If we send out an event which refers to `prev_events` which other servers in
the federation are missing, then (after a round or two of backfill attempts),
they will end up asking us for `/state_ids` at a particular point in the DAG.
As per https://github.com/matrix-org/synapse/issues/7893, this is quite
expensive, and we tend to see lots of very similar requests around the same
time.
We can therefore handle this much more efficiently by using a cache, which (a)
ensures that if we see the same request from multiple servers (or even the same
server, multiple times), then they share the result, and (b) any other servers
that miss the initial excitement can also benefit from the work.
[It's interesting to note that `/state` has a cache for exactly this
reason. `/state` is now essentially unused and replaced with `/state_ids`, but
evidently when we replaced it we forgot to add a cache to the new endpoint.]
For inbound federation requests, if a given remote server makes too many
requests at once, we start stacking them up rather than processing them
immediatedly.
However, that means that there is a fair chance that the requesting server will
disconnect before we start processing the request. In that case, if it was a
read-only request (ie, a GET request), there is absolutely no point in
building a response (and some requests are quite expensive to handle).
Even in the case of a POST request, one of two things will happen:
* Most likely, the requesting server will retry the request and we'll get the
information anyway.
* Even if it doesn't, the requesting server has to assume that we didn't get
the memo, and act accordingly.
In short, we're better off aborting the request at this point rather than
ploughing on with what might be a quite expensive request.
The [postgres setup docs](https://github.com/matrix-org/synapse/blob/develop/docs/postgres.md#set-up-database) recommend setting up your database with user `synapse_user`.
However, uncommenting the postgres defaults in the sample config leave you with user `synapse`.
This PR switches the sample config to recommend `synapse_user`. Took a me a second to figure this out, so assume this will beneficial to others.
It serves no purpose and updating everytime we write to the device inbox
stream means all such transactions will conflict, causing lots of
transaction failures and retries.
When we get behind on replication, we tend to stack up background processes
behind a linearizer. Bg processes are heavy (particularly with respect to
prometheus metrics) and linearizers aren't terribly efficient once the queue
gets long either.
A better approach is to maintain a queue of requests to be processed, and
nominate a single process to work its way through the queue.
Fixes: #7444
It was correct at the time of our friend Jorik writing it (checking
git blame), but the world has moved now and it is no longer a
generator.
Signed-off-by: Olivier Wilkinson (reivilibre) <olivier@librepush.net>
When considering rooms to clean up in `delete_old_current_state_events`, skip
rooms which we are creating, which otherwise look a bit like rooms we have
left.
Fixes#7834.
