Initial TCP protocol implementation

This defines the low level TCP replication protocol

docs/tcp_replication.rst

@@ -0,0 +1,174 @@
+TCP Replication
+===============
+
+This describes the TCP replication protocol that replaces the HTTP protocol.
+
+Motivation
+----------
+
+The HTTP API used long poll from the workers to the master, this has the problem
+of causing a lot of duplicate work on the server. This TCP protocol aims to
+solve.
+
+Overview
+--------
+
+The protocol is based on fire and forget, line based commands. An example flow
+would be (where '>' indicates master->worker and '<' worker->master flows)::
+
+    > SERVER example.com
+    < REPLICATE events 53
+    > RDATA events 54 ["$foo1:bar.com", ...]
+    > RDATA events 55 ["$foo4:bar.com", ...]
+
+The example shows the server accepting a new connection and sending its identity
+with the ``SERVER`` command, followed by the client asking to subscribe to the
+``events`` stream from the token ``53``. The server then periodically sends ``RDATA``
+commands which have the format ``RDATA <stream_name> <token> <row>```, where the
+format of ``<row>`` is defined by the individual streams.
+
+Error reporting happens by either the client or server sending an `ERROR`
+command, and usually the connection will be closed.
+
+
+Since the protocol is a simple line based, its possible to manually connect to
+the server using a tool like netcat. A few things should be noted when manually
+using the protocol:
+ * When subscribing to a stream using ``REPLICATE``, the special token ``NOW`` can
+   be used to get all future updates. The special stream name ``ALL`` can be used
+   with ``NOW`` to subscribe to all available streams.
+ * The federation stream is only available if federation sending has been
+   disabled on the main process.
+ * The server will only time connections out that have sent a ``PING`` command.
+   If a ping is sent then the connection will be closed if no further commands
+   are receieved within 15s. Both the client and server protocol implementations
+   will send an initial PING on connection and ensure at least one command every
+   5s is sent (not necessarily ``PING``).
+ * ``RDATA`` commands *usually* include a numeric token, however if the stream
+   has multiple rows to replicate per token the server will send multiple
+   ``RDATA`` commands, with all but the last having a token of ``batch``. See
+   the documentation on ``commands.RdataCommand`` for further details.
+
+
+Architecture
+------------
+
+The basic structure of the protocol is line based, where the initial word of
+each line specifies the command. The rest of the line is parsed based on the
+command. For example, the `RDATA` command is defined as::
+
+    RDATA <stream_name> <token> <row_json>
+
+(Note that `<row_json>` may contains spaces, but cannot contain newlines.)
+
+Blank lines are ignored.
+
+
+Keep alives
+~~~~~~~~~~~
+
+Both sides are expected to send at least one command every 5s or so, and
+should send a ``PING`` command if necessary. If either side do not receive a
+command within e.g. 15s then the connection should be closed.
+
+Because the server may be connected to manually using e.g. netcat, the timeouts
+aren't enabled until an initial ``PING`` command is seen. Both the client and
+server implementations below send a ``PING`` command immediately on connection to
+ensure the timeouts are enabled.
+
+This ensures that both sides can quickly realize if the tcp connection has gone
+and handle the situation appropriately.
+
+
+Start up
+~~~~~~~~
+
+When a new connection is made, the server:
+ * Sends a ``SERVER`` command, which includes the identity of the server, allowing
+   the client to detect if its connected to the expected server
+ * Sends a ``PING`` command as above, to enable the client to time out connections
+   promptly.
+
+The client:
+ * Sends a ``NAME`` command, allowing the server to associate a human friendly
+   name with the connection. This is optional.
+ * Sends a ``PING`` as above
+ * For each stream the client wishes to subscribe to it sends a ``REPLICATE``
+   with the stream_name and token it wants to subscribe from.
+ * On receipt of a ``SERVER`` command, checks that the server name matches the
+   expected server name.
+
+
+Error handling
+~~~~~~~~~~~~~~
+
+If either side detects an error it can send an ``ERROR`` command and close the
+connection.
+
+If the client side loses the connection to the server it should reconnect,
+following the steps above.
+
+
+Congestion
+~~~~~~~~~~
+
+If the server sends messages faster than the client can consume them the server
+will first buffer a (fairly large) number of commands and then disconnect the
+client. This ensures that we don't queue up an unbounded number of commands in
+memory and gives us a potential oppurtunity to squawk loudly. When/if the client
+recovers it can reconnect to the server and ask for missed messages.
+
+
+Reliability
+~~~~~~~~~~~
+
+In general the replication stream should be consisdered an unreliable transport
+since e.g. commands are not resent if the connection disappears.
+
+The exception to that are the replication streams, i.e. RDATA commands, since
+these include tokens which can be used to restart the stream on connection
+errors.
+
+The client should keep track of the token in the last RDATA command received
+for each stream so that on reconneciton it can start streaming from the correct
+place. Note: not all RDATA have valid tokens due to batching. See
+``RdataCommand`` for more details.
+
+
+Example
+~~~~~~~
+
+An example iteraction is shown below. Each line is prefixed with '>' or '<' to
+indicate which side is sending, these are *not* included on the wire::
+
+    * connection established *
+    > SERVER localhost:8823
+    > PING 1490197665618
+    < NAME synapse.app.appservice
+    < PING 1490197665618
+    < REPLICATE events 1
+    < REPLICATE backfill 1
+    < REPLICATE caches 1
+    > POSITION events 1
+    > POSITION backfill 1
+    > POSITION caches 1
+    > RDATA caches 2 ["get_user_by_id",["@01register-user:localhost:8823"],1490197670513]
+    > RDATA events 14 ["$149019767112vOHxz:localhost:8823",
+        "!AFDCvgApUmpdfVjIXm:localhost:8823","m.room.guest_access","",null]
+    < PING 1490197675618
+    > ERROR server stopping
+    * connection closed by server *
+
+The ``POSITION`` command sent by the server is used to set the clients position
+without needing to send data with the ``RDATA`` command.
+
+
+An example of a batched set of ``RDATA`` is::
+
+    > RDATA caches batch ["get_user_by_id",["@test:localhost:8823"],1490197670513]
+    > RDATA caches batch ["get_user_by_id",["@test2:localhost:8823"],1490197670513]
+    > RDATA caches batch ["get_user_by_id",["@test3:localhost:8823"],1490197670513]
+    > RDATA caches 54 ["get_user_by_id",["@test4:localhost:8823"],1490197670513]
+
+In this case the client shouldn't advance their caches token until it sees the
+the last ``RDATA``.

synapse/replication/tcp/__init__.py

@@ -12,3 +12,35 @@
 # 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.
+
+"""This module implements the TCP replication protocol used by synapse to
+communicate between the master process and its workers (when they're enabled).
+
+The protocol is based on fire and forget, line based commands. An example flow
+would be (where '>' indicates master->worker and '<' worker->master flows)::
+
+    > SERVER example.com
+    < REPLICATE events 53
+    > RDATA events 54 ["$foo1:bar.com", ...]
+    > RDATA events 55 ["$foo4:bar.com", ...]
+
+The example shows the server accepting a new connection and sending its identity
+with the `SERVER` command, followed by the client asking to subscribe to the
+`events` stream from the token `53`. The server then periodically sends `RDATA`
+commands which have the format `RDATA <stream_name> <token> <row>`, where the
+format of `<row>` is defined by the individual streams.
+
+Error reporting happens by either the client or server sending an `ERROR`
+command, and usually the connection will be closed.
+
+
+Structure of the module:
+ * client.py   - the client classes used for workers to connect to master
+ * 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
+
+Further details can be found in docs/tcp_replication.rst
+"""

synapse/replication/tcp/commands.py

@@ -0,0 +1,341 @@
+# -*- coding: utf-8 -*-
+# Copyright 2017 Vector Creations Ltd
+#
+# 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.
+"""Defines the various valid commands
+
+The VALID_SERVER_COMMANDS and VALID_CLIENT_COMMANDS define which commands are
+allowed to be sent by which side.
+"""
+
+import logging
+import ujson as json
+
+
+logger = logging.getLogger(__name__)
+
+
+class Command(object):
+    """The base command class.
+
+    All subclasses must set the NAME variable which equates to the name of the
+    command on the wire.
+
+    A full command line on the wire is constructed from `NAME + " " + to_line()`
+
+    The default implementation creates a command of form `<NAME> <data>`
+    """
+    NAME = None
+
+    def __init__(self, data):
+        self.data = data
+
+    @classmethod
+    def from_line(cls, line):
+        """Deserialises a line from the wire into this command. `line` does not
+        include the command.
+        """
+        return cls(line)
+
+    def to_line(self):
+        """Serialises the comamnd for the wire. Does not include the command
+        prefix.
+        """
+        return self.data
+
+
+class ServerCommand(Command):
+    """Sent by the server on new connection and includes the server_name.
+
+    Format::
+
+        SERVER <server_name>
+    """
+    NAME = "SERVER"
+
+
+class RdataCommand(Command):
+    """Sent by server when a subscribed stream has an update.
+
+    Format::
+
+        RDATA <stream_name> <token> <row_json>
+
+    The `<token>` may either be a numeric stream id OR "batch". The latter case
+    is used to support sending multiple updates with the same stream ID. This
+    is done by sending an RDATA for each row, with all but the last RDATA having
+    a token of "batch" and the last having the final stream ID.
+
+    The client should batch all incoming RDATA with a token of "batch" (per
+    stream_name) until it sees an RDATA with a numeric stream ID.
+
+    `<token>` of "batch" maps to the instance variable `token` being None.
+
+    An example of a batched series of RDATA::
+
+        RDATA presence batch ["@foo:example.com", "online", ...]
+        RDATA presence batch ["@bar:example.com", "online", ...]
+        RDATA presence 59 ["@baz:example.com", "online", ...]
+    """
+    NAME = "RDATA"
+
+    def __init__(self, stream_name, token, row):
+        self.stream_name = stream_name
+        self.token = token
+        self.row = row
+
+    @classmethod
+    def from_line(cls, line):
+        stream_name, token, row_json = line.split(" ", 2)
+        return cls(
+            stream_name,
+            None if token == "batch" else int(token),
+            json.loads(row_json)
+        )
+
+    def to_line(self):
+        return " ".join((
+            self.stream_name,
+            str(self.token) if self.token is not None else "batch",
+            json.dumps(self.row),
+        ))
+
+
+class PositionCommand(Command):
+    """Sent by the client to tell the client the stream postition without
+    needing to send an RDATA.
+    """
+    NAME = "POSITION"
+
+    def __init__(self, stream_name, token):
+        self.stream_name = stream_name
+        self.token = token
+
+    @classmethod
+    def from_line(cls, line):
+        stream_name, token = line.split(" ", 1)
+        return cls(stream_name, int(token))
+
+    def to_line(self):
+        return " ".join((self.stream_name, str(self.token),))
+
+
+class ErrorCommand(Command):
+    """Sent by either side if there was an ERROR. The data is a string describing
+    the error.
+    """
+    NAME = "ERROR"
+
+
+class PingCommand(Command):
+    """Sent by either side as a keep alive. The data is arbitary (often timestamp)
+    """
+    NAME = "PING"
+
+
+class NameCommand(Command):
+    """Sent by client to inform the server of the client's identity. The data
+    is the name
+    """
+    NAME = "NAME"
+
+
+class ReplicateCommand(Command):
+    """Sent by the client to subscribe to the stream.
+
+    Format::
+
+        REPLICATE <stream_name> <token>
+
+    Where <token> may be either:
+        * a numeric stream_id to stream updates from
+        * "NOW" to stream all subsequent updates.
+
+    The <stream_name> can be "ALL" to subscribe to all known streams, in which
+    case the <token> must be set to "NOW", i.e.::
+
+        REPLICATE ALL NOW
+    """
+    NAME = "REPLICATE"
+
+    def __init__(self, stream_name, token):
+        self.stream_name = stream_name
+        self.token = token
+
+    @classmethod
+    def from_line(cls, line):
+        stream_name, token = line.split(" ", 1)
+        if token in ("NOW", "now"):
+            token = "NOW"
+        else:
+            token = int(token)
+        return cls(stream_name, token)
+
+    def to_line(self):
+        return " ".join((self.stream_name, str(self.token),))
+
+
+class UserSyncCommand(Command):
+    """Sent by the client to inform the server that a user has started or
+    stopped syncing. Used to calculate presence on the master.
+
+    Format::
+
+        USER_SYNC <user_id> <state>
+
+    Where <state> is either "start" or "stop"
+    """
+    NAME = "USER_SYNC"
+
+    def __init__(self, user_id, is_syncing):
+        self.user_id = user_id
+        self.is_syncing = is_syncing
+
+    @classmethod
+    def from_line(cls, line):
+        user_id, state = line.split(" ", 1)
+
+        if state not in ("start", "end"):
+            raise Exception("Invalid USER_SYNC state %r" % (state,))
+
+        return cls(user_id, state == "start")
+
+    def to_line(self):
+        return " ".join((self.user_id, "start" if self.is_syncing else "end"))
+
+
+class FederationAckCommand(Command):
+    """Sent by the client when it has processed up to a given point in the
+    federation stream. This allows the master to drop in-memory caches of the
+    federation stream.
+
+    This must only be sent from one worker (i.e. the one sending federation)
+
+    Format::
+
+        FEDERATION_ACK <token>
+    """
+    NAME = "FEDERATION_ACK"
+
+    def __init__(self, token):
+        self.token = token
+
+    @classmethod
+    def from_line(cls, line):
+        return cls(int(line))
+
+    def to_line(self):
+        return str(self.token)
+
+
+class SyncCommand(Command):
+    """Used for testing. The client protocol implementation allows waiting
+    on a SYNC command with a specified data.
+    """
+    NAME = "SYNC"
+
+
+class RemovePusherCommand(Command):
+    """Sent by the client to request the master remove the given pusher.
+
+    Format::
+
+        REMOVE_PUSHER <app_id> <push_key> <user_id>
+    """
+    NAME = "REMOVE_PUSHER"
+
+    def __init__(self, app_id, push_key, user_id):
+        self.user_id = user_id
+        self.app_id = app_id
+        self.push_key = push_key
+
+    @classmethod
+    def from_line(cls, line):
+        app_id, push_key, user_id = line.split(" ", 2)
+
+        return cls(app_id, push_key, user_id)
+
+    def to_line(self):
+        return " ".join((self.app_id, self.push_key, self.user_id))
+
+
+class InvalidateCacheCommand(Command):
+    """Sent by the client to invalidate an upstream cache.
+
+    THIS IS NOT RELIABLE, AND SHOULD *NOT* BE USED ACCEPT FOR THINGS THAT ARE
+    NOT DISASTROUS IF WE DROP ON THE FLOOR.
+
+    Mainly used to invalidate destination retry timing caches.
+
+    Format::
+
+        INVALIDATE_CACHE <cache_func> <keys_json>
+
+    Where <keys_json> is a json list.
+    """
+    NAME = "INVALIDATE_CACHE"
+
+    def __init__(self, cache_func, keys):
+        self.cache_func = cache_func
+        self.keys = keys
+
+    @classmethod
+    def from_line(cls, line):
+        cache_func, keys_json = line.split(" ", 1)
+
+        return cls(cache_func, json.loads(keys_json))
+
+    def to_line(self):
+        return " ".join((self.cache_func, json.dumps(self.keys)))
+
+
+# Map of command name to command type.
+COMMAND_MAP = {
+    cmd.NAME: cmd
+    for cmd in (
+        ServerCommand,
+        RdataCommand,
+        PositionCommand,
+        ErrorCommand,
+        PingCommand,
+        NameCommand,
+        ReplicateCommand,
+        UserSyncCommand,
+        FederationAckCommand,
+        SyncCommand,
+        RemovePusherCommand,
+        InvalidateCacheCommand,
+    )
+}
+
+# The commands the server is allowed to send
+VALID_SERVER_COMMANDS = (
+    ServerCommand.NAME,
+    RdataCommand.NAME,
+    PositionCommand.NAME,
+    ErrorCommand.NAME,
+    PingCommand.NAME,
+    SyncCommand.NAME,
+)
+
+# The commands the client is allowed to send
+VALID_CLIENT_COMMANDS = (
+    NameCommand.NAME,
+    ReplicateCommand.NAME,
+    PingCommand.NAME,
+    UserSyncCommand.NAME,
+    FederationAckCommand.NAME,
+    RemovePusherCommand.NAME,
+    InvalidateCacheCommand.NAME,
+    ErrorCommand.NAME,
+)

synapse/replication/tcp/protocol.py

@@ -0,0 +1,601 @@
+# -*- coding: utf-8 -*-
+# Copyright 2017 Vector Creations Ltd
+#
+# 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.
+"""This module contains the implementation of both the client and server
+protocols.
+
+The basic structure of the protocol is line based, where the initial word of
+each line specifies the command. The rest of the line is parsed based on the
+command. For example, the `RDATA` command is defined as::
+
+    RDATA <stream_name> <token> <row_json>
+
+(Note that `<row_json>` may contains spaces, but cannot contain newlines.)
+
+Blank lines are ignored.
+
+# Example
+
+An example iteraction is shown below. Each line is prefixed with '>' or '<' to
+indicate which side is sending, these are *not* included on the wire::
+
+    * connection established *
+    > SERVER localhost:8823
+    > PING 1490197665618
+    < NAME synapse.app.appservice
+    < PING 1490197665618
+    < REPLICATE events 1
+    < REPLICATE backfill 1
+    < REPLICATE caches 1
+    > POSITION events 1
+    > POSITION backfill 1
+    > POSITION caches 1
+    > RDATA caches 2 ["get_user_by_id",["@01register-user:localhost:8823"],1490197670513]
+    > RDATA events 14 ["$149019767112vOHxz:localhost:8823",
+        "!AFDCvgApUmpdfVjIXm:localhost:8823","m.room.guest_access","",null]
+    < PING 1490197675618
+    > ERROR server stopping
+    * connection closed by server *
+"""
+
+from twisted.internet import defer
+from twisted.protocols.basic import LineOnlyReceiver
+
+from commands import (
+    COMMAND_MAP, VALID_CLIENT_COMMANDS, VALID_SERVER_COMMANDS,
+    ErrorCommand, ServerCommand, RdataCommand, PositionCommand, PingCommand,
+    NameCommand, ReplicateCommand, UserSyncCommand, SyncCommand,
+)
+from streams import STREAMS_MAP
+
+from synapse.util.stringutils import random_string
+
+import logging
+import synapse.metrics
+import struct
+import fcntl
+
+
+metrics = synapse.metrics.get_metrics_for(__name__)
+
+inbound_commands_counter = metrics.register_counter(
+    "inbound_commands", labels=["command", "name", "conn_id"],
+)
+outbound_commands_counter = metrics.register_counter(
+    "outbound_commands", labels=["command", "name", "conn_id"],
+)
+
+
+# A list of all connected protocols. This allows us to send metrics about the
+# connections.
+connected_connections = []
+
+
+logger = logging.getLogger(__name__)
+
+
+PING_TIME = 5000
+
+
+class ConnectionStates(object):
+    CONNECTING = "connecting"
+    ESTABLISHED = "established"
+    PAUSED = "paused"
+    CLOSED = "closed"
+
+
+class BaseReplicationStreamProtocol(LineOnlyReceiver):
+    """Base replication protocol shared between client and server.
+
+    Reads lines (ignoring blank ones) and parses them into command classes,
+    asserting that they are valid for the given direction, i.e. server commands
+    are only sent by the server.
+
+    On receiving a new command it calls `on_<COMMAND_NAME>` with the parsed
+    command.
+
+    It also sends `PING` periodically, and correctly times out remote connections
+    (if they send a `PING` command)
+    """
+    delimiter = b'\n'
+
+    VALID_INBOUND_COMMANDS = []  # Valid commands we expect to receive
+    VALID_OUTBOUND_COMMANDS = []  # Valid commans we can send
+
+    max_line_buffer = 10000
+
+    def __init__(self, clock):
+        self.clock = clock
+
+        self.last_received_command = self.clock.time_msec()
+        self.last_sent_command = 0
+        self.time_we_closed = None  # When we requested the connection be closed
+
+        self.received_ping = False  # Have we reecived a ping from the other side
+
+        self.state = ConnectionStates.CONNECTING
+
+        self.name = "anon"  # The name sent by a client.
+        self.conn_id = random_string(5)  # To dedupe in case of name clashes.
+
+        # List of pending commands to send once we've established the connection
+        self.pending_commands = []
+
+        # The LoopingCall for sending pings.
+        self._send_ping_loop = None
+
+    def connectionMade(self):
+        logger.info("[%s] Connection established", self.id())
+
+        self.state = ConnectionStates.ESTABLISHED
+
+        connected_connections.append(self)  # Register connection for metrics
+
+        self.transport.registerProducer(self, True)  # For the *Producing callbacks
+
+        self._send_pending_commands()
+
+        # Starts sending pings
+        self._send_ping_loop = self.clock.looping_call(self.send_ping, 5000)
+
+        # Always send the initial PING so that the other side knows that they
+        # can time us out.
+        self.send_command(PingCommand(self.clock.time_msec()))
+
+    def send_ping(self):
+        """Periodically sends a ping and checks if we should close the connection
+        due to the other side timing out.
+        """
+        now = self.clock.time_msec()
+
+        if self.time_we_closed:
+            if now - self.time_we_closed > PING_TIME * 3:
+                logger.info(
+                    "[%s] Failed to close connection gracefully, aborting", self.id()
+                )
+                self.transport.abortConnection()
+        else:
+            if now - self.last_sent_command >= PING_TIME:
+                self.send_command(PingCommand(now))
+
+            if self.received_ping and now - self.last_received_command > PING_TIME * 3:
+                logger.info(
+                    "[%s] Connection hasn't received command in %r ms. Closing.",
+                    self.id(), now - self.last_received_command
+                )
+                self.send_error("ping timeout")
+
+    def lineReceived(self, line):
+        """Called when we've received a line
+        """
+        if line.strip() == "":
+            # Ignore blank lines
+            return
+
+        line = line.decode("utf-8")
+        cmd_name, rest_of_line = line.split(" ", 1)
+
+        if cmd_name not in self.VALID_INBOUND_COMMANDS:
+            logger.error("[%s] invalid command %s", self.id(), cmd_name)
+            self.send_error("invalid command: %s", cmd_name)
+            return
+
+        self.last_received_command = self.clock.time_msec()
+
+        inbound_commands_counter.inc(cmd_name, self.name, self.conn_id)
+
+        cmd_cls = COMMAND_MAP[cmd_name]
+        try:
+            cmd = cmd_cls.from_line(rest_of_line)
+        except Exception as e:
+            logger.exception(
+                "[%s] failed to parse line %r: %r", self.id(), cmd_name, rest_of_line
+            )
+            self.send_error(
+                "failed to parse line for  %r: %r (%r):" % (cmd_name, e, rest_of_line)
+            )
+            return
+
+        # Now lets try and call on_<CMD_NAME> function
+        try:
+            getattr(self, "on_%s" % (cmd_name,))(cmd)
+        except Exception:
+            logger.exception("[%s] Failed to handle line: %r", self.id(), line)
+
+    def close(self):
+        self.time_we_closed = self.clock.time_msec()
+        self.transport.loseConnection()
+        self.on_connection_closed()
+
+    def send_error(self, error_string, *args):
+        """Send an error to remote and close the connection.
+        """
+        self.send_command(ErrorCommand(error_string % args))
+        self.close()
+
+    def send_command(self, cmd, do_buffer=True):
+        """Send a command if connection has been established.
+
+        Args:
+            cmd (Command)
+            do_buffer (bool): Whether to buffer the message or always attempt
+                to send the command. This is mostly used to send an error
+                message if we're about to close the connection due our buffers
+                becoming full.
+        """
+        if self.state == ConnectionStates.CLOSED:
+            logger.info("[%s] Not sending, connection closed", self.id())
+            return
+
+        if do_buffer and self.state != ConnectionStates.ESTABLISHED:
+            self._queue_command(cmd)
+            return
+
+        outbound_commands_counter.inc(cmd.NAME, self.name, self.conn_id)
+
+        string = "%s %s" % (cmd.NAME, cmd.to_line(),)
+        if "\n" in string:
+            raise Exception("Unexpected newline in command: %r", string)
+
+        self.sendLine(string.encode("utf-8"))
+
+        self.last_sent_command = self.clock.time_msec()
+
+    def _queue_command(self, cmd):
+        """Queue the command until the connection is ready to write to again.
+        """
+        logger.info("[%s] Queing as conn %r, cmd: %r", self.id(), self.state, cmd)
+        self.pending_commands.append(cmd)
+
+        if len(self.pending_commands) > self.max_line_buffer:
+            # The other side is failing to keep up and out buffers are becoming
+            # full, so lets close the connection.
+            # XXX: should we squawk more loudly?
+            logger.error("[%s] Remote failed to keep up", self.id())
+            self.send_command(ErrorCommand("Failed to keep up"), do_buffer=False)
+            self.close()
+
+    def _send_pending_commands(self):
+        """Send any queued commandes
+        """
+        pending = self.pending_commands
+        self.pending_commands = []
+        for cmd in pending:
+            self.send_command(cmd)
+
+    def on_PING(self, line):
+        self.received_ping = True
+
+    def on_ERROR(self, cmd):
+        logger.error("[%s] Remote reported error: %r", self.id(), cmd.data)
+
+    def pauseProducing(self):
+        """This is called when both the kernel send buffer and the twisted
+        tcp connection send buffers have become full.
+
+        We don't actually have any control over those sizes, so we buffer some
+        commands ourselves before knifing the connection due to the remote
+        failing to keep up.
+        """
+        logger.info("[%s] Pause producing", self.id())
+        self.state = ConnectionStates.PAUSED
+
+    def resumeProducing(self):
+        """The remote has caught up after we started buffering!
+        """
+        logger.info("[%s] Resume producing", self.id())
+        self.state = ConnectionStates.ESTABLISHED
+        self._send_pending_commands()
+
+    def stopProducing(self):
+        """We're never going to send any more data (normally because either
+        we or the remote has closed the connection)
+        """
+        logger.info("[%s] Stop producing", self.id())
+        self.on_connection_closed()
+
+    def connectionLost(self, reason):
+        logger.info("[%s] Replication connection closed: %r", self.id(), reason)
+
+        try:
+            # Remove us from list of connections to be monitored
+            connected_connections.remove(self)
+        except ValueError:
+            pass
+
+        # Stop the looping call sending pings.
+        if self._send_ping_loop and self._send_ping_loop.running:
+            self._send_ping_loop.stop()
+
+        self.on_connection_closed()
+
+    def on_connection_closed(self):
+        logger.info("[%s] Connection was closed", self.id())
+
+        self.state = ConnectionStates.CLOSED
+        self.pending_commands = []
+
+        if self.transport:
+            self.transport.unregisterProducer()
+
+    def __str__(self):
+        return "ReplicationConnection<name=%s,conn_id=%s,addr=%s>" % (
+            self.name, self.conn_id, self.addr,
+        )
+
+    def id(self):
+        return "%s-%s" % (self.name, self.conn_id)
+
+
+class ServerReplicationStreamProtocol(BaseReplicationStreamProtocol):
+    VALID_INBOUND_COMMANDS = VALID_CLIENT_COMMANDS
+    VALID_OUTBOUND_COMMANDS = VALID_SERVER_COMMANDS
+
+    def __init__(self, server_name, clock, streamer, addr):
+        BaseReplicationStreamProtocol.__init__(self, clock)  # Old style class
+
+        self.server_name = server_name
+        self.streamer = streamer
+        self.addr = addr
+
+        # The streams the client has subscribed to and is up to date with
+        self.replication_streams = set()
+
+        # The streams the client is currently subscribing to.
+        self.connecting_streams = set()
+
+        # Map from stream name to list of updates to send once we've finished
+        # subscribing the client to the stream.
+        self.pending_rdata = {}
+
+    def connectionMade(self):
+        self.send_command(ServerCommand(self.server_name))
+        BaseReplicationStreamProtocol.connectionMade(self)
+        self.streamer.new_connection(self)
+
+    def on_NAME(self, cmd):
+        self.name = cmd.data
+
+    def on_USER_SYNC(self, cmd):
+        self.streamer.on_user_sync(self.conn_id, cmd.user_id, cmd.is_syncing)
+
+    def on_REPLICATE(self, cmd):
+        stream_name = cmd.stream_name
+        token = cmd.token
+
+        if stream_name == "ALL":
+            # Subscribe to all streams we're publishing to.
+            for stream in self.streamer.streams_by_name.iterkeys():
+                self.subscribe_to_stream(stream, token)
+        else:
+            self.subscribe_to_stream(stream_name, token)
+
+    def on_FEDERATION_ACK(self, cmd):
+        self.streamer.federation_ack(cmd.token)
+
+    def on_REMOVE_PUSHER(self, cmd):
+        self.streamer.on_remove_pusher(cmd.app_id, cmd.push_key, cmd.user_id)
+
+    def onINVALIDATE_CACHE(self, cmd):
+        self.streamer.on_invalidate_cache(cmd.cache_func, cmd.keys)
+
+    @defer.inlineCallbacks
+    def subscribe_to_stream(self, stream_name, token):
+        """Subscribe the remote to a streams.
+
+        This invloves checking if they've missed anything and sending those
+        updates down if they have. During that time new updates for the stream
+        are queued and sent once we've sent down any missed updates.
+        """
+        self.replication_streams.discard(stream_name)
+        self.connecting_streams.add(stream_name)
+
+        try:
+            # Get missing updates
+            updates, current_token = yield self.streamer.get_stream_updates(
+                stream_name, token,
+            )
+
+            # Send all the missing updates
+            for update in updates:
+                token, row = update[0], update[1]
+                self.send_command(RdataCommand(stream_name, token, row))
+
+            # Now we can send any updates that came in while we were subscribing
+            pending_rdata = self.pending_rdata.pop(stream_name, [])
+            for token, update in pending_rdata:
+                self.send_command(RdataCommand(stream_name, token, update))
+
+            # We send a POSITION command to ensure that they have an up to
+            # date token (especially useful if we didn't send any updates
+            # above)
+            self.send_command(PositionCommand(stream_name, current_token))
+
+            # They're now fully subscribed
+            self.replication_streams.add(stream_name)
+        except Exception as e:
+            logger.exception("[%s] Failed to handle REPLICATE command", self.id())
+            self.send_error("failed to handle replicate: %r", e)
+        finally:
+            self.connecting_streams.discard(stream_name)
+
+    def stream_update(self, stream_name, token, data):
+        """Called when a new update is available to stream to clients.
+
+        We need to check if the client is interested in the stream or not
+        """
+        if stream_name in self.replication_streams:
+            # The client is subscribed to the stream
+            self.send_command(RdataCommand(stream_name, token, data))
+        elif stream_name in self.connecting_streams:
+            # The client is being subscribed to the stream
+            logger.info("[%s] Queuing RDATA %r %r", self.id(), stream_name, token)
+            self.pending_rdata.setdefault(stream_name, []).append((token, data))
+        else:
+            # The client isn't subscribed
+            logger.debug("[%s] Dropping RDATA %r %r", self.id(), stream_name, token)
+
+    def send_sync(self, data):
+        self.send_command(SyncCommand(data))
+
+    def on_connection_closed(self):
+        BaseReplicationStreamProtocol.on_connection_closed(self)
+        logger.info("[%s] Replication connection closed", self.id())
+        self.streamer.lost_connection(self)
+
+
+class ClientReplicationStreamProtocol(BaseReplicationStreamProtocol):
+    VALID_INBOUND_COMMANDS = VALID_SERVER_COMMANDS
+    VALID_OUTBOUND_COMMANDS = VALID_CLIENT_COMMANDS
+
+    def __init__(self, client_name, server_name, clock, handler):
+        BaseReplicationStreamProtocol.__init__(self, clock)
+
+        self.client_name = client_name
+        self.server_name = server_name
+        self.handler = handler
+
+        # Map of stream to batched updates. See RdataCommand for info on how
+        # batching works.
+        self.pending_batches = {}
+
+    def connectionMade(self):
+        self.send_command(NameCommand(self.client_name))
+        BaseReplicationStreamProtocol.connectionMade(self)
+
+        # Once we've connected subscribe to the necessary streams
+        for stream_name, token in self.handler.get_streams_to_replicate().iteritems():
+            self.replicate(stream_name, token)
+
+        # Tell the server if we have any users currently syncing (should only
+        # happen on synchrotrons)
+        currently_syncing = self.handler.get_currently_syncing_users()
+        for user_id in currently_syncing:
+            self.send_command(UserSyncCommand(user_id, True))
+
+        # We've now finished connecting to so inform the client handler
+        self.handler.update_connection(self)
+
+    def on_SERVER(self, cmd):
+        if cmd.data != self.server_name:
+            logger.error("[%s] Connected to wrong remote: %r", self.id(), cmd.data)
+            self.transport.abortConnection()
+
+    def on_RDATA(self, cmd):
+        try:
+            row = STREAMS_MAP[cmd.stream_name].ROW_TYPE(*cmd.row)
+        except Exception:
+            logger.exception(
+                "[%s] Failed to parse RDATA: %r %r",
+                self.id(), cmd.stream_name, cmd.row
+            )
+            raise
+
+        if cmd.token is None:
+            # 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(cmd.stream_name, []).append(row)
+        else:
+            # Check if this is the last of a batch of updates
+            rows = self.pending_batches.pop(cmd.stream_name, [])
+            rows.append(row)
+
+            self.handler.on_rdata(cmd.stream_name, cmd.token, rows)
+
+    def on_POSITION(self, cmd):
+        self.handler.on_position(cmd.stream_name, cmd.token)
+
+    def on_SYNC(self, cmd):
+        self.handler.on_sync(cmd.data)
+
+    def replicate(self, stream_name, token):
+        """Send the subscription request to the server
+        """
+        if stream_name not in STREAMS_MAP:
+            raise Exception("Invalid stream name %r" % (stream_name,))
+
+        logger.info(
+            "[%s] Subscribing to replication stream: %r from %r",
+            self.id(), stream_name, token
+        )
+
+        self.send_command(ReplicateCommand(stream_name, token))
+
+    def on_connection_closed(self):
+        BaseReplicationStreamProtocol.on_connection_closed(self)
+        self.handler.update_connection(None)
+
+
+# The following simply registers metrics for the replication connections
+
+metrics.register_callback(
+    "pending_commands",
+    lambda: {
+        (p.name, p.conn_id): len(p.pending_commands)
+        for p in connected_connections
+    },
+    labels=["name", "conn_id"],
+)
+
+
+def transport_buffer_size(protocol):
+    if protocol.transport:
+        size = len(protocol.transport.dataBuffer) + protocol.transport._tempDataLen
+        return size
+    return 0
+
+
+metrics.register_callback(
+    "transport_send_buffer",
+    lambda: {
+        (p.name, p.conn_id): transport_buffer_size(p)
+        for p in connected_connections
+    },
+    labels=["name", "conn_id"],
+)
+
+
+def transport_kernel_read_buffer_size(protocol, read=True):
+    SIOCINQ = 0x541B
+    SIOCOUTQ = 0x5411
+
+    if protocol.transport:
+        fileno = protocol.transport.getHandle().fileno()
+        if read:
+            op = SIOCINQ
+        else:
+            op = SIOCOUTQ
+        size = struct.unpack("I", fcntl.ioctl(fileno, op, '\0\0\0\0'))[0]
+        return size
+    return 0
+
+
+metrics.register_callback(
+    "transport_kernel_send_buffer",
+    lambda: {
+        (p.name, p.conn_id): transport_kernel_read_buffer_size(p, False)
+        for p in connected_connections
+    },
+    labels=["name", "conn_id"],
+)
+
+
+metrics.register_callback(
+    "transport_kernel_read_buffer",
+    lambda: {
+        (p.name, p.conn_id): transport_kernel_read_buffer_size(p, True)
+        for p in connected_connections
+    },
+    labels=["name", "conn_id"],
+)