Building Elixir/Phoenix Release With Docker

Jun 20, 2020

Estimated Reading Time: 7 minutes (1309 words)

This is a short post about how I build my Elixir/Phoenix releases with Docker and extract the tarball that will be deployed to production. In this approach, we are just building the release with Docker. We are not building the image to run our application in a Docker container.

This post assume that you have the basic knowledge of Docker, building Elixir release and using Elixir 1.9.3 and above, where :tar options is supported in Elixir releases. Your mix.exs should also have the similar configuration as below:

def project do
    releases: [
      app_name: [
        # Ask mix release to build tarball of the release.
        steps: [:assemble, :tar]

For more, can refer to the mix release documentation.

This post is break down into the following sections:

By the end of this post, you should be able to build your Elixir/Phoenix application by just running:


Why Docker?

To deploy a release to a target (your production server), most of the time you are required to build the release in the host with the same environment. To quote the Mix Release documentation:

…to deploy straight from a host to a separate target, the Erlang Runtime System (ERTS), and any native dependencies (NIFs), must be compiled for the same target triple.

Hence, if you are using a Macbook, and want to build you release and deploy to a Ubuntu 18.04 server, you’ll need to build your release on a Ubuntu 18.04 virtual machine (VM).

Unless you configure it to include_erts: false. Not going to dive deep into this, but if you are interested into it, feel free to refer to the documentation at here.

Without Docker

Without using Docker, normally the common approaches are:

I previously setup a Ubuntu VM with Vagrant and have Ansible script that provision the VM and build the release in the VM locally. This approach have more dependencies. You’ll need to understand and install both Vagrant and Ansible to make this happen.

With Docker

With Docker, all you need is to learn and install Docker.

After getting familiar with Docker, I experiment with building Elixir release with Docker, which turns out to be fairly simple, thanks to the resource available online. I end up gluing it all together with some bash script to build the release in Docker and extract the tarball from the Docker images.

Writing the Dockerfile

To build your Docker image, you’ll first need to write the Dockerfile. In the process of writing these Dockerfile, there are a few references that I refer to, which are:

While we are not using distillery to generate our releases, the documentation is still quite relevant especially when we want to build our release with Docker. It’s not until I am more familiar with Docker and wrote my own bash scripts that I realized that the documentation of distillery is really good.

Parent Image

Depending on your production environment , you might just want to use the official Elixir image as your parent image, which is based on Debian.

FROM elixir:1.9.0 AS build

and skip to the next section.

However if you are using Ubuntu 18.04 on your production machine, you can use the following Dockerfile as the base image for Elixir in Ubuntu 18.04.

FROM ubuntu:18.04

  apt-get update -y && \
  apt-get install -y git curl wget locales gnupg2 build-essential && \
  locale-gen en_US.UTF-8 && \
  wget && \
  dpkg -i erlang-solutions_2.0_all.deb && \
  rm erlang-solutions_2.0_all.deb && \
  apt-get update -y && \
  curl -sL | bash - && \
  apt-get install -y esl-erlang elixir nodejs && node -v && npm -v

CMD ["/bin/bash"]

I named it as Dockerfile.ubuntu in my application root directory and run the following command to build the parent image:

docker build -t ubuntu-elixir -f Dockerfile.ubuntu .

Build Image

Writing the rest of the Dockerfile for building release is fairly straightforward as I am referring to the Phoenix “Deploying with Releases” documentation.

FROM ubuntu-elixir as build

# ===========
# Application
# ===========

# prepare build dir

# install hex + rebar
RUN mix local.hex --force && \
    mix local.rebar --force

# set build ENV

# install mix dependencies
COPY mix.exs mix.lock ./
COPY config config
RUN mix do deps.get, deps.compile

# build assets
COPY assets/package.json assets/package-lock.json ./assets/
RUN npm --prefix ./assets ci --progress=false --no-audit --loglevel=error

COPY priv priv
COPY assets assets
RUN npm run --prefix ./assets deploy
RUN mix phx.digest

# compile and build release
COPY lib lib
# uncomment COPY if rel/ exists
# COPY rel rel
RUN mix do compile, release

FROM scratch AS app

COPY --from=build /app/_build/prod/app_name-*.tar.gz ./

CMD ["/bin/bash"]

There are some minor differences in this Dockerfile compared to the one in the documentation. For example:

Now you can build your release by running:

docker build -t app_name_server .

Extracting tar file

After building the image, we can then extract the tar file from the image by running the following command:

# You could also just manually specifying your app name and version.
APP_NAME="$(grep 'app:' mix.exs | sed -e 's/\[//g' -e 's/ //g' -e 's/app://' -e 's/[:,]//g')"
APP_VSN="$(grep 'version:' mix.exs | cut -d '"' -f2)"

id=$(docker create ${APP_NAME}_server)
docker cp $id:/app/${TAR_FILENAME} .
docker rm $id

Here are the explanation of the main commands we run:

Glue it all together with a simple bash script

Lastly, with some bash script, we can glue the build and extraction process all into a single script build:


# Setting the flag to exit on error

# Without this, for example, when we didn't run Docker daemon
# the script will still continue to execute despite of the error.
set -e

# Get App info (which is copied from Distillery documentation)
APP_NAME="$(grep 'app:' mix.exs | sed -e 's/\[//g' -e 's/ //g' -e 's/app://' -e 's/[:,]//g')"
APP_VSN="$(grep 'version:' mix.exs | cut -d '"' -f2)"

# Build image
docker build -t ${APP_NAME}_server .

# Extract tar
id=$(docker create ${APP_NAME}_server)
docker cp $id:/app/${TAR_FILENAME} .
docker rm $id

After saving the build file, you need to make it executable by running:

chmod +x build

Now, you can just build your Phoenix application by running ./build.

Wrap Up

Building the release and getting the tarball is the very first step of deployment. The next steps of deployment normally involves:

I’ll write up a upcoming post in the future to cover on all these steps in the future. Stay tuned.

The same deployment process is also used in my open source TIL project. While it's not documented clearly, the build and deploy script are there for references.

At the time of writing, the deploy script includes buildkite command to download the tar file, so you might need to comment out that specific line to run directly