TCP Replication

Motivation

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.

Overview

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):

> 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 considered 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.

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