Current state of Presence #15877

In order to track all the 'stuff' I'm finding about presence, let's have a central location to aggregate the data.

Known Historical Issues:

• massive storm of presence EDUs when joining a large room #3120
  • hopefully mitigated by Presence should send less presence when joining a room #15818
• Presence filter in sync ignored #2298
• synchrotron sometimes returns stale presence #1658
• Should we return m.presence events in /sync at all for offline users? (SYN-682) #1535
• last_active_ago is updated on sync when it shouldn't be #12424
  • hopefully mitigated by Presence fix for idle mode #15989
• Allow sending presence only to whitelisted servers or keeping it local only #8055
• [meta] Improving Presence Performance #9478
  • Synchrotrons always wake up sync streams for presence updates #8955
  • Presence logic for handling newly joined rooms is very inefficient and does duplicate work #8956
    • hopefully mitigated by Presence should send less presence when joining a room #15818
  • Not all presence updates should be sent out over federation immediately. #9415

Known Current Issues:

• Could not serialize access due to concurrent DELETE from presence_stream #15827
  • hopefully mitigated by Fix could not serialize access due to concurrent DELETE from presence_stream #15826
• [Presence] Huge spike in CPU usage/Federation traffic approximately every 25 minutes #15878
• [Presence] Flicker when using multiple devices simultaneously #15879
• Sliding Sync Proxy + Presence enabled leads to high CPU usage #15980

Other related:

• getting rid of the replication "stream" tables #13456
• sending typing notice event doesn't update m.presence (SYN-541) #1439
• Add tests that notify Application Services about ephemeral events such as presence #10950
• Optionally hide online status #14618
• Postgres indexes differ on matrix.org #11893
• Faster joins: handle presence correctly #13008
• Initial and incremental syncs with presence enabled eat up RAM until an OOM hits #13901
• A filtered /sync for a specific room does not include account_data or ephemeral events in its response #13648
• Status messages aren't included in initial syncs if the sender is offline #13507
• Crashes on startup while generating presence stream #10106
• It's a real pain that synapse doesn't log stats on the nature of the PDUs/EDUs it receives #2652
• Admins should always be able to get user presence #5492
• Deactivated user's presence online #15623
• And I'm sure there are others, plus all the ones that have been closed