Httpoison, for example, starts its own supervision tree to manage client process pools (not obvious that an http client should do that, definitely usecases where it shouldn't) and there is no indication either in mix.exs, or `MyApplication.Application.start/2` that Httpoison needs the tree. For most deployments, it's probably not a big deal. Some process will try to read http content, fail if httpoison isn't quite ready, and be restarted by its supervisor.
However, if you try to use it early in compilation or test, say, in your test_helper.exs file, speaking from experience, you could wind up with a very difficult to understand race condition where the httpoison process tree hasn't fully booted and you're trying to fetch something off the internet, and you don't have the same level of supervision protection -- if test_helper fails the whole test suite gives up and doesn't restart -- for obvious reason.
For the http case thankfully the elixir ecosystem is getting Mint as a base http library, which doesn't require a process tree out the gate, and several interesting explicit process-pool-libaries (finch, mojito) which are tuned for their own use cases that derive from mint.
However, if you try to use it early in compilation or test, say, in your test_helper.exs file, speaking from experience, you could wind up with a very difficult to understand race condition where the httpoison process tree hasn't fully booted and you're trying to fetch something off the internet, and you don't have the same level of supervision protection -- if test_helper fails the whole test suite gives up and doesn't restart -- for obvious reason.
For the http case thankfully the elixir ecosystem is getting Mint as a base http library, which doesn't require a process tree out the gate, and several interesting explicit process-pool-libaries (finch, mojito) which are tuned for their own use cases that derive from mint.