The Hex team has been working on building our own Elixir and Erlang Docker images. We think they can be useful for the general community so I am sharing them in this post. The images are built by Bob which is the build service that provides the builds when you use Travis CI, GitHub CI, asdf, and other tools and services.

The reason we are building our own Docker images is because we have identified and run into a few issues with the existing image offerings that we are hoping to address with these images.

Issues with existing offerings

To sum it up:

1. Image tags are not immutable
2. Delay in availability of new versions
3. Cannot pick the combination of versions you want

Our main issue is that image tags are not immutable. If you pull the image elixir:1.9.4 you do not know which Erlang version or OS version you are getting. This is specially a problem when the tag the image points to changes to update the Erlang version. Recently Erlang introduced a breaking change in a minor version that made it an error when you started a client side SSL connection with the honor_cipher_order option, the Elixir image was updated to the new Erlang version which caused hackney to break and our deploys stopped working without any changes to either our dependencies or our Dockerfile.

The images we build with Bob are immutable and the versions used are explicit. An example of an Elixir image tag is 1.10.0-erlang-22.2.4-alpine-3.11.3, here we can see that the Elixir, Erlang and Alpine versions are explicit and there is no reason to update any versions behind the scenes.

Sometimes there are delays in the availability of images for new releases. As I am writing this the new images for Elixir 1.10.0 are not yet available on the official Docker images even though 1.10.0 was released 3 days ago. The images from Bob are built automatically and should be available within 30 minutes of tagged releases.

Our final issue has been that we cannot pick the exact versions we want. From the official images I can install Elixir 1.9.4 but I have no idea what Erlang or OS version will be used. Sometimes we want to use the latest features from Erlang or stay on an older version for stability but this is not an option. The images from Bob are built with all compatible versions of the underlying OS, Erlang, and Elixir.

Possible downsides with Bob’s images

First off, they are currently experimental. This means you should take care before running them in production. We are running them in production for all Hex services without issue so far. While they are experimental the immutability guarantee of image tags may not hold if we find issues that require such changes.

We currently only provide images on the latest Alpine versions. In the future we may build on more systems.

Since we build for all compatible OS, Erlang, and Elixir versions we produce many, many images that use up disk spaces. We haven’t run into any limitations on DockerHub yet, but it is possible we do in the future. As far as I can tell there are no documented limitations.

To build the images faster we use the pre-compiled versions of Erlang and Elixir that Bob already provides. Because of this the original Makefiles are not available so we cannot use make install to install files in the correct locations. Currently we install Elixir and Erlang in the root of the filesystem at /elixir and /erlang respectively. This is obviously not ideal so if anyone has a solution to this problem we are all ears.

Links

To read more about Bob’s docker images go to our README: GitHub - hexpm/bob: The Builder · GitHub. Links to the DockerHub repositories can be found here: hexpm/erlang - Docker Image and here: hexpm/elixir - Docker Image.