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