Since FOSDEM we've seen even more interest in Matrix than normal, and we've been having some problems getting the Matrix.org homeserver to keep up with demand. This has resulted in performance being slightly slower than normal at peak times, but the main impact has been the additional traffic exacerbating outages on the homeserver - either by revealing new failure modes, or making it harder to recover rapidly after something goes wrong.
Specifically: on Friday afternoon we had a service disruption caused by someone sending an unusual event into Matrix HQ. It turns out that both matrix-android-sdk and matrix-ios-sdk based clients (e.g. Riot/Android and iOS) handled this naively by simply resyncing the room state... which has been fine in the past, but not when you have several hundred clients actively syncing the room, and resulted in a thundering herd effect which overloaded the server for ~10 mins or so whilst they all resynced the room (which, in turn, nowadays, involves calculating and syncing several MB of JSON state to each client). The traffic load was then high enough that it took the server a further 10-20 minutes for the server to fully catch up and recover after the herd had dissipated. We then had a repeat performance on Monday morning of the same failure mode.
Similarly, we had disruption last night after a user who hadn't used the service for ages logged on for the first time and rapidly caught up on a few rooms which literally had millions of unread messages in them. Generally this would be okay, but the combination of loaded DB and the sheer number of notifications being deleted ended up with 4 long-running DB deletes in parallel. This seems to have caused postgres to lock the event_actions_table more aggressively than we'd expect, blocking other queries which were trying to access it... causing most requests to block until the deletes were over. At the current traffic volumes this meant that the main synapse process tried to serve thousands of simultaneous requests as they stacked up and ran out of filehandles within about 10 minutes and wedged the whole synapse solid before the DB could unblock. Irritatingly, it turns out our end-to-end monitoring has a bug where it in turn can crash on receiving a 500 from synapse, so despite having PagerDuty all set up and running (and having been receiving pages for traffic delays over the last few weeks)... we didn't get paged when we got actual failed traffic rather than slow traffic, which delayed resolving the issue. Finally, whilst rolling out a fix this afternoon, we again hit issues with the traffic load causing more problems than we were expecting, making a routine redeploy distinctly more disruptive.
So, what are we doing about this?
Meanwhile, please accept our apologies for the instability and be assured that we're doing everything we can to get out this turbulence as rapidly as possible.