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Merge pull request #2082 from matrix-org/erikj/repl_tcp_server
Replace HTTP replication with TCP replication (Server side part)
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TCP Replication | ||
=============== | ||
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Motivation | ||
---------- | ||
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Previously the workers used an HTTP long poll mechanism to get updates from the | ||
master, which had the problem of causing a lot of duplicate work on the server. | ||
This TCP protocol replaces those APIs with the aim of increased efficiency. | ||
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Overview | ||
-------- | ||
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The protocol is based on fire and forget, line based commands. An example flow | ||
would be (where '>' indicates master to worker and '<' worker to master flows):: | ||
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> SERVER example.com | ||
< REPLICATE events 53 | ||
> RDATA events 54 ["$foo1:bar.com", ...] | ||
> RDATA events 55 ["$foo4:bar.com", ...] | ||
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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. | ||
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Error reporting happens by either the client or server sending an `ERROR` | ||
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 | ||
the server using a tool like netcat. A few things should be noted when manually | ||
using the protocol: | ||
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* 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. | ||
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Architecture | ||
------------ | ||
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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:: | ||
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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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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. | ||
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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. | ||
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This ensures that both sides can quickly realize if the tcp connection has gone | ||
and handle the situation appropriately. | ||
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Start up | ||
~~~~~~~~ | ||
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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 | ||
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. | ||
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The client: | ||
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* 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. | ||
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Error handling | ||
~~~~~~~~~~~~~~ | ||
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If either side detects an error it can send an ``ERROR`` command and close the | ||
connection. | ||
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If the client side loses the connection to the server it should reconnect, | ||
following the steps above. | ||
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Congestion | ||
~~~~~~~~~~ | ||
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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. | ||
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Reliability | ||
~~~~~~~~~~~ | ||
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In general the replication stream should be considered an unreliable transport | ||
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 | ||
these include tokens which can be used to restart the stream on connection | ||
errors. | ||
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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. | ||
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Example | ||
~~~~~~~ | ||
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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:: | ||
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* 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 * | ||
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The ``POSITION`` command sent by the server is used to set the clients position | ||
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] | ||
> 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] | ||
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In this case the client shouldn't advance their caches token until it sees the | ||
the last ``RDATA``. | ||
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List of commands | ||
~~~~~~~~~~~~~~~~ | ||
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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) | ||
Sent at the start to identify which server the client is talking to | ||
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RDATA (S) | ||
A single update in a stream | ||
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POSITION (S) | ||
The position of the stream has been updated | ||
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ERROR (S, C) | ||
There was an error | ||
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PING (S, C) | ||
Sent periodically to ensure the connection is still alive | ||
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NAME (C) | ||
Sent at the start by client to inform the server who they are | ||
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REPLICATE (C) | ||
Asks the server to replicate a given stream | ||
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USER_SYNC (C) | ||
A user has started or stopped syncing | ||
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FEDERATION_ACK (C) | ||
Acknowledge receipt of some federation data | ||
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REMOVE_PUSHER (C) | ||
Inform the server a pusher should be removed | ||
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INVALIDATE_CACHE (C) | ||
Inform the server a cache should be invalidated | ||
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SYNC (S, C) | ||
Used exclusively in tests | ||
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See ``synapse/replication/tcp/commands.py`` for a detailed description and the | ||
format of each command. |
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