Building docker images for Elixir applications

TL;DR: Use exrm to speed up working with Elixir and Docker. Time of running docker pull dropped from 5m to 16s.

Docker among many other things solves problem of deploys. It makes them easy to perform and repeatable. I can deploy the same docker image many times on different machines. Developers, testers, QAs and Ops can work with almost identical environment. Performing manual tests, automated tests and stress tests in parallel saves a lot of time.

Up to last week during docker pull, docker had to perform number of steps:

During pull docker waits for layers, if next layer depends on it. It means, that it can’t pull Elixir layer before Erlang layer is ready and it can’t pull application before Elixir is ready.
On my development machine (with the base image precached) docker pull took about five minutes to complete. In case, you are interested, we used this docker image, that installed Elixir and Erlang.

As I said before those 5 minutes pulls are performed many times during the day.

5 minutes * number of environments * number of features we want to push to production adds up quickly.

Can we somehowe speed up the process? Yes! Erlang introduces concept of releases. A release is a minimal self contained build. Releases include Erlang runtime, so you don’t have to have Elixir or Erlang installed.

Releases were historically painful to build, so there are tools that do if for you using sane defaults. relx for Erlang and exrm for Elixir.

Now docker pull performs only two steps:

  • Get the base image
  • Pull our application files

And it takes only 16s!

Is there a catch? Yes, there is.

We also liked to perform unit tests in docker image with mix tests, but exrm contains only application code. No tests code, no mix at all.

We use the old image with all dependencies for unit tests. After they finish, we build a release and create new docker image, which is then pulled many times by other teams.

If you are working with Phoenix web framework, there is a great step by step guide for setting up relx with Phoenix.

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