[thomastay]

Having dealt with this problem at work for several years now, I feel the pain of keeping different clients in sync - it's extremely difficult. Not sure if it's possible in Matrix, but consider having a message ID that increments by one on every message in a room. That lets the client know pretty quickly if there's a gap or a misordering.

Not really getting this point though:

  The /sync API returns events in an order "according to the arrival time of the event on the homeserver".

  The spec for /messages says it returns events "in chronological order. (The exact definition of chronological is dependent on the server implementation.)".

Why would those two return different results? When does the chronological order of two messages differ from the arrival time of the event on the homeserver?

[duskwuff]

What I think you're missing is that Matrix runs as a distributed system. There's no central authority to assign IDs to messages, and it's possible for a single group chat to run in a split-brain configuration if two homeservers lose connectivity to each other. When those homeservers reconnect, users connected to each one will see messages appear "in the past" which were sent by users on the other side of the split.

[wolrah]

Perhaps I'm wrong about how Matrix works, but my understanding was that at least public rooms still had a "primary" homeserver, like for example I can connect to #debian:matrix.org from any number of federated servers but matrix.org is still where that room "lives".

If that understanding is correct, then IMO the answer is simply that the canonical timeline is what that server says it is. Poorly connected users or those on other servers experiencing issues or delays with federation may temporarily see a different sequence of events but once everyone's had a chance to sync back up the state should generally be what the primary server for the room saw it as.

Perhaps there should be some sort of flag for "this message has been reordered during a resync" that clients which initially had a different state due to whatever reason could store to make it clear what happened, and likewise if the central homeserver receives messages with a timestamp significantly off real time it could flag those messages as possibly having been received out of order while still displaying them in the order they were received.

[mycall]

Split-brain scenarios can be resolved using an odd number of nodes (or voters) to achieve a majority consensus to agree on the state of the system, stopping the services on the minority side to prevent conflicting operations. Once communication is restored, the stopped nodes can rejoin the cluster and synchronize their data. Vector clocks are a great abstraction for ensuring correct ordering as well.

[thomastay]

yeah, having eventual consistency for messages across homeservers makes the work on the client harder. I guess they just have to accept that messages will "appear in the past" as you said.

But at least for messages sent within the same homeserver, I would think that those two apis should return the same data

[adastra22]

That makes the problem harder, but not impossible.

[nine_k]

What's mysterious here? One ordering is dictated (arrival time), another left for the consideration of the server (likely allowing for stuff like pinned messages, etc, that break the strict ordering).

If a Matrix server allows to delete messages (by the poster or by a moderator), then increasing IDs with no gaps become impossible. If the server allows editing of existing messages, then a sequence with no gaps is not sufficient to reflect all changes. Ideally a server does not do either, but uses more messages to augment existing messages, or mark some as deleted; with that, a sequence with no gaps would suffice.

[wkrp]

/messages might be a legacy endpoint compared to a newer /sync. I know Matrix has been working hard on their sliding sync api.

[AlotOfReading]

Non-monotonic clocks?