[scraplab]

Hi Chris - this is superb, really pleased to see LiveView getting so much attention, and thanks for all your hard work.

Can you talk a little about the current story with deployments and managing state across/during those?

Obviously Erlang/OTP supports hot upgrades, but those are hard to design correctly and not supported by container/VM environments like Heroku and (I presume?) Fly.io.

[chrismccord]

Hot upgrades are a powerful and essential feature – when you have hard requirements for it. As you said, hot upgrades aren't simple and introduce a lot of work to get right. I like to tell folks, imagine you are building a pusher.com competitor, so your customers are paying for you to host active connections for their end-users. In this scenario, bouncing hundreds of thousands of connections for a cold deploy is a terrible offering because your clients are paying you or that reliable wire to their user's browser. Hot upgrades makes total sense here. Equally, if you're building a game server in Elixir, you likely don't want to teardown and rebuild your world state and bounce connected players.

So you opt-in to this complexity when you have a clear requirement for it. They other key point in this, is in either hot upgrade or cold deploy cases, you still have to design your stateful system to recover from complete restart. Servers catch on fire and things go bad sometimes, so the cold deploy approach of rebuilding the state on restart is not only completely viable by itself, but you're doing it anyway even with hot upgrades. Hope that helps!

[ramchip]

You can store state client-side. Say you have a view to edit users, with a modal dialog. A typical LiveView app will change the URL when you open the modal, so it's like /users?action=edit&user=bob. When a server goes down the client reconnects, and the LV receives the URL params in its handle_param callback, so it can recover the earlier state. Forms recover automatically in a similar way.

With this and a DB to store permanent data, an LV app can be deployed just like any other standard container app without user impact.

[modal-soul]

If LiveView is backed by PubSub, then I think you can offload persistence to Redis.

[ramchip]

LiveView in its standard configuration does not use Redis, it uses built-in Erlang distribution. But you can use Redis if you want to or if direct connections between nodes aren't possible.