EDIB — Attempt to create the possibly smallest docker image for an Elixir release
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README.md

Elixir Docker Image Builder (EDIB)

Attempt to create the possibly smallest Docker image for an Elixir release.


Use it in your Elixir project with mix-edib, a mix task to easily package your release image.

(Note: This repo is for documentation only.)


TOC

Showdown

Showdown (shell output of EDIB packaging)

Prerequisites

Next to your Elixir application you need docker and most likely mix-edib. Also do not forget to add distillery as a dependency in your project.

Usage

Either install the mix archive or add mix-edib as a dependency in your Elixir project.

In both cases you definitely need to add distillery as a project dependency!

Mix archive installation

mix archive.install http://git.io/edib-0.9.0.ez

Do not forget to add distillery to your project (in mix.exs):

defp deps do
  [
    {:distillery, "~> 0.10"},
  ]
end

Project dependency

In mix.exs:

defp deps do
  [
    {:distillery, "~> 0.10"},
    {:edib, "~> 0.9"}
  ]
end

Then run:

mix deps.get edib && mix deps.compile edib

Without any mix task (CI environment)

If you want to keep your CI build setup clean, you do not neccessarily have to install Elixir and mix-edib at all.

The docker commands are pretty simple. The information for application/release name and version might be less trivial. (If your CI server/service supports useful ENV vars you should use them.)

artifact creation:

docker run --rm \
  -v /path/to/my_awesome_app:/source \
  -v /path/to/my_awesome_app/tarballs:/stage/tarballs \
  edib/edib-tool:1.0

image creation:

cat /path/to/my_awesome_app/tarballs/my_awesome_app-0.1.0.tar.gz | \
  docker import \
    --change 'CMD trap exit TERM; /app/bin/my_awesome_app foreground & wait' - \
    local/my_awesome_app:0.1.0

tagging with "latest":

docker tag --force local/my_awesome_app:0.1.0 local/my_awesome_app:latest

Test

Start the app in foreground mode:

docker run --rm -it local/my_awesome_app

If everything went well, your release should be up and running now.

How it works

The whole build/packaging process happens in a Docker container. Therefore the only system dependency next to Elixir (for running the mix task) is docker itself.

(If you plan to use EDIB tool in a CI environment you can manually trigger the artifact and image creation, then no Elixir in the host environment of the CI build is needed.)

Step 0: Downloading the EDIB tool

This is just a docker image.

The setup of it can be found at edib-tool/docker-edib-tool.

At Docker Hub: https://hub.docker.com/r/edib/edib-tool/

Because nesting of docker containers can be sometimes quite cumbersome, we split the packaging into two steps, so we can stay on the host were the docker daemon is running.

If you run everything already within containers keep in mind to properly propagate the docker environment with --privileged and volumes for the docker executable and the socket.

Step 1: Creating the artifact (tarball archive)

In the first step only the so-called artifact is created. This is a tarball archive containing the application release and its system dependencies (because the Erlang VM is not statically linked and therefore needs some libraries).

Step 2: Creating the final docker image

With the artifact from the previous step we can simply create a docker image of it.

This is a pretty easy step (utilizing docker import).

Caveats

Beware of hex packages which have dependencies to the system (shared libraries or tools/binaries). If step 1 fails then it's probably because of missing stuff.

Because the docker image is based on an Alpine Linux it also might be possible that not everything you would need is available. Sometimes it can be manually compiled, as long as the library or application has no mandatory dependency to glibc (Alpine is using musl libc instead).

FAQ

Why?

Not just for fun, but to save my sanity by not using overly huge, crazily humongous, tremendously gigantic Ubuntu images.*

There is no need for custom application images which reach the 1 GB mark.

And there is no need for images with risky and unnecessary cruft in it.

*) Yes, I know, the Ubuntu base image is less than 200 MB, but when you start building your custom image onto it, then it will quickly grow as hell even if you try to be careful.

Does it work for Phoenix apps?

If you ignore the static asset compilation step, then yes, it does. (You can adjust the package/Makefile.app to your needs.)

After a successful build you can test it like this:

docker run --rm -e "PORT=4000" -p 4000:4000 local/release-image

You need to set the PORT environment variable, otherwise the app will just crash (only in Phoenix's default config).

How different is it from ...?

I'm glad you asked! (And if you find similar projects, point me to them; I'd like to have a look at them, too.)

msaraiva's "Erlang/Elixir on Alpine Linux"

Under msaraiva/alpine-erlang you'll find a pretty good guide on how to build pretty small images/containers for your Erlang or Elixir application. Yes, you've read it correctly, I said application, not release.

Of course, there is an example for a Phoenix app with exrm, but unfortunatelly it requires dependencies on your host. I'd like to avoid or at least to minimalize this burden. While in development of your application, you don't care so much about this downside, but I think here more about CI/CD (continuous integration/deployment).

The other example "Hello world (compilation inside the container)" is more the flavor of build process I'd like to see. I think this is the much better approach we should aim for.

When you try this and inspect the container content, you will notice some differences from this project:

  • the source code is visible
  • there are still tools present which are not needed (like apk)

In conclusion: Either you have a bit crufty image or a crufty system. Mostly this is not a big deal, but was never my goal.

Is it free?

Of course it is!