Getting your Elixir application ready for CI/CD

Posted by Alex Koutmos on Monday, February 10, 2020


In today’s post we’ll be going over what exactly continuous integration and continuous delivery are, the benefits that come along with employing CI/CD, and some best practices that you should follow. In addition, we’ll also explore a wide array of Elixir ecosystem tools that can help you to create top notch CI pipelines. In order to experiment with a handful of the tools that we will be discussing, we will use a Git hooks Elixir library to execute our CI/CD validation steps, but on our local machine. Let’s jump right in!

What is CI/CD and when do you use it?

Continuous integration is the process by which new features and bug fixes are frequently merged into mainline branches. For each change to the code base, the project’s validation suite is executed to make sure it is up to team standards and does not introduce any regressions.

Continuous delivery is the process by which validated code, once merged, generates a new build artifact. The build artifact with the new code is then automatically deployed to your non-production environments. Note that continuous delivery deploys to non-production environments. The act of deploying to production directly is called–confusingly enough–continuous deployment.

CI and CD are critical tools to have at your disposal as they allow you and your team to move faster and with a higher degree of confidence. It enables you to catch bugs early on and ensure that they don’t impact customers.

How does CI/CD influence application design?

With our vocabulary explained, let’s move on to discussing the 12 Factor App [2]. The 12 Factor App is collection of techniques and guidelines that can be used to create sustainable and scalable web services. As the name implies, there are 12 concepts that make up the manifesto. For the purposes of CI/CD, we’ll focus on “Config” and “Codebase”, but I highly recommend reading up on the 12 Factor App if you are unfamiliar.

The Codebase section states that a single code repository should only contain a single application. If multiple applications make up your entire system, then each of those applications should be contained within their own repositories. This is particularly relevant to CI/CD since, by following this convention, it is very easy to ensure that only the necessary applications are built, tested, and deployed. If multiple applications are contained within the same repository, it becomes a bit more difficult to ascertain which application should be built, tested, and deployed.

The Config section states that any kind of configuration data, credentials and secrets need to be kept out of the code. Instead, these bits of data should be set in the application’s environment via environment variables. This is important from a CI/CD perspective since you will be able to spin up supporting services for testing, and can easily point your application to those services via runtime configuration. You can do this with a wide array of services such as Postgres, Redis, RabbitMQ and it will not only make your application portable between higher up environments, but it will also make it easy to test.

CI/CD best practices

Before jumping into some Elixir ecosystem tooling, let’s review some best practices that we should adhere to when designing our CI/CD pipelines (for clarification, a CI/CD pipeline is defined as a series of steps that are performed in order take code from commit to deployment):

  • Ensure that your tests are deterministic, as having an unreliable suite of tests can have terrible consequences for the velocity of a team and the reliability of the software that is being validated.

  • Run cheap validations first and expensive validations last so that the CI/CD pipeline fails as early as possible. You don’t want to take up CI/CD server resources running tests suites over and over again, when you have other cheaper issues that need to be sorted out.

  • Promote the same build artifact to higher level environments as it gives you the greatest amount of confidence that what you tested is what you are deploying. By leveraging the Config philosophies from the 12 Factor App, it should be possible to promote the same build artifact by only changing some environment variables.

Elixir tools for continuous integration

Luckily, there are many tools at our disposal that come out of the box with Elixir. Some of these tools include:

  • mix compile --warnings-as-errors - Running this will return a non-zero exist status if your code contains any warnings.
  • mix xref unreachable --abort-if-any - Running this will return a non-zero exit status if your code makes any references to functions/modules that do no exist (if you are using Elixir 1.10+ this Mix task has been deprecated and its functionality has been rolled into mix compile --warnings-as-errors).
  • mix xref deprecated --abort-if-any - Running this will return a non-zero exit status if your code leverages any functions that have been marked as deprecated (if you are using Elixir 1.10+ this Mix task has been deprecated and its functionality has been rolled into mix compile --warnings-as-errors).
  • mix format --check-formatted - Running this will return a non-zero exist status if any of your source files do not adhere to the format configuration specified in your .formatter.exs. This helps ensure that the codebase has a uniform look and feel for all team members.
  • mix test - Elixir has an amazing builtin testing framework called ExUnit [3]. By running the preceding command you can execute all of your project’s tests and the exit status will be non-zero if any tests failed.

After you’ve incorporated the aforementioned items into your CI flow, it is time to reach for some community tools. Luckily the Elixir ecosystem is packed full of great tools! Below is a listing of a few of the tools that I use day in and day out:

  • Credo [4] - Credo is a configurable static analysis tool that checks your code for design, readability, and consistency issues. In addition, it also enforces its own style guide which can help a codebase stay uniform even if there are many developers in and out of the code.
  • Dialyxir [5] - Dialyzer is an amazing static analysis tool that we inherit from Erlang. In order to streamline its usage in Elixir, the Dialyxir library provides a nice wrapper around Dialyzer which makes it easier to consume from an Elixir perspective. If you make heavy usage of typespecs then this is a great tool for you to ensure that you aren’t introducing any type related errors into your code.
  • Doctor [6] - Doctor is a static analysis tool that focuses on scanning your project’s documentation. It can be used to enforce the presence of typespecs, function docs, and module docs. In addition, it can be configured to return a non-zero exit status if documentation coverage falls below a certain threshold.
  • Bypass [7] - Bypass is a useful library for when you want to mock out external HTTP services for the purposes of your tests. Your mock server can be brought up during your tests and can return pre-canned responses.
  • Faker [8] - Faker is an excellent library for when you need to generate fake data like names, address, phone numbers, etc.
  • ExMachina [9] - If you leverage Ecto in your application and want a streamlined way of generating test data along with associations, then ExMachina is what you are looking for. When paired with Faker, ExMachina can make a great test data creation tool.

Elixir tools for continuous delivery

Once your application has been tested and statically analyzed, you’ll want to deploy it somehow. Below are a few options that are available to you for doing so:

  • Mix Releases [10] - As of Elixir 1.9, the ability to create self contained application releases has been brought into Elixir core (historically you would need to use separate tools like Distillery [11]). Elixir is interesting from a deployment standpoint given that you have the ability to package your application, and the Erlang virtual machine (the BEAM) all within a build artifact. You need to make sure that the machine you build on is the same as the machine that you deploy to, but seeing as though the application+runtime are all bundled, you will not need Elixir or Erlang installed on the target machine.
  • Docker - Like many programming languages available today, Elixir plays nicely with Docker and it is a relatively straightforward task to create a Docker image with your application. You can leverage the base Elixir Docker image and run your application via Mix inside the container for development purposes, or you can leverage Mix Releases and multistage builds [12] to create lightweight images with only your application and the bundled Erlang virtual machine.
  • eDeliver [13] - eDeliver is a tool that can be used to deploy Elixir applications to remote hosts using hot-code upgrades. By using hot-code upgrades, you can update your application with zero downtime on one of more hosts.

Performing CI validations on your local machine

In order to play around with some of the ideas presented here without trying to learn a new CI/CD system, we’ll instead experiment with validating a sample Elixir project using Git hooks. Leveraging Git hooks is a good habit to get into as it will help you validate your code locally before pushing it to your team’s repository. It makes fixing any errors easier given that you don’t need to dig through logs to figure out why builds failed.

To begin, start by cloning a sample repository that I put together on GitHub:

$ git clone

Once we have our project cloned locally, we’ll want to open up our mix.exs file and add the following dependency:

defp deps do
    {:git_hooks, "~> 0.4.0", only: [:test, :dev], runtime: false}

With that in place, we can now open up our config/config.exs file and add the following (feel free to omit the mix xref tasks if you are using Elixir 1.10+):


if Mix.env() != :prod do
  config :git_hooks,
    verbose: true,
    hooks: [
      pre_commit: [
        tasks: [
          "mix clean",
          "mix compile --warnings-as-errors",
          "mix xref deprecated --abort-if-any",
          "mix xref unreachable --abort-if-any",
          "mix format --check-formatted",
          "mix credo --strict",
          "mix doctor --summary",
          "mix test"

With that in place, let’s fetch our new dependency and attempt to commit our code via the terminal:

$ mix deps.get
$ mix git_hooks.install
$ git add .
$ git commit -m "Added git hooks to sample math project"
↗ Running hooks for :pre_commit
✔ `mix clean` was successful
Compiling 1 file (.ex)
warning: variable "num_2" is unused (if the variable is not meant to be used, prefix it with an underscore)
  lib/sample_math.ex:10: SampleMath.sum/2

Compilation failed due to warnings while using the --warnings-as-errors option
× pre_commit failed on `mix`
** (exit) 1
    lib/mix/tasks/git_hooks/run.ex:175: Mix.Tasks.GitHooks.Run.error_exit/1
    lib/mix/tasks/git_hooks/run.ex:128: Mix.Tasks.GitHooks.Run.run_task/3
    (elixir) lib/enum.ex:783: Enum."-each/2-lists^foreach/1-0-"/2
    (elixir) lib/enum.ex:783: Enum.each/2
    (mix) lib/mix/task.ex:331: Mix.Task.run_task/3
    (mix) lib/mix/cli.ex:79: Mix.CLI.run_task/2

As you may have guessed, there is an issue with our project that requires fixing! Luckily, our commit will not go through until we are able to rectify the issue. I’ll leave the fixing of issues in your capable hands, but as we can see there is a problem with the compile step because of the presence of an unused variable. Our validation steps worked just as expected!

While we only performed our validation steps on our local machine, taking these same validation steps to an actual CI/CD pipeline is a relatively simple task. The Git hooks library that we leveraged allows us to run all of our configured steps via mix all. In other words, we can run this command in our CI/CD solution of choice and validate that our code changes pass team standards. The benefit of this is that our CI/CD validation steps can be easily run locally for quick and easy debugging.


Thanks for sticking with me to the end and hopefully you learned a thing or two related to CI/CD and how to go about doing it with an Elixir application. If you would like to learn more about CI/CD or any of the tools that I mentioned, I suggest going through the following resources:

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