Guaranteeing order with phoenix channels

For a project, I’m using channels to handle realtime notifications and synchronization between clients. I send the initial state embedded in the DOM and use channels to listen to updates from the server. However, there is a time between the HTML leaving the server and the channel being setup on the client during which I’m “blind” and won’t receive events.

Is there any way to guarantee that I won’t have such a window during which events are not acknowledged?

I was thinking of the following: instead of embedding the state in the DOM, I could ask for the initial state through the channel. Does this guarantee that I will only receive the events in the browser after receiving the initial state? Where can I read abou the “ordering” garanteed of phoenix channel communication?

So the order of messages received by client of channel will always be the order of events that were sent to given channel. The client will always also receive all messages, even if they are outdated.

It looks like a good thing in your case. You are right, that you will have a gap window between the time when user loads initial HTML page, and when the channel connection is established, and you can miss some updates, yes.

If this is mission critical not to loose any events that happen in the meantime, you can either do what you suggest - load whole state, including initial state via channel. Alternatively, you can start a process on each GET request, that would be tied to that get request and future channel by some UUID for example. You generate UUID, send with your HTML, start a process with the same UUID, and start a channel with the same UUID. Rather than pushing the messages to channel directly, you use your newly created process as a buffer. But it’s way more complicated than simply loading all the state via phoenix channel.


Thanks! I think that maybe something like this should be added to the channels documentation. It doesn’t say anything there about preserving order of events.

The order of events is incidental in that it is the BEAM that assures that messages are passed to the network stack in order and TCP is an in-order protocol. Ordering between ‘different’ process are not ordered though, and that is reflected in channels. Different Channel messages can arrive in any order.

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