Allow sharing assigns between live navigation

[SteffenDE]

Considerations

To support sharing assigns between live navigations, there are multiple ways we could implement this:

1. Add a way to store values in the transport

We could have the transport own an ETS table or provide an API to assign values to it. Then LV could store assigns that use assign_new into this storage. Drawbacks: assign_new would have a messaging overhead, sending (potentially big) data to another process, even if it is not needed again. If an assign is updated later, we'd either need to sync those updates (keeping track which assigned keys used assign_new) or live with potentially old data.

2. Add a way for the old LV to be kept alive on navigation

This is the solution that this PR implements (together with a PR in Phoenix). This solves all the drawbacks mentioned above: it only copies the data that is actually used for the optimization and it also always gets the most recent data from the old LV's assigns. The only new drawback that I can come up with is that the old LV needs to be in memory just a little bit longer than usual, but I think that's fine.

Changes

This PR relies on a yet to be finalized feature in Phoenix Channels (phoenixframework/phoenix#5959) to perform a custom handover between channel rejoins.

It changes the LV code to not leave the old channel before rejoining and also instructs Phoenix to not kill the old process before starting the new channel process. After the new channel is joined, the old one is killed. Any assign_new calls in the LV mount will try to fetch assigns from the old LV.

This is a backwards compatible optimization, so if a version of Phoenix is used that does not support handover, it just falls back to calling the function supplied to assign_new as usual.

Closes #3357.

cc @josevalim

[josevalim]

@SteffenDE I originally thought about another way to implement this, which is to let the server say "I have spawned up a new channel" and the client assumes it has taken over control. The benefit of doing it this way is that you don't need to keep the process around. You immediately spawn the new process passing the assigns it needs and that's it. Do you have thoughts about this route?

[SteffenDE]

I'll actually need to handle push_navigate as well. Forgot about that.

I need to think about the consequences of your proposal (e.g. we'll still want to start the new process under the channel's supervisor). Currently, I cannot really imagine what an API for this would look like.

We cannot really shorten the time that the process needs to wait though, I think, as we don't know the keys from assign_new upfront - at least without looking through the LV mount's AST or just passing all assigns, which could be rather expensive. So we need to at least wait for the initial mount of the new LV, don't we?

[josevalim]

We will wait for the initial mount but, because we are also the one who called the initial mount, we will know if the initial mount either succeed or failed. We don't need to have a timeout that says "in case I am not pinged in 15 seconds, terminate".

[SteffenDE]

We don’t wait in the current implementation :)

[SteffenDE]

So yeah I think the current implementation is fine for client-initiated live navigation. What you described is what we need for push_navigate.

[josevalim]

Oh, I see, we don't wait because it is client initiated!

@SteffenDE if we need the version I proposed for push_navigate, it can then be used for both push_navigate and live_navigate, right? 🤔 So it should be more general?

[josevalim]

Another downside of the current implementation is that it only works if you rejoin the same channel. The other version should be a bit more general... but it may also be (much?) harder to implement. :(

[greven]

Sorry to disrupt the conversation, but I couldn't understand the end-goal with the issue.
Is the idea by sharing assigns to reduce costly queries for example?
And would this only work inside a live session or between any Live Views?

[SteffenDE]

@greven yes, the end goal is to reduce things like querying the current user from the DB on each live navigation by expanding assign_new to access previous assigns. It will only work inside a live session.

[SteffenDE]

@josevalim I implemented a proof of concept for server initiated handovers in 2ec6c6a and phoenixframework/phoenix@cd99fcf. It breaks a couple of tests, so very much not finished, but it shows the concept. I did not change the client handovers to use this yet, but we could do it by changing the client from issuing a rejoin to instead send a message and trigger the same path.