Heroku app recompiles after restarts using Elixir 1.15

I upgraded to Elixir 1.15 and am having issues deploying on Heroku now. Whenever the app restarts, the whole application recompiles. On our large application, this recompile takes longer than 30 seconds, which causes the app to crash.

I’m able to reproduce this on a bare bones Phoenix app. The line where it says “Compiling 15 files” should not be happening during a restart.

2023-07-06T14:42:59.587968+00:00 heroku[web.1]: Restarting
2023-07-06T14:42:59.650735+00:00 heroku[web.1]: State changed from up to starting
2023-07-06T14:43:00.295043+00:00 heroku[web.1]: Stopping all processes with SIGTERM
2023-07-06T14:43:00.322078+00:00 app[web.1]: 14:43:00.321 [notice] SIGTERM received - shutting down
2023-07-06T14:43:00.322086+00:00 app[web.1]: 
2023-07-06T14:43:01.502645+00:00 heroku[web.1]: Process exited with status 0
2023-07-06T14:43:03.720077+00:00 heroku[web.1]: Starting process with command `mix phx.server`
2023-07-06T14:43:05.363804+00:00 app[web.1]: Compiling 15 files (.ex)

Where should I report this issue? I don’t know if it’s something specific to Elixir or if the Heroku buildpack we’re using needs to be updated to support Elixir 1.15. GitHub - HashNuke/heroku-buildpack-elixir: Heroku Buildpack for Elixir with nitro boost

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Have you considered deploying elixir through mix releases on heroku? That should prevent having to recompile and also bring some other benefits.

It might not answer your question but i can recommend looking i to it as this problem will reoccur even if you manage to solve it for now :slight_smile:. I would post links but I’m currently on my phone so i have limited resources. I will try to do so later

Thanks for the suggestion. I was able to get our app running on Heroku with mix releases with minimal changes. One thing that seems a bit odd is that the app recompiles during Heroku’s release phase when we run migrations. It only does this in 1.15 and not 1.14, so it think there’s still something “off” here. The app at least starts up, but adds a few minutes to deploy time.

Yes, running the migrations through mix will force recompilation. There is a way around it though, i think its described quite well under the phoenix release docs. In short its a helper function that is called via the release through the eval command

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If you haven’t found it already: Deploying with Releases — Phoenix v1.7.6 :slight_smile:

That’s perfect. Thanks for your help!

For those coming here with a setup similar to mine:

  • Elixir 1.15
  • OTP 26
  • Heroku
  • Phoenix 1.7
  • And Umbrella app (4 apps)

The solution to make it work with Heroku, as explained previously, is to migrate to use Releases, but then, Node and npm weren’t being run so statics assets were not being compiled (i.e. apps/xxx/assets).

In the end I made it work in Heroku by adding a Procfile manually, keeping the Elixir and Phoenix static assets buildpacks, adding an additional “elixir mix release” buildpack and setting Phoenix Endpoint as a server.

Configuration step by step:

Phoenix Endpoint

I had a lot of trouble making Phoenix start in Heroku when building the mix release. In the end, this is what I needed to put inside config/runtime.exs. The secret lies in adding server: true, otherwise the Elixir release won’t start Phoenix as a server and Heroku will timeout on startup.

if config_env() == :prod do
  config :fb_web, Farmbackup.Endpoint,
    http: [
      port: {:system, "PORT"},
      compress: true,
      timeout: 60_000,
      protocol_options: [
        idle_timeout: 60_000
    url: [scheme: "https", host: System.fetch_env!("FARMBACKUP_HOST"), port: 443],
    debug_errors: false,
    cache_static_manifest: "priv/static/cache_manifest.json",
    secret_key_base: System.fetch_env!("SECRET_KEY_BASE"),
    server: true

mix.exs release

In the main mix.exs file add all your umbrella apps in the releases config key:

  def project do
      apps_path: "apps",
      start_permanent: Mix.env() == :prod,
      deps: deps(),
      aliases: aliases(),
      releases: [
        fb_task: [
          applications: [
            fb_web: :permanent,
            fb_db: :permanent,
            fb_invoice: :permanent,
            fb_salary: :permanent
          version: DateTime.utc_now() |> DateTime.to_iso8601()

Heroku Buildpacks

You are going to need these 3 buildpacks installed in order:


Buildpack config




phoenix_relative_path=apps/fb_web (i.e. app in the umbrella which has `/assets`)


YOU need to manually create a Heroku Procfile for Umbrellas.

In our case it is:

web: _build/prod/rel/fb_task/bin/fb_task start
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