Docker role mhutter.docker-systemd-service

Generic role for creating systemd services to manage docker containers.

Example

- name: Start WebApp
  include_role:
    name: mhutter.docker-systemd-service
  vars:
    container_name: myapp
    container_image: myapp:latest
    container_links: [ 'mysql' ]
    container_volumes:
      - '/data/uploads:/data/uploads'
    container_ports:
      - '3000:3000'
    container_env:
      MYSQL_ROOT_PASSWORD: "{{ mysql_root_pw }}"
    container_labels:
      - 'traefik.enable=true'

This will create:

• A file containing the env vars (either /etc/sysconfig/mysql or /etc/default/mysql).
• A systemd unit which starts and stops the container. The unit will be called <name>_container.service to avoid name clashes.

Role variables

• container_name (required) - name of the container

Docker container specifics

• container_image (required) - Docker image the service uses
• container_args - arbitrary list of arguments to the docker run command
• container_cmd - optional command to the container run command (the part after the image name)
• container_env - key/value pairs of ENV vars that need to be present
• container_volumes (default: []) - List of -v arguments
• container_ports (default: []) - List of -p arguments
• container_link (default: []) - List of --link arguments
• container_labels (default: []) - List of -l arguments
• container_docker_pull (default: yes) - whether the docker image should be pulled

Systemd service specifics

• enabled (default: yes) - whether the service should be enabled
• masked (default: no) - whether the service should be masked
• state (default: started) - state the service should be in - set to absent to remove the service.

Installation

This role requires the docker python module. Install it with pip3 install docker or apt install python3-docker (or drop the 3 for python 2.x).

Put this in your requirements.yml:

- role: mhutter.docker-systemd-service

and run ansible-galaxy install -r requirements.yml.

Gotchas

• When the unit or env file is changed, systemd gets reloaded but existing containers are NOT restarted.
• Make sure to quote values for container_ports, container_volumes and so on, especially if they contain colons (:). Otherwise YAML will interpret them as hashes/maps and ansible will throw up.

About orchestrating Docker containers using systemd.

The concept behind this is to define systemd units for every docker container. This has some benefits:

• systemd is a well-known interface
• all services are controllable via the same tool (systemctl)
• all logs are accessible via the same tool (journalctl)
• dependencies can be defined
• startup behaviour can be defined
• by correctly defining the unit (see below), we can ensure we always have a clean container.

Here is an example myapp_container.service unit file (about what's produced by above code):

[Unit]
# define dependencies
After=docker.service
PartOf=docker.service
Requires=docker.service

[Service]
# Load ENV vars from a file. Note that this env vars will only be
# accessible in the context of the Exec* commands, and not within the
# container itself. To make env-vars accessible within the Container, we use
# the `--env-file` flag for the `docker run` command.
EnvironmentFile=/etc/sysconfig/myapp

# Even though we explicitly run the container using the `--rm` flag, there
# may be leftover containers (eg. after a system-, docker- or app-crash).
# Starting a container with an existing name will always fail.

ExecStartPre=-/usr/bin/docker rm -f myapp

# actually run the container.
# `--name` to identify the container
# `--rm` ensure the container is removed after stopping
# `--env-file` make ENV vars accessible to app
# `--link mysql` link to a container named `mysql`. The DB will then be
#                accesible at `mysql:3306`
# `-v` mount `/data/uploads` into the container
# `-p 3000:3000` expose port 3000 on the network
ExecStart=/usr/bin/docker run --name myapp --rm --env-file /etc/sysconfig/myapp --link mysql -v /data/uploads:/data/uploads -p 3000:3000 registry.cust.net/myapp/myapp:latest
# note that there is no `--restart` parameter. This is because restarting
# is taken care of by `systemd`.

# Stop command.
ExecStop=/usr/bin/docker stop myapp

# Ensure log messages are correctly tagged in the system log.
SyslogIdentifier=myapp

# Auto-Restart the container after a crash.
Restart=always


[Install]
# make sure service is started after docker is up
WantedBy=docker.service