Want to test Ansible playbooks that require systemd in Docker? Try this.

Kubernetes and other cloud-native strategies might be putting configuration management out to pasture, but I found myself writing a playbook recently while learning how to create infrastructure as code for Azure. I needed to start my Flask web server and Postgres database with systemd, which isn’t a pattern that’s easily supported by Docker. I got this working with Docker Compose, however, and this post will show you how!

Create a Docker Compose file with the following services:

version: '2.2'
services:
  ansible:
    tty: true # this adds colorized output; you can disable this if you prefer
    privileged: true # required to volume-mount the cgroups pseudofile, as systemd requires it
    build:
      context: .
    volumes:
      - $PWD:/workdir:ro
      - /sys/fs/cgroup:/sys/fs/cgroup:ro # required by systemd
    working_dir: /workdir
  ansible-playbook-under-test:
    extends: ansible

Create a Dockerfile that installs Ansible and systemd. Your base layer should match the actual system onto which this playbook will be deployed.

FROM ubuntu:19.04

# Install Ansible
RUN apt-get -y update && \
    apt-get -y install software-properties-common && \
    apt-add-repository --yes --update ppa:ansible/ansible && \
    apt-get -y install ansible

# Install SystemD since our app uses this upon starting up.
# Cribbed from: https://github.com/j8r/dockerfiles/blob/master/systemd/ubuntu/18.04.Dockerfile
ENV LC_ALL C
RUN apt-get update \
    && apt-get install -y systemd systemd-sysv \
    && apt-get clean \
    && rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*

RUN cd /lib/systemd/system/sysinit.target.wants/ \
    && ls | grep -v systemd-tmpfiles-setup | xargs rm -f $1

RUN rm -f /lib/systemd/system/multi-user.target.wants/* \
    /etc/systemd/system/*.wants/* \
    /lib/systemd/system/local-fs.target.wants/* \
    /lib/systemd/system/sockets.target.wants/*udev* \
    /lib/systemd/system/sockets.target.wants/*initctl* \
    /lib/systemd/system/basic.target.wants/* \
    /lib/systemd/system/anaconda.target.wants/* \
    /lib/systemd/system/plymouth* \
    /lib/systemd/system/systemd-update-utmp*

VOLUME [ "/sys/fs/cgroup" ]
ENTRYPOINT [ "/sbin/init" ]

You might notice that the ENTRYPOINT for this container is /sbin/init. systemd requires init to be PID 1. This is problematic for Docker containers because their ENTRYPOINTs are the only things that can run when they start up and init cannot be backgrounded.

To get around this, run docker-compose up -d ansible-playbook-under-test. This will “boot” the Docker container while allowing you to run something else against it with docker-compose exec.
Run your playbook!
```
docker-compose exec ansible-playbook-under-test ansible-playbook-under-test \
 -vvv \
 playbook.yml
```
The container under test will stay up after your playbook run completes. If your systemd service fails to start, you can look into it using standard tooling (I usually look at the systemd journal first by running journalctl -u $name_of_service).
Turn down your container under test: docker-compose down.

I hope this helps!

neurons firing from a keyboard

thoughts about devops, technology, and faster business from a random guy from dallas.

Want to test Ansible playbooks that require systemd in Docker? Try this.