How I fixed my compilations issues in my Ash Project

Hi all. I did not want to start my Ash series with this article, but I think it’s worth it.

I’ve been building a multitenant app for years now Servixo. It’s an application aimed at businesses selling Services and Products. We aim to provide a clean an easy invoicing, billing, ordering, online services etc… platform.

It was a Phoenix, Vue.js project initially and we decided to move it to Ash (more on this in new articles).

Everything went well until I encountered compilations issues. I was modifying a single liveview file, and had all of my domains (7 of them) recompile; Domains are one of the heaviest things to recompile in Ash. That’s about 5s lost everytime I’m saving my liveview files. I always though Ash was doing something anoying behind the scenes, and that somehow there was some relations between my domains I was not seeing.

This is not related to Ash only, if you feel you don’t understand your Elixir recompile cycles, they’re somewhere in your code that’s causing that bad DX.

Where did my troubles come from?

The first trouble I had was having all the resources in one Domain. It was intentionnal because we were migrating from an existing structure with Phoenix Contexts, and putting everything in a single Domain was the easiest thing to do and easy to refactor. I never intended the compilation to be as impacted. Splitting the app in the corresponding domains solved the first issue we faced.

The secound trouble was not detectable from eyesigth. I have a notifier.ex in my app, which build emails to send to users. And because I wanted to have route verified using Phoenix.VerifiedRoutes (~p"/route), I did this naive thing in my notifier code:

use Phoenix.VerifiedRoutes,
    router: MyApp.Router,
    endpoint: MyApp.Endpoint,
    statics: ~w(images assets)

Little would I have known it will be the cause of all my recompilation cycles.

How did I solve the issue?

I proceeded in steps:

1. Identify recompiling files related to a touched/modified file
2. Identify the relationship between the two files.

This article Avoiding recompilation hell in Elixir with mix xref by Renato Massaro paved the solution I found.

Identify recompiling files related to the touched/modified file

$ mix compile --verbose 

This will list the files that are recompiled after your file changes.

Compiled lib/myapp/file1.ex
Compiled lib/myapp/file2.ex
Compiled lib/myapp_web/live/budget/file_live.ex
Compiled lib/myapp/file3.ex
Compiled lib/myapp/file4.ex

Identify the relationship between the two files

Renato then suggests to find the relation between the last file file4.ex and the file I just touched/modified file_live.ex.

$ mix xref graph --source lib/myapp/file4.ex --sink lib/myapp_web/live/budget/file_live.ex

This will list a dependency tree having file4.ex at the top. Locate your file_live.ex and find its ancestors in the tree bottom to top. You’ll find the cause of the recompilation of that file. In most cases transitive compile-time dependencies are the reason you have many files recompiling.

In my case I just had to remove the use Phoenix.VerifiedRoutes... from my notifier module and just use plain urls.

Final notes

I also found that I had another dependency in my app:

defmodule MyApp.Token do
  @moduledoc false
  @salt "very_long_salt_used_here"
  @max_age 60 * 60 * 48

  def sign(data) do
    Phoenix.Token.sign(MyApp.Endpoint, @salt, data)
  end

  def verify(token, max_age \\ @max_age) do
    Phoenix.Token.verify(MyApp.Endpoint, @salt, token, max_age: max_age)
  end
end

I was calling MyApp.Token in a Resource, for signing and verifying tokens. Since my resource is a compile time deps for my Domain, and MyApp.Endpoint has a dep with MyApp.Router, I had a transitive dep with my Liveview file too!!!. I just deleted the file, and called directly Phoenix.Token.sign... directly in my resource, which solved that case.