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

0 426 13

2025-10-17 12:39:34 UTC

First Post!

Show Best Posts
Show All (oldest first)
Show All (newest first)

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

Most Liked

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

jswanner

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

Post #8

Last Post!

s-stepien

Hmmm

In fail case:

iex(2)> Application.started_applications()
[
{:myapp, [], []},
{:hex, ~c"hex", ~c"2.2.2"},
{:inets, ~c"INETS  CXC 138 49", ~c"9.4.2"},
{:ssl, ~c"Erlang/OTP SSL application", ~c"11.4"},
{:public_key, ~c"Public key infrastructure", ~c"1.18.3"},
{:asn1, ~c"The Erlang ASN1 compiler version 5.4.2", ~c"5.4.2"},
{:crypto, ~c"CRYPTO", ~c"5.7"},
{:mix, ~c"mix", ~c"1.18.4"},
{:iex, ~c"iex", ~c"1.18.4"},
{:elixir, ~c"elixir", ~c"1.18.4"},
{:compiler, ~c"ERTS  CXC 138 10", ~c"9.0.2"},
{:stdlib, ~c"ERTS  CXC 138 10", ~c"7.1"},
{:kernel, ~c"ERTS  CXC 138 10", ~c"10.4"}
]

iex(4)> Application.ensure_started(:phoenix_live_reload)
{:error, {:not_started, :logger}}
iex(5)> Application.ensure_started(:logger)
:ok
iex(6)> Application.ensure_started(:phoenix_live_reload)
{:error, {:not_started, :phoenix}}
iex(7)> Application.ensure_started(:phoenix)
{:error, {:not_started, :eex}}
iex(8)> Application.ensure_started(:eex)
:ok
iex(9)> Application.ensure_started(:phoenix)
{:error, {:not_started, :plug}}
iex(10)> Application.ensure_started(:plug)
{:error, {:not_started, :mime}}
iex(11)> Application.ensure_started(:mime)
:ok
iex(12)> Application.ensure_started(:plug)
{:error, {:not_started, :plug_crypto}}
iex(13)> Application.ensure_started(:plug_crypto)
:ok
iex(14)> Application.ensure_started(:plug)
{:error, {:not_started, :telemetry}}
iex(15)> Application.ensure_started(:telemetry)
:ok
iex(16)> Application.ensure_started(:plug)
:ok
iex(17)> Application.ensure_started(:phoenix)
{:error, {:not_started, :phoenix_pubsub}}
iex(18)> Application.ensure_started(:phoenix_pubsub)
:ok
iex(19)> Application.ensure_started(:phoenix)
{:error, {:not_started, :phoenix_template}}
iex(20)> Application.ensure_started(:phoenix_templete)
{:error, {~c"no such file or directory", ~c"phoenix_templete.app"}}

Update: in the end I was able to start them all and made sure that the file is there. So I do not know if this is related (or if I’m doing it correctly anyway).

Post #13