[kelnos]

Agreed, absolutely. I was surprised by the author's aside that mentioned a colleague that disagreed, claiming that it's ok or even useful (what?) that two clients (or even the same client in different situations) could show different message orderings... and acknowledging that this point is debatable.

It's not debatable! There is, actually, in reality, one true message ordering (at this time I think we can safely ignore relativistic effects), and that message ordering should be the one that is always displayed. Our technology and the nature of distributed systems may make it impossible to always faithfully determine what the true message ordering is, but at the very least, the implementation should decide on a single message ordering, and always present that same message ordering. (This does mean that clients connected to different homeservers might see different orderings; this is unfortunate but probably unavoidable. But clients connected to the same homeserver should all see the same ordering.)

Anything else is just terrible, awful, horrible UX, that will ultimately confuse users. And it will also reduce user faith in the system as a whole: if they notice the inconsistent ordering, they will assume the system is buggy and unreliable.

At the risk of sounding way too absolutist: this is not debatable, and anyone who thinks it is, is wrong.

[OskarS]

> (This does mean that clients connected to different homeservers might see different orderings; this is unfortunate but probably unavoidable. But clients connected to the same homeserver should all see the same ordering.)

I don't think it's unavoidable at all, this is exactly the kind of problems CRDTs solve beautifully. If you model your chatting as a CRDT (basically, each message has a Lamport timestamp on it, and you have some sensible way to resolve ties deterministically), then assuming all peers have gotten all messages, the ordering should be perfectly consistent. What may happen is that you type a message, press "enter", and then some message might pop up before that when everyone finally sync, but I think that is acceptible UX, I see version of this in things like Slack and Discord frequently.

All peers having the same message order assuming they all have received the same messages is absolutely required for a chat application in 2024, distributed or not.

[bostik]

> clients connected to the same homeserver should all see the same ordering

Ideally, yes. In the real world you get to deal with such inconveniences as unreliable transport, slow networks, server-to-server communications, eventual consistency, routing glitches, reconnections, clock skew, queues, concurrency, retries, and always, ALWAYS the infuriatingly slow speed of light. Oh yes, also multiple client implementations. For the record, many years ago I was involved in writing both a chat server and a client. (Albeit those were two different projects.)

From pure UX standpoint, you want the client to always show any messages it has sent but has not received back. Even for a single server and two clients, all synchronised to the same clock, you can get ordering conflicts. Let's say that you are sending a message at fixed two-second interval, and the other client is sending messages at non-fixed, power law distribution intervals. That's your happy path.

Now consider the same with dozens, or hundreds of clients across hundreds of different networks, each with their own debatable quality.

You want to see the messages you've sent, so they need to be visible on your screen. Having your own messages disappear into the void and only appear once they have been sent back is terrible UX. So you keep a local order and interleave received messages as they come in. But once you receive the message back from the server, you obviously want to reorder the known quantities. With two clients you will the occasional "jump" where one of your messages is moved to its canonical position. With hundreds of clients each user will see those jumps constantly - and with sufficient volume a decent fraction of their own sent messages can "disappear" at any time from their screen as they are reordered and don't fit on the screen.

Now add lots of bad networks and latency floor in hundreds of milliseconds. Network connections/route fluctuate constantly, so even messages sent by the same client less than two seconds apart can arrive in different order at the server. (The client reconnected between the two, and the message sent over the first connection arrives several seconds later than the other one.) The user is confused, because the server is very clearly showing them their own messages in the wrong order.

For one inconvenienced user the server being wrong occasionally is mildly annoying. But when that can happen to any number of users, concurrently, at any time, the overall effect is outright infuriating.

> this is not debatable, and anyone who thinks it is, is wrong

Your server has a known order it sent the messages out. Any disagreement means the client must be wrong.

Each of the clients connected to your server has a known order in which they sent their messages out. Any disagreement means the server must be wrong.