Exploring ArchUnit-like Architecture Testing for Elixir

Like most engineers these days, I’ve been thinking about the intersection of LLM driven development and using Elixir to do so. After being particular inspired by this blog post, I’ve been looking at implementing architecture testing in Elixir projects and wondering if there’s an equivalent to Java’s ArchUnit. After some research and brainstorming, I’d like to share some thoughts and potential approaches to spark a community discussion.

The Problem

In larger Elixir codebases, maintaining architectural boundaries becomes increasingly important - especially as we navigate the limitations of pumping LLM agents full of context. While Elixir’s design encourages good practices, we still need ways to codify:

1. Enforcing domain boundaries

2. Maintaining layered architectures

3. Preventing unwanted dependencies / insecure patterns

4. Ensuring architectural decisions are continually followed by the team (or even LLM coding agents)

Java developers have ArchUnit, which provides a fluent API for defining and testing architectural rules:

noClasses().that().resideInAPackage("..source..")
    .should().dependOnClassesThat().resideInAPackage("..foo..")

Now tell me that doesn’t look a lot like a functional pipeline chain…

Potential Approaches in Elixi

I see two main paths forward:

1. Custom Credo Rules

We could extend Credo with custom rules for architecture testing:

# In .credo.exs
%{
  configs: [
    %{
      name: "default",
      checks: [
        {MyProject.ArchitectureCheck.DomainBoundaries, []},
      ]
    }
  ]
}

With implementation like:

defmodule MyProject.ArchitectureCheck.DomainBoundaries do
  use Credo.Check, category: :design, base_priority: :high

  def run(%SourceFile{} = source_file, params) do
    # Parse AST and check for violations
    # Return list of issues found
  end
end

2. Custom DSL with Metaprogramming

Alternatively (and where I’m personally leaning), we could leverage Elixir’s metaprogramming to create an expressive DSL similar to ArchUnit and further leans into pipelines:

defmodule MyApp.ArchitectureTest do
  use ExUnit.Case
  use ArchElixir.DSL
  
  test "domain modules should not depend on web modules" do
    architecture_rule do
      modules_that()
      |> reside_in_namespace("MyApp.Domain")
      |> should()
      |> not_depend_on_modules_that()
      |> reside_in_namespace("MyApp.Web")
    end
  end
  
  test "layered architecture is respected" do
    architecture_rule do
      layers()
      |> define("Web", "MyApp.Web")
      |> define("Application", "MyApp.Application")
      |> define("Domain", "MyApp.Domain")
      |> where_layer("Web")
      |> may_only_access_layers(["Application"])
      |> where_layer("Application")
      |> may_only_access_layers(["Domain"])
    end
  end
end

This would require building:

• A fluent rule builder API

• A dependency analyzer (walking the AST similar to Credo and Sobelow)

• A rule evaluator

• ExUnit integration

Comparing Approaches

Custom DSL

Strengths

• Highly expressive and readable rules

• Flexible and extensible for complex patterns

• Seamless ExUnit integration

• Runtime analysis catches dynamic dependencies

• Customized reporting

• Paves the path for Dynamic Application Security Testing (which is worth exploring for me as a security professional)

Weaknesses

• Significant development effort

• Potential performance overhead

• Limited community support

• Complex dependency analysis

Credo Rules

Strengths

• Integration with existing tooling

• Static analysis benefits (speed, early feedback)

• Community and ecosystem support

• Lower implementation barrier

• IDE integration

Weaknesses

• Less expressive API

• Limited analysis depth

• Constrained by Credo’s design

• Less natural for complex rules

Questions for the Community

1. Has anyone built something like this already and I’m just not finding it in my searches?

2. Which approach seems most promising?

3. What architectural patterns would you want to enforce?

4. Would this be valuable as a standalone package?

5. Are there specific challenges in Elixir’s compilation model that would make this difficult?

6. How could we handle dependencies across umbrella apps?

Again, I’m particularly interested in Elixir’s metaprogramming capabilities and how they could enable an elegant DSL for architecture testing. The ability to define rules that match the mental model of our architectural constraints seems valuable.

What are your thoughts? Would you even use such a tool?