as returned by
+                Stream.parse_row.
+        """
+        logger.debug("Received rdata %s -> %s", stream_name, token)
+        await self._replication_data_handler.on_rdata(stream_name, token, rows)
+
+    async def on_POSITION(self, cmd: PositionCommand):
+        stream = self._streams.get(cmd.stream_name)
+        if not stream:
+            logger.error("Got POSITION for unknown stream: %s", cmd.stream_name)
+            return
+
+        # We protect catching up with a linearizer in case the replication
+        # connection reconnects under us.
+        with await self._position_linearizer.queue(cmd.stream_name):
+            # We're about to go and catch up with the stream, so mark as connecting
+            # to stop RDATA being handled at the same time by removing stream from
+            # list of connected streams. We also clear any batched up RDATA from
+            # before we got the POSITION.
+            self._streams_connected.discard(cmd.stream_name)
+            self._pending_batches.clear()
+
+            # Find where we previously streamed up to.
+            current_token = self._replication_data_handler.get_streams_to_replicate().get(
+                cmd.stream_name
+            )
+            if current_token is None:
+                logger.warning(
+                    "Got POSITION for stream we're not subscribed to: %s",
+                    cmd.stream_name,
+                )
+                return
+
+            # Fetch all updates between then and now.
+            limited = True
+            while limited:
+                updates, current_token, limited = await stream.get_updates_since(
+                    current_token, cmd.token
+                )
+                if updates:
+                    await self.on_rdata(
+                        cmd.stream_name,
+                        current_token,
+                        [stream.parse_row(update[1]) for update in updates],
+                    )
+
+            # We've now caught up to position sent to us, notify handler.
+            await self._replication_data_handler.on_position(cmd.stream_name, cmd.token)
+
+            # Handle any RDATA that came in while we were catching up.
+            rows = self._pending_batches.pop(cmd.stream_name, [])
+            if rows:
+                await self._replication_data_handler.on_rdata(
+                    cmd.stream_name, rows[-1].token, rows
+                )
+
+            self._streams_connected.add(cmd.stream_name)
+
+    async def on_SYNC(self, cmd: SyncCommand):
+        pass
+
+    async def on_REMOTE_SERVER_UP(self, cmd: RemoteServerUpCommand):
+        """"Called when get a new REMOTE_SERVER_UP command."""
+        self._replication_data_handler.on_remote_server_up(cmd.data)
+
+    def get_currently_syncing_users(self):
+        """Get the list of currently syncing users (if any). This is called
+        when a connection has been established and we need to send the
+        currently syncing users.
+        """
+        return self._presence_handler.get_currently_syncing_users()
+
+    def update_connection(self, connection):
+        """Called when a connection has been established (or lost with None).
+        """
+        self._connection = connection
+
+    def finished_connecting(self):
+        """Called when we have successfully subscribed and caught up to all
+        streams we're interested in.
+        """
+        logger.info("Finished connecting to server")
+
+        # We don't reset the delay any earlier as otherwise if there is a
+        # problem during start up we'll end up tight looping connecting to the
+        # server.
+        if self._factory:
+            self._factory.resetDelay()
+
+    def send_command(self, cmd: Command):
+        """Send a command to master (when we get establish a connection if we
+        don't have one already.)
+        """
+        if self._connection:
+            self._connection.send_command(cmd)
+        else:
+            logger.warning("Dropping command as not connected: %r", cmd.NAME)
+
+    def send_federation_ack(self, token: int):
+        """Ack data for the federation stream. This allows the master to drop
+        data stored purely in memory.
+        """
+        self.send_command(FederationAckCommand(token))
+
+    def send_user_sync(
+        self, instance_id: str, user_id: str, is_syncing: bool, last_sync_ms: int
+    ):
+        """Poke the master that a user has started/stopped syncing.
+        """
+        self.send_command(
+            UserSyncCommand(instance_id, user_id, is_syncing, last_sync_ms)
+        )
+
+    def send_remove_pusher(self, app_id: str, push_key: str, user_id: str):
+        """Poke the master to remove a pusher for a user
+        """
+        cmd = RemovePusherCommand(app_id, push_key, user_id)
+        self.send_command(cmd)
+
+    def send_invalidate_cache(self, cache_func: Callable, keys: tuple):
+        """Poke the master to invalidate a cache.
+        """
+        cmd = InvalidateCacheCommand(cache_func.__name__, keys)
+        self.send_command(cmd)
+
+    def send_user_ip(
+        self,
+        user_id: str,
+        access_token: str,
+        ip: str,
+        user_agent: str,
+        device_id: str,
+        last_seen: int,
+    ):
+        """Tell the master that the user made a request.
+        """
+        cmd = UserIpCommand(user_id, access_token, ip, user_agent, device_id, last_seen)
+        self.send_command(cmd)
+
+    def send_remote_server_up(self, server: str):
+        self.send_command(RemoteServerUpCommand(server))
diff --git a/synapse/replication/tcp/protocol.py b/synapse/replication/tcp/protocol.py
index dae246825f..f2a37f568e 100644
--- a/synapse/replication/tcp/protocol.py
+++ b/synapse/replication/tcp/protocol.py
@@ -46,12 +46,11 @@ indicate which side is sending, these are *not* included on the wire::
     > ERROR server stopping
     * connection closed by server *
 """
-import abc
 import fcntl
 import logging
 import struct
 from collections import defaultdict
-from typing import Any, DefaultDict, Dict, List, Set
+from typing import TYPE_CHECKING, DefaultDict, List
 
 from six import iteritems
 
@@ -78,13 +77,12 @@ from synapse.replication.tcp.commands import (
     SyncCommand,
     UserSyncCommand,
 )
-from synapse.replication.tcp.streams import STREAMS_MAP, Stream
 from synapse.types import Collection
 from synapse.util import Clock
 from synapse.util.stringutils import random_string
 
-MYPY = False
-if MYPY:
+if TYPE_CHECKING:
+    from synapse.replication.tcp.handler import ReplicationCommandHandler
     from synapse.server import HomeServer
 
 
@@ -475,71 +473,6 @@ class ServerReplicationStreamProtocol(BaseReplicationStreamProtocol):
         self.streamer.lost_connection(self)
 
 
-class AbstractReplicationClientHandler(metaclass=abc.ABCMeta):
-    """
-    The interface for the handler that should be passed to
-    ClientReplicationStreamProtocol
-    """
-
-    @abc.abstractmethod
-    async def on_rdata(self, stream_name, token, rows):
-        """Called to handle a batch of replication data with a given stream token.
-
-        Args:
-            stream_name (str): name of the replication stream for this batch of rows
-            token (int): stream token for this batch of rows
-            rows (list): a list of Stream.ROW_TYPE objects as returned by
-                Stream.parse_row.
-        """
-        raise NotImplementedError()
-
-    @abc.abstractmethod
-    async def on_position(self, stream_name, token):
-        """Called when we get new position data."""
-        raise NotImplementedError()
-
-    @abc.abstractmethod
-    def on_sync(self, data):
-        """Called when get a new SYNC command."""
-        raise NotImplementedError()
-
-    @abc.abstractmethod
-    async def on_remote_server_up(self, server: str):
-        """Called when get a new REMOTE_SERVER_UP command."""
-        raise NotImplementedError()
-
-    @abc.abstractmethod
-    def get_streams_to_replicate(self):
-        """Called when a new connection has been established and we need to
-        subscribe to streams.
-
-        Returns:
-            map from stream name to the most recent update we have for
-            that stream (ie, the point we want to start replicating from)
-        """
-        raise NotImplementedError()
-
-    @abc.abstractmethod
-    def get_currently_syncing_users(self):
-        """Get the list of currently syncing users (if any). This is called
-        when a connection has been established and we need to send the
-        currently syncing users."""
-        raise NotImplementedError()
-
-    @abc.abstractmethod
-    def update_connection(self, connection):
-        """Called when a connection has been established (or lost with None).
-        """
-        raise NotImplementedError()
-
-    @abc.abstractmethod
-    def finished_connecting(self):
-        """Called when we have successfully subscribed and caught up to all
-        streams we're interested in.
-        """
-        raise NotImplementedError()
-
-
 class ClientReplicationStreamProtocol(BaseReplicationStreamProtocol):
     VALID_INBOUND_COMMANDS = VALID_SERVER_COMMANDS
     VALID_OUTBOUND_COMMANDS = VALID_CLIENT_COMMANDS
@@ -550,7 +483,7 @@ class ClientReplicationStreamProtocol(BaseReplicationStreamProtocol):
         client_name: str,
         server_name: str,
         clock: Clock,
-        handler: AbstractReplicationClientHandler,
+        command_handler: "ReplicationCommandHandler",
     ):
         BaseReplicationStreamProtocol.__init__(self, clock)
 
@@ -558,20 +491,7 @@ class ClientReplicationStreamProtocol(BaseReplicationStreamProtocol):
 
         self.client_name = client_name
         self.server_name = server_name
-        self.handler = handler
-
-        self.streams = {
-            stream.NAME: stream(hs) for stream in STREAMS_MAP.values()
-        }  # type: Dict[str, Stream]
-
-        # Set of stream names that have been subscribe to, but haven't yet
-        # caught up with. This is used to track when the client has been fully
-        # connected to the remote.
-        self.streams_connecting = set(STREAMS_MAP)  # type: Set[str]
-
-        # Map of stream to batched updates. See RdataCommand for info on how
-        # batching works.
-        self.pending_batches = {}  # type: Dict[str, List[Any]]
+        self.handler = command_handler
 
     def connectionMade(self):
         self.send_command(NameCommand(self.client_name))
@@ -589,89 +509,39 @@ class ClientReplicationStreamProtocol(BaseReplicationStreamProtocol):
 
         # We've now finished connecting to so inform the client handler
         self.handler.update_connection(self)
+        self.handler.finished_connecting()
 
-    async def on_SERVER(self, cmd):
-        if cmd.data != self.server_name:
-            logger.error("[%s] Connected to wrong remote: %r", self.id(), cmd.data)
-            self.send_error("Wrong remote")
-
-    async def on_RDATA(self, cmd):
-        stream_name = cmd.stream_name
-        inbound_rdata_count.labels(stream_name).inc()
-
-        try:
-            row = STREAMS_MAP[stream_name].parse_row(cmd.row)
-        except Exception:
-            logger.exception(
-                "[%s] Failed to parse RDATA: %r %r", self.id(), stream_name, cmd.row
-            )
-            raise
-
-        if cmd.token is None or stream_name in self.streams_connecting:
-            # I.e. this is part of a batch of updates for this stream. Batch
-            # until we get an update for the stream with a non None token
-            self.pending_batches.setdefault(stream_name, []).append(row)
-        else:
-            # Check if this is the last of a batch of updates
-            rows = self.pending_batches.pop(stream_name, [])
-            rows.append(row)
-            await self.handler.on_rdata(stream_name, cmd.token, rows)
-
-    async def on_POSITION(self, cmd: PositionCommand):
-        stream = self.streams.get(cmd.stream_name)
-        if not stream:
-            logger.error("Got POSITION for unknown stream: %s", cmd.stream_name)
-            return
-
-        # Find where we previously streamed up to.
-        current_token = self.handler.get_streams_to_replicate().get(cmd.stream_name)
-        if current_token is None:
-            logger.warning(
-                "Got POSITION for stream we're not subscribed to: %s", cmd.stream_name
-            )
-            return
-
-        # Fetch all updates between then and now.
-        limited = True
-        while limited:
-            updates, current_token, limited = await stream.get_updates_since(
-                current_token, cmd.token
-            )
-
-            # Check if the connection was closed underneath us, if so we bail
-            # rather than risk having concurrent catch ups going on.
-            if self.state == ConnectionStates.CLOSED:
-                return
-
-            if updates:
-                await self.handler.on_rdata(
-                    cmd.stream_name,
-                    current_token,
-                    [stream.parse_row(update[1]) for update in updates],
-                )
+    async def handle_command(self, cmd: Command):
+        """Handle a command we have received over the replication stream.
 
-        # We've now caught up to position sent to us, notify handler.
-        await self.handler.on_position(cmd.stream_name, cmd.token)
+        Delegates to `command_handler.on_<COMMAND>`, which must return an
+        awaitable.
 
-        self.streams_connecting.discard(cmd.stream_name)
-        if not self.streams_connecting:
-            self.handler.finished_connecting()
+        Args:
+            cmd: received command
+        """
+        handled = False
 
-        # Check if the connection was closed underneath us, if so we bail
-        # rather than risk having concurrent catch ups going on.
-        if self.state == ConnectionStates.CLOSED:
-            return
+        # First call any command handlers on this instance. These are for TCP
+        # specific handling.
+        cmd_func = getattr(self, "on_%s" % (cmd.NAME,), None)
+        if cmd_func:
+            await cmd_func(cmd)
+            handled = True
 
-        # Handle any RDATA that came in while we were catching up.
-        rows = self.pending_batches.pop(cmd.stream_name, [])
-        if rows:
-            await self.handler.on_rdata(cmd.stream_name, rows[-1].token, rows)
+        # Then call out to the handler.
+        cmd_func = getattr(self.handler, "on_%s" % (cmd.NAME,), None)
+        if cmd_func:
+            await cmd_func(cmd)
+            handled = True
 
-    async def on_SYNC(self, cmd):
-        self.handler.on_sync(cmd.data)
+        if not handled:
+            logger.warning("Unhandled command: %r", cmd)
 
-    async def on_REMOTE_SERVER_UP(self, cmd: RemoteServerUpCommand):
-        self.handler.on_remote_server_up(cmd.data)
+    async def on_SERVER(self, cmd):
+        if cmd.data != self.server_name:
+            logger.error("[%s] Connected to wrong remote: %r", self.id(), cmd.data)
+            self.send_error("Wrong remote")
 
     def replicate(self):
         """Send the subscription request to the server
@@ -768,8 +638,3 @@ tcp_outbound_commands = LaterGauge(
         for k, count in iteritems(p.outbound_commands_counter)
     },
 )
-
-# number of updates received for each RDATA stream
-inbound_rdata_count = Counter(
-    "synapse_replication_tcp_protocol_inbound_rdata_count", "", ["stream_name"]
-)