Replication Architecture
========================

Motivation
----------

We'd like to be able to split some of the work that synapse does into multiple
python processes. In theory multiple synapse processes could share a single
postgresql database and we'd scale up by running more synapse processes.
However much of synapse assumes that only one process is interacting with the
database, both for assigning unique identifiers when inserting into tables,
notifying components about new updates, and for invalidating its caches.

So running multiple copies of the current code isn't an option. One way to
run multiple processes would be to have a single writer process and multiple
reader processes connected to the same database. In order to do this we'd need
a way for the reader process to invalidate its in-memory caches when an update
happens on the writer. One way to do this is for the writer to present an
append-only log of updates which the readers can consume to invalidate their
caches and to push updates to listening clients or pushers.

Synapse already stores much of its data as an append-only log so that it can
correctly respond to /sync requests so the amount of code changes needed to
expose the append-only log to the readers should be fairly minimal.

Architecture
------------

The Replication Protocol
~~~~~~~~~~~~~~~~~~~~~~~~

See ``tcp_replication.rst``


The Slaved DataStore
~~~~~~~~~~~~~~~~~~~~

There are read-only version of the synapse storage layer in
``synapse/replication/slave/storage`` that use the response of the replication
API to invalidate their caches.
