[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!