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 The client will always also receive all messages, even if they are outdated.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.