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``.


List of commands
~~~~~~~~~~~~~~~~

The list of valid commands, with which side can send it: server (S) or client (C):

SERVER (S)
    Sent at the start to identify which server the client is talking to

RDATA (S)
    A single update in a stream

POSITION (S)
    The position of the stream has been updated

ERROR (S, C)
    There was an error

PING (S, C)
    Sent periodically to ensure the connection is still alive

NAME (C)
    Sent at the start by client to inform the server who they are

REPLICATE (C)
    Asks the server to replicate a given stream

USER_SYNC (C)
    A user has started or stopped syncing

FEDERATION_ACK (C)
    Acknowledge receipt of some federation data

REMOVE_PUSHER (C)
    Inform the server a pusher should be removed

INVALIDATE_CACHE (C)
    Inform the server a cache should be invalidated

SYNC (S, C)
    Used exclusively in tests


See ``synapse/replication/tcp/commands.py`` for a detailed description and the
format of each command.


Cache Invalidation Stream
~~~~~~~~~~~~~~~~~~~~~~~~~

The cache invalidation stream is used to inform workers when they need to
invalidate any of their caches in the data store. This is done by streaming all
cache invalidations done on master down to the workers, assuming that any caches
on the workers also exist on the master.

Each individual cache invalidation results in a row being sent down replication,
which includes the cache name (the name of the function) and they key to
invalidate. For example::

    > RDATA caches 550953771 ["get_user_by_id", ["@bob:example.com"], 1550574873251]

However, there are times when a number of caches need to be invalidated at the
same time with the same key. To reduce traffic we batch those invalidations into
a single poke by defining a special cache name that workers understand to mean
to expand to invalidate the correct caches.

Currently the special cache names are declared in ``synapse/storage/_base.py``
and are:

1. ``cs_cache_fake`` ─ invalidates caches that depend on the current state
