Application module `start` function not called?

s-stepien

I have fairly complicated Phoenix app (but it doubt that this is Phoenix related issue?). In it I have the MyApp.Application module with start(_type, _args) function. I noticed today (after long time without working on it), that the start() function isn’t called at all. In the mix.exs file, the mod: is set correctly to that module - I haven’t changed anything there.

How one can try to debug this? What is happening before call to start() - maybe something there hangs?

I’ve tried rebuild from scratch, re-get deps, etc. And nothing, the function will not be called.
When I created new project, all seems OK in it.

I’m using Erlang/OTP 28.1 and Elixir 1.18.4

Update:
If I use MIX_ENV=prod, then I can see print at the beginning of the start() function. But I want to use the dev env

13 comments

#mix #supervisor #application

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2025-10-17 12:39:34 UTC

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garrison

That start callback is what starts the Endpoint, so if it isn’t being called you have no server/app at all. Is that what you’re experiencing?

Post #1

s-stepien

Well I think it is not about Endpoint (I have more children, not just the Phoenix ones). I have IO.inspect(“here”) as 1st instruction in start() function (before the children definition). And it is not printed.

And I have mix.exs with application/server definitions that are OK (I haven’t changed them for like years now).

Post #2

garrison

I asked a specific question because I am trying to narrow down what is going on. If start/2 is not being called then your app must not be starting.

Is your application starting? Is the web server running? Does it work?

Post #3

jswanner

I believe @garrison is trying to get at is if any of the children in your application are running, if so then your application is running, and your endpoint is an easy one to test if it’s running.

How are you running your application? mix phx.server from the terminal, or some other way? Is it possible something is swallowing that IO you are using to debug? Instead of IO.inspect("here") you can try raise "here"

Post #4

s-stepien

@garrison none of the children are started. The function is not called/entered.

@jswanner even with raise “here” it will not stop it. I’m using iex -S mix phx.server with some env variables.

Note that with MIX_ENV=prod it works, every time. With MIX_ENV=dev it does not. Only this MIX_ENV is different in invocation.

Post #5

garrison

That is odd. I don’t know off the top of my head anything that would cause that behavior, though perhaps someone else will think of something.

Given that the behavior is different between environments you may want to comb through the configs for anything that could be different, though again I’m not sure what that might be.

As for your question about what happens when the application starts, it’s described quite thoroughly in the Application docs.

Post #6

dimitarvp

Then the culprit should be config/prod.exs or config/runtime.exs or any code that branches on the environment variables.

Post #7

jswanner

diff _build/{dev,prod}/lib/<app_name>/ebin/<app_name>.app

Post #8

s-stepien

It seems that the issue is with:

# Watch static and templates for browser reloading.
config :myapp, MyAppWeb.Endpoint,
  live_reload: [
    patterns: [
      ~r"priv/static/.*(js|css|png|jpeg|jpg|gif|svg)$",
      ~r"priv/gettext/.*(po)$",
      ...
    ]
  ]

Without this feature configured, the dev comes up. I wonder what might be the issue here. I will double check my paths here.

Post #9