diff --git a/changelog.d/7185.misc b/changelog.d/7185.misc
new file mode 100644
index 000000000000..deb9ca702122
--- /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 1c7c6ec0c876..a37818fe9ab2 100644
--- a/synapse/app/admin_cmd.py
+++ b/synapse/app/admin_cmd.py
@@ -43,7 +43,6 @@
 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 @@ def _listen_http(self, listener_config):
     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 1ee266f7c5e1..588db40e86e4 100644
--- a/synapse/app/generic_worker.py
+++ b/synapse/app/generic_worker.py
@@ -64,7 +64,7 @@
 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 @@ def start_listening(self, listeners):
     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 @@ def build_typing_handler(self):
         return GenericWorkerTyping(self)
 
 
-class GenericWorkerReplicationHandler(ReplicationClientHandler):
+class GenericWorkerReplicationHandler(ReplicationDataHandler):
     def __init__(self, hs):
         super(GenericWorkerReplicationHandler, self).__init__(hs.get_datastore())
 
@@ -644,9 +644,6 @@ def get_streams_to_replicate(self):
             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 81c2ea7ee9f6..523a1358d4e3 100644
--- a/synapse/replication/tcp/__init__.py
+++ b/synapse/replication/tcp/__init__.py
@@ -20,11 +20,31 @@
 
 
 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 e86d9805f11c..700ae7915852 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 @@ def startedConnecting(self, connector):
     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 @@ def clientConnectionFailed(self, connector, reason):
         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 @@ async def on_rdata(self, stream_name, token, rows):
             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 @@ def get_streams_to_replicate(self) -> Dict[str, int]:
             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 000000000000..12a1cfd6d1c4
--- /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 dae246825fb0..f2a37f568e30 100644
--- a/synapse/replication/tcp/protocol.py
+++ b/synapse/replication/tcp/protocol.py
@@ -46,12 +46,11 @@
     > 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 @@
     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 @@ def on_connection_closed(self):
         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 @@ def __init__(
         client_name: str,
         server_name: str,
         clock: Clock,
-        handler: AbstractReplicationClientHandler,
+        command_handler: "ReplicationCommandHandler",
     ):
         BaseReplicationStreamProtocol.__init__(self, clock)
 
@@ -558,20 +491,7 @@ def __init__(
 
         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 @@ def connectionMade(self):
 
         # 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 @@ def transport_kernel_read_buffer_size(protocol, read=True):
         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 cd86475d6bb0..d54e4cedf8a4 100644
--- a/synapse/server.py
+++ b/synapse/server.py
@@ -87,6 +87,8 @@
 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 @@ def build_DEPENDENCY(self)
         "password_policy_handler",
         "storage",
         "replication_streamer",
+        "replication_data_handler",
     ]
 
     REQUIRED_ON_MASTER_STARTUP = ["user_directory_handler", "stats_handler"]
@@ -468,7 +471,7 @@ def build_read_marker_handler(self):
         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 @@ def build_storage(self) -> Storage:
     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 9d1dfa71e792..9013e9bac9dd 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 2a1e7c7166de..8902a5ab6946 100644
--- a/tests/replication/slave/storage/_base.py
+++ b/tests/replication/slave/storage/_base.py
@@ -17,8 +17,9 @@
 
 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 @@ def prepare(self, reactor, clock, hs):
         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 a755fe28794f..32238fe79a25 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 @@
 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 @@ def replicate(self):
         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 @@ def get_streams_to_replicate(self):
                 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 0ec0825a0e62..a0206f7363e6 100644
--- a/tests/replication/tcp/streams/test_receipts.py
+++ b/tests/replication/tcp/streams/test_receipts.py
@@ -24,7 +24,6 @@ def test_receipt(self):
         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