Homeservers don't catch up with missed traffic until someone sends another event

[richvdh]

If a homeserver goes offline, it may miss events. There is then no reliable means for it to catch up. Currently we rely on someone else sending an event in the room, which will then make it do a backfill.

Potential solutions include:

• Proactively sending the data once we hear the server is back online (We should retry pending transactions when we hear from a server which was offline #2526)
• Making /pull work properly, and use it (federation/v1/pull/ doesn't seem to work #2527)

[ara4n]

some keywords: federation outage, resync room, server offline, recover

[OlegGirko]

some keywords: federation outage, resync room, server offline, recover

This is not just about temporary outage with following recovery. Partial netsplit can exist by design during transition to another underlying networking protocol. Imagine nodes that exist in I2P network only (or any other privacy-oriented network). These nodes can not communicate directly to normal HTTPS-based nodes that are not I2P-aware, but messages can be relayed through nodes that are dual-stack.

I think, partial netsplit should be considered not a temporary outage, but part of normal functioning.

[ara4n]

i'm guessing this is the cause of https://twitter.com/jomatv6/status/1213161573759541256.

The fact we don't retry transactions when a server reappears seems very worthwhile (especially in a p2p world).

[erikjohnston]

To limit the number of events we send we can limit ourselves to only sending the most recent N events for each room there are missed updates in.

Side note: sending old events down /send will cause the remote server and clients to think the event is "new" and notify users as normal. This may or may not be desired