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.

12 files changed, 378 insertions, 371 deletions

diff --git a/changelog.d/7185.misc b/changelog.d/7185.misc
new file mode 100644
index 0000000000..deb9ca7021
--- /dev/null
+++ b/changelog.d/7185.misc
@@ -0,0 +1 @@
+Move client command handling out of TCP protocol.
diff --git a/synapse/app/admin_cmd.py b/synapse/app/admin_cmd.py
index 1c7c6ec0c8..a37818fe9a 100644
--- a/synapse/app/admin_cmd.py
+++ b/synapse/app/admin_cmd.py
@@ -43,7 +43,6 @@ from synapse.replication.slave.storage.push_rule import SlavedPushRuleStore
 from synapse.replication.slave.storage.receipts import SlavedReceiptsStore
 from synapse.replication.slave.storage.registration import SlavedRegistrationStore
 from synapse.replication.slave.storage.room import RoomStore
-from synapse.replication.tcp.client import ReplicationClientHandler
 from synapse.server import HomeServer
 from synapse.util.logcontext import LoggingContext
 from synapse.util.versionstring import get_version_string
@@ -79,17 +78,6 @@ class AdminCmdServer(HomeServer):
     def start_listening(self, listeners):
         pass
 
-    def build_tcp_replication(self):
-        return AdminCmdReplicationHandler(self)
-
-
-class AdminCmdReplicationHandler(ReplicationClientHandler):
-    async def on_rdata(self, stream_name, token, rows):
-        pass
-
-    def get_streams_to_replicate(self):
-        return {}
-
 
 @defer.inlineCallbacks
 def export_data_command(hs, args):
diff --git a/synapse/app/generic_worker.py b/synapse/app/generic_worker.py
index 174bef360f..dcd0709a02 100644
--- a/synapse/app/generic_worker.py
+++ b/synapse/app/generic_worker.py
@@ -64,7 +64,7 @@ from synapse.replication.slave.storage.receipts import SlavedReceiptsStore
 from synapse.replication.slave.storage.registration import SlavedRegistrationStore
 from synapse.replication.slave.storage.room import RoomStore
 from synapse.replication.slave.storage.transactions import SlavedTransactionStore
-from synapse.replication.tcp.client import ReplicationClientHandler
+from synapse.replication.tcp.client import ReplicationDataHandler
 from synapse.replication.tcp.commands import ClearUserSyncsCommand
 from synapse.replication.tcp.streams import (
     AccountDataStream,
@@ -603,7 +603,7 @@ class GenericWorkerServer(HomeServer):
     def remove_pusher(self, app_id, push_key, user_id):
         self.get_tcp_replication().send_remove_pusher(app_id, push_key, user_id)
 
-    def build_tcp_replication(self):
+    def build_replication_data_handler(self):
         return GenericWorkerReplicationHandler(self)
 
     def build_presence_handler(self):
@@ -613,7 +613,7 @@ class GenericWorkerServer(HomeServer):
         return GenericWorkerTyping(self)
 
 
-class GenericWorkerReplicationHandler(ReplicationClientHandler):
+class GenericWorkerReplicationHandler(ReplicationDataHandler):
     def __init__(self, hs):
         super(GenericWorkerReplicationHandler, self).__init__(hs.get_datastore())
 
@@ -644,9 +644,6 @@ class GenericWorkerReplicationHandler(ReplicationClientHandler):
             args.update(self.send_handler.stream_positions())
         return args
 
-    def get_currently_syncing_users(self):
-        return self.presence_handler.get_currently_syncing_users()
-
     async def process_and_notify(self, stream_name, token, rows):
         try:
             if self.send_handler:
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"]
-)
diff --git a/synapse/server.py b/synapse/server.py
index 9228e1c892..9d273c980c 100644
--- a/synapse/server.py
+++ b/synapse/server.py
@@ -87,6 +87,8 @@ from synapse.http.matrixfederationclient import MatrixFederationHttpClient
 from synapse.notifier import Notifier
 from synapse.push.action_generator import ActionGenerator
 from synapse.push.pusherpool import PusherPool
+from synapse.replication.tcp.client import ReplicationDataHandler
+from synapse.replication.tcp.handler import ReplicationCommandHandler
 from synapse.replication.tcp.resource import ReplicationStreamer
 from synapse.rest.media.v1.media_repository import (
     MediaRepository,
@@ -206,6 +208,7 @@ class HomeServer(object):
         "password_policy_handler",
         "storage",
         "replication_streamer",
+        "replication_data_handler",
     ]
 
     REQUIRED_ON_MASTER_STARTUP = ["user_directory_handler", "stats_handler"]
@@ -468,7 +471,7 @@ class HomeServer(object):
         return ReadMarkerHandler(self)
 
     def build_tcp_replication(self):
-        raise NotImplementedError()
+        return ReplicationCommandHandler(self)
 
     def build_action_generator(self):
         return ActionGenerator(self)
@@ -562,6 +565,9 @@ class HomeServer(object):
     def build_replication_streamer(self) -> ReplicationStreamer:
         return ReplicationStreamer(self)
 
+    def build_replication_data_handler(self):
+        return ReplicationDataHandler(self.get_datastore())
+
     def remove_pusher(self, app_id, push_key, user_id):
         return self.get_pusherpool().remove_pusher(app_id, push_key, user_id)
 
diff --git a/synapse/server.pyi b/synapse/server.pyi
index 9d1dfa71e7..9013e9bac9 100644
--- a/synapse/server.pyi
+++ b/synapse/server.pyi
@@ -19,6 +19,7 @@ import synapse.handlers.set_password
 import synapse.http.client
 import synapse.notifier
 import synapse.replication.tcp.client
+import synapse.replication.tcp.handler
 import synapse.rest.media.v1.media_repository
 import synapse.server_notices.server_notices_manager
 import synapse.server_notices.server_notices_sender
@@ -106,7 +107,11 @@ class HomeServer(object):
         pass
     def get_tcp_replication(
         self,
-    ) -> synapse.replication.tcp.client.ReplicationClientHandler:
+    ) -> synapse.replication.tcp.handler.ReplicationCommandHandler:
+        pass
+    def get_replication_data_handler(
+        self,
+    ) -> synapse.replication.tcp.client.ReplicationDataHandler:
         pass
     def get_federation_registry(
         self,
diff --git a/tests/replication/slave/storage/_base.py b/tests/replication/slave/storage/_base.py
index 2a1e7c7166..8902a5ab69 100644
--- a/tests/replication/slave/storage/_base.py
+++ b/tests/replication/slave/storage/_base.py
@@ -17,8 +17,9 @@ from mock import Mock, NonCallableMock
 
 from synapse.replication.tcp.client import (
     ReplicationClientFactory,
-    ReplicationClientHandler,
+    ReplicationDataHandler,
 )
+from synapse.replication.tcp.handler import ReplicationCommandHandler
 from synapse.replication.tcp.resource import ReplicationStreamProtocolFactory
 from synapse.storage.database import make_conn
 
@@ -51,15 +52,19 @@ class BaseSlavedStoreTestCase(unittest.HomeserverTestCase):
         self.event_id = 0
 
         server_factory = ReplicationStreamProtocolFactory(self.hs)
-        self.streamer = server_factory.streamer
+        self.streamer = hs.get_replication_streamer()
 
-        handler_factory = Mock()
-        self.replication_handler = ReplicationClientHandler(self.slaved_store)
-        self.replication_handler.factory = handler_factory
+        # We now do some gut wrenching so that we have a client that is based
+        # off of the slave store rather than the main store.
+        self.replication_handler = ReplicationCommandHandler(self.hs)
+        self.replication_handler._replication_data_handler = ReplicationDataHandler(
+            self.slaved_store
+        )
 
         client_factory = ReplicationClientFactory(
             self.hs, "client_name", self.replication_handler
         )
+        client_factory.handler = self.replication_handler
 
         server = server_factory.buildProtocol(None)
         client = client_factory.buildProtocol(None)
diff --git a/tests/replication/tcp/streams/_base.py b/tests/replication/tcp/streams/_base.py
index a755fe2879..32238fe79a 100644
--- a/tests/replication/tcp/streams/_base.py
+++ b/tests/replication/tcp/streams/_base.py
@@ -15,7 +15,7 @@
 
 from mock import Mock
 
-from synapse.replication.tcp.commands import ReplicateCommand
+from synapse.replication.tcp.handler import ReplicationCommandHandler
 from synapse.replication.tcp.protocol import ClientReplicationStreamProtocol
 from synapse.replication.tcp.resource import ReplicationStreamProtocolFactory
 
@@ -26,15 +26,20 @@ from tests.server import FakeTransport
 class BaseStreamTestCase(unittest.HomeserverTestCase):
     """Base class for tests of the replication streams"""
 
+    def make_homeserver(self, reactor, clock):
+        self.test_handler = Mock(wraps=TestReplicationDataHandler())
+        return self.setup_test_homeserver(replication_data_handler=self.test_handler)
+
     def prepare(self, reactor, clock, hs):
         # build a replication server
-        server_factory = ReplicationStreamProtocolFactory(self.hs)
-        self.streamer = server_factory.streamer
+        server_factory = ReplicationStreamProtocolFactory(hs)
+        self.streamer = hs.get_replication_streamer()
         self.server = server_factory.buildProtocol(None)
 
-        self.test_handler = Mock(wraps=TestReplicationClientHandler())
+        repl_handler = ReplicationCommandHandler(hs)
+        repl_handler.handler = self.test_handler
         self.client = ClientReplicationStreamProtocol(
-            hs, "client", "test", clock, self.test_handler,
+            hs, "client", "test", clock, repl_handler,
         )
 
         self._client_transport = None
@@ -69,13 +74,9 @@ class BaseStreamTestCase(unittest.HomeserverTestCase):
         self.streamer.on_notifier_poke()
         self.pump(0.1)
 
-    def replicate_stream(self):
-        """Make the client end a REPLICATE command to set up a subscription to a stream"""
-        self.client.send_command(ReplicateCommand())
-
 
-class TestReplicationClientHandler(object):
-    """Drop-in for ReplicationClientHandler which just collects RDATA rows"""
+class TestReplicationDataHandler:
+    """Drop-in for ReplicationDataHandler which just collects RDATA rows"""
 
     def __init__(self):
         self.streams = set()
@@ -88,18 +89,9 @@ class TestReplicationClientHandler(object):
                 positions[stream] = max(token, positions.get(stream, 0))
         return positions
 
-    def get_currently_syncing_users(self):
-        return []
-
-    def update_connection(self, connection):
-        pass
-
-    def finished_connecting(self):
-        pass
-
-    async def on_position(self, stream_name, token):
-        """Called when we get new position data."""
-
     async def on_rdata(self, stream_name, token, rows):
         for r in rows:
             self._received_rdata_rows.append((stream_name, token, r))
+
+    async def on_position(self, stream_name, token):
+        pass
diff --git a/tests/replication/tcp/streams/test_receipts.py b/tests/replication/tcp/streams/test_receipts.py
index 0ec0825a0e..a0206f7363 100644
--- a/tests/replication/tcp/streams/test_receipts.py
+++ b/tests/replication/tcp/streams/test_receipts.py
@@ -24,7 +24,6 @@ class ReceiptsStreamTestCase(BaseStreamTestCase):
         self.reconnect()
 
         # make the client subscribe to the receipts stream
-        self.replicate_stream()
         self.test_handler.streams.add("receipts")
 
         # tell the master to send a new receipt