When packaging an application as a Docker container it is too easy to just be lazy and put FROM debian (other distributions are available, replace debian with your distribution of choice). For sure it is going to work, but you have just included dozens of libraries and binaries that your application just does not need. An image that could be tens of megabytes is now at least several hundred - we are building containers not virtual machines here!

One of the things I like about Go is that typical application binaries are small with no runtime dependencies. Fewer dependencies mean less patching and security headaches. The less friction in the CI build cycle, the better. Go achieves this by having statically linked applications meaning that just one binary is necessary in ADD, and they are typically built from scratch (etcd as a good example).

Erlang was designed to be embedded in telecoms equipment, so we must be able to package applications in Docker with a small footprint too?

This repository contains a regular erlang application that is packaged in a docker container from scratch. You will need both erlang installed and docker service running preferably on a Linux environment. The release needs to be built on Linux to be able to run on Linux because we are going include the ERTS.

On MacOS you might want to run shortishly/docker-erlang which will give you erlang and docker packaged together in a shell (brew install docker-machine if you don't already have it):

docker run \
       -v /var/run/docker.sock:/var/run/docker.sock \
       -t \
       -i \
       --rm \
       shortishly/docker-erlang \
       /bin/bash

Clone and build this erlang-in-docker-from-scratch repository, which contains a minimal erlang application that builds a release into the _rel directory:

git clone https://github.com/shortishly/erlang-in-docker-from-scratch.git eidfs
cd eidfs
make

The eidfs application includes a simple cowboy "Hello World!" resource to show the container working. It also provides a secure shell daemon using shelly into the BEAM enabling maintenance on the dockerised application.

At the end of make a standard erlang release for the eidfs application is now present in the _rel directory. To make it run inside a scratch container we need to include any runtime dependencies too. This is where mkimage comes in:

./bin/mkimage

The ./bin/mkimage script copies in any dynamic libraries that ERTS needs to run the erlang release:

'/lib/x86_64-linux-gnu/libc.so.6' -> '_rel/eidfs/lib/x86_64-linux-gnu/libc.so.6'
'/lib/x86_64-linux-gnu/libdl.so.2' -> '_rel/eidfs/lib/x86_64-linux-gnu/libdl.so.2'
'/lib/x86_64-linux-gnu/libm.so.6' -> '_rel/eidfs/lib/x86_64-linux-gnu/libm.so.6'
'/lib/x86_64-linux-gnu/libpthread.so.0' -> '_rel/eidfs/lib/x86_64-linux-gnu/libpthread.so.0'
'/lib/x86_64-linux-gnu/librt.so.1' -> '_rel/eidfs/lib/x86_64-linux-gnu/librt.so.1'
'/lib/x86_64-linux-gnu/libtinfo.so.5' -> '_rel/eidfs/lib/x86_64-linux-gnu/libtinfo.so.5'
'/lib/x86_64-linux-gnu/libutil.so.1' -> '_rel/eidfs/lib/x86_64-linux-gnu/libutil.so.1'
'/lib/x86_64-linux-gnu/libz.so.1' -> '_rel/eidfs/lib/x86_64-linux-gnu/libz.so.1'

It also copies /bin/sh so that we can run the release too. We can build a docker image for the release using the following command:

docker build \
       --build-arg REL_NAME=$(bin/release_name) \
       --build-arg ERTS_VSN=$(bin/system_version) \
       --pull=true \
       --no-cache=true \
       --force-rm=true \
       -t $(bin/release_name):$(bin/version) .

The $(bin/release_name) $(bin/system_version) and $(bin/version) are short escripts that respond with the release name, system ERTS version and the application version respectively.

Quite a lot of effort, what is the reward? Try docker images and look at the size of the resultant container:

REPOSITORY                 TAG                 IMAGE ID            CREATED             SIZE
eidfs                      0.0.1               6748931f94e4        4 seconds ago       16.74 MB

We have a docker packaged erlang release in ~17MB. Lets run it!

docker run \
       --name $(bin/release_name) \
       -e SHELLY_AUTHORIZED_KEYS="$(cat ~/.ssh/authorized_keys)" \
       -d \
       $(bin/release_name):$(bin/version)

The SHELLY_AUTHORIZED_KEYS will copy your public keys into the Docker container so that you can ssh directly into the BEAM to perform any operational maintenance. If you'd prefer not to, just remove that line from docker run.

Check the logs using docker logs $(bin/release_name) and you will see lots of application startup messages from SASL.

You can find the IP address of the containes that is running by using docker inspect:

docker inspect --format={{.NetworkSettings.IPAddress}} $(bin/release_name)

We can curl the static "Hello World!" cowboy response by running:

curl http://$(docker inspect --format={{.NetworkSettings.IPAddress}} $(bin/release_name))/hello

If you included the SHELLY_AUTHORIZED_KEYS in your docker run above you can also ssh directly into the BEAM running inside the docker container:

ssh $(docker inspect --format={{.NetworkSettings.IPAddress}} $(bin/release_name))

The first time you connect you will need accept the ssh host key for the application:

ssh $(docker inspect --format={{.NetworkSettings.IPAddress}} $(bin/release_name))
The authenticity of host '172.17.0.2 (172.17.0.2)' can't be established.
RSA key fingerprint is SHA256:1EIgSYLN9pP9mwvnBf8ibQ/1bpEangTKprKKWJ9jQ7s.
RSA key fingerprint is MD5:69:b8:52:22:30:c7:56:b2:f9:66:e6:39:68:7f:9e:b3.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added '172.17.0.2' (RSA) to the list of known hosts.
Eshell V7.2.1  (abort with ^G)
(minerl@127.0.0.1)1>

You can type exit(). to quit the shell, keeping the container running.

You might notice that the ENTRYPOINT used in the Dockerfile directly invokes erlexec. I have done this to reduce dependencies further so that the release, ERTS dynamic libraries, and /bin/bash only are present in the container.

FROM scratch
MAINTAINER Peter Morgan <peter.james.morgan@gmail.com>

ARG REL_NAME
ARG REL_VSN=1
ARG ERTS_VSN

ENV BINDIR /erts-${ERTS_VSN}/bin
ENV BOOT /releases/${REL_VSN}/${REL_NAME}
ENV CONFIG /releases/${REL_VSN}/sys.config
ENV ARGS_FILE /releases/${REL_VSN}/vm.args

ENV TZ=GMT

ENTRYPOINT exec ${BINDIR}/erlexec \
           -boot_var /lib \
           -boot ${BOOT} \
           -noinput \
           -config ${CONFIG} \
           -args_file ${ARGS_FILE}

ADD _rel/${REL_NAME}/ /