Mar 12, 2019 4 min read

Docker with Ansible Part II : using variables

Ansible supports variables which allows you to add a level of abstraction in an Ansible file and to manage it more efficiently in case of change: a change will be applied globally rather than having to change one by one the options.

This is the second post in a series of 3 articles about Docker and Ansible:
Part I: from Docker Compose to Ansible
Part II: using variables
Part III: using vault to encrypt sensitive information

Understand variables

Here is an example :

---
- hosts: localhost

  vars:
    var_1: variable

  tasks:

  - debug:
      msg: "This is a {{ var_1 }}."

Result:

PLAY [localhost] *************************************************************************************************************************************************************************************************************************

TASK [Gathering Facts] *******************************************************************************************************************************************************************************************************************
ok: [localhost]

TASK [debug] *****************************************************************************************************************************************************************************************************************************
ok: [localhost] => {
    "msg": "This is a variable."
}

PLAY RECAP *******************************************************************************************************************************************************************************************************************************
localhost                  : ok=2    changed=0    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0

File without variables

We will use the file created in the previous post as a reference:

---
- hosts: localhost

  tasks:

  - name: Create network
    docker_network:
      name: network_app
      ipam_options:
        subnet: '172.16.98.0/24'
        
  - name: Run Nginx container
    docker_container:
      name: 'nginx'
      recreate: true
      restart_policy: unless-stopped
      image: 'nginx:latest'
      published_ports:
        - "80:80"
        - "443:443"
      volumes:
        - "./app.conf:/etc/nginx/conf.d/app.conf"
        - "php-app:/var/www/app"
      networks:
        - name: "network_app"        - 

  - name: Run PHP container
    docker_container:
      name: 'php'
      recreate: true
      restart_policy: unless-stopped
      image: 'php:7-fpm'
      volumes:
        - "php-app:/var/www/app"
      networks:
        - name: "network_app"
        
  - name: Run Percona container
    docker_container:
      name: 'percona'
      recreate: true
      restart_policy: unless-stopped
      image: 'percona:latest'
      volumes:
        - "percona:/var/lib/mysql"
      env:
        MYSQL_ROOT_PASSWORD: "secret_root"
        MYSQL_DATABASE: "db"
        MYSQL_USER: "db_user"
        MYSQL_PASSWORD: "secret_user"
      networks:
        - name: "network_app"

Play with variables

To add variables we will place a vars block above the tasks block.

---
- hosts: localhost

  vars:
    docker_network: network_app
    docker_network_backend: backend
    mysql_db_name: db
    mysql_db_user: ghost
    mysql_db_host: db
    mysql_db_password: secret_user
    mysql_db_root_password: secret_root
  tasks:
    [...]

And for include in the tasks part :

  - name: Create network
    docker_network:
      name: "{{ docker_network }}"
      ipam_options:
        subnet: '172.16.98.0/24'

File with variables

Here an example with few variables :

---
- hosts: localhost

  vars:
    docker_network: network_app
    mysql_db_name: db
    mysql_db_user: ghost
    mysql_db_host: db
    mysql_db_password: secret_user
    mysql_db_root_password: secret_root

  tasks:

  - name: Create network
    docker_network:
      name: "{{ docker_network }}"
      ipam_options:
        subnet: '172.16.98.0/24'
        
  - name: Run Nginx container
    docker_container:
      name: 'nginx'
      recreate: true
      restart_policy: unless-stopped
      image: 'nginx:latest'
      published_ports:
        - "80:80"
        - "443:443"
      volumes:
        - "./app.conf:/etc/nginx/conf.d/app.conf"
        - "php-app:/var/www/app"
      networks:
        - name: "{{ docker_network }}"

  - name: Run PHP container
    docker_container:
      name: 'php'
      recreate: true
      restart_policy: unless-stopped
      image: 'php:7-fpm'
      volumes:
        - "php-app:/var/www/app"
      networks:
        - name: "{{ docker_network }}"
        
  - name: Run Percona container
    docker_container:
      name: 'percona'
      recreate: true
      restart_policy: unless-stopped
      image: 'percona:latest'
      volumes:
        - "percona:/var/lib/mysql"
      env:
        MYSQL_ROOT_PASSWORD: "{{ mysql_db_root_password }}"
        MYSQL_DATABASE: "{{ mysql_db_name }}"
        MYSQL_USER: "{{ mysql_db_user }}"
        MYSQL_PASSWORD: "{{ mysql_db_password }}"
      networks:
        - name: "{{ docker_network }}"

Multiples variables

Here are examples with multiples variables :

---
- hosts: localhost

  vars:
    var_1: "the first variable"
    var_2: "the second variable"

  tasks:

  - debug:
      msg: "This is {{ var_1 }} and this is {{ var_2 }}"

and the result that goes with it:

PLAY [localhost] *************************************************************************************************************************************************************************************************************************

TASK [Gathering Facts] *******************************************************************************************************************************************************************************************************************
ok: [localhost]

TASK [debug] *****************************************************************************************************************************************************************************************************************************
ok: [localhost] => {
    "msg": "This is the first variable and this is the second variable"
}

PLAY RECAP *******************************************************************************************************************************************************************************************************************************
localhost                  : ok=2    changed=0    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0

Variables are very flexible :

---
- hosts: localhost

  vars:
    var_1: app_
    var_2: network

  tasks:

  - debug:
      msg: "{{ var_1 }}{{ var_2 }}"

and the result :

PLAY [localhost] *************************************************************************************************************************************************************************************************************************

TASK [Gathering Facts] *******************************************************************************************************************************************************************************************************************
ok: [localhost]

TASK [debug] *****************************************************************************************************************************************************************************************************************************
ok: [localhost] => {
    "msg": "app_network"
}

PLAY RECAP *******************************************************************************************************************************************************************************************************************************
localhost                  : ok=2    changed=0    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0

Prefix in Docker Compose way

With variables you can prefix resources really easily like Docker Compose do on a deployment (for a stack named app all of the resources will be prefixed with the stack name (example : app_$VOLUME, app_$NETWORK, app_$CONTAINER. Simply add a variables and prefix the resources you want.

For example, you want to prefix with app_ :

---
- hosts: localhost

  vars:
    docker_network: network
    stack_prefix_name: app_

  tasks:

  - name: Create network
    docker_network:
      name: "{{ stack_name}}{{ docker_network }}"
      ipam_options:
        subnet: '172.16.98.0/24'
  - name: Run Nginx container
    docker_container:
      name: '{{ stack_name}}nginx'
      recreate: true
      restart_policy: unless-stopped
      image: 'nginx:latest'
      published_ports:
        - "80:80"
        - "443:443"
      volumes:
        - "./app.conf:/etc/nginx/conf.d/app.conf"
        - "{{ stack_name }}_php-app:/var/www/app"
      networks:
        - name: "{{ stack_name}}{{ docker_network }}"

Docker Compose up and down

Now that we have a super functionnal Ansible may be we want to easily up and down the stack. It's possible with the state option.

For docker_network there's 2 state : absent and present. For docker_container there's 4 states : absent, present, stopped, started.

We'll set the default state with 2 variables : network_state (the force disconnect all containers from network when the network is deleted) and container_state and add the option to the different resources :

---
- hosts: localhost
  
  vars:
    container_state: started
    network_state: present
  
  tasks:
  - name: Create network
    docker_network:
      name: "{{ docker_network }}"
      ipam_options:
        subnet: '172.16.98.0/24'
      state: "{{ network_state }}"
      force: yes
      
[...]

  - name: Run PHP container
    docker_container:
      name: 'php'
      recreate: true
      restart_policy: unless-stopped
      image: 'php:7-fpm'
      volumes:
        - "php-app:/var/www/app"
      networks:
        - name: "{{ docker_network }}"
      state: "{{ container_state }}"

[...]

By default the stack will be created. If we want to delete it, just change the state with the --extra-vars or -e option :

$ ansible-playbook playbook.yml --extra-vars "container_state=absent network_state=absent"

Thanks to these option you can do whatever you want, for example just create the containers :

$ ansible-playbook playbook.yml --extra-vars "container_state=present"

Stop the containers :

$ ansible-playbook playbook.yml --extra-vars "container_state=stopped"

Or delete the containers but keep the network :

$ ansible-playbook playbook.yml --extra-vars "container_state=absent"

Resources :
https://docs.ansible.com/ansible/latest/user_guide/playbooks_variables.html
https://docs.ansible.com/ansible/latest/modules/docker_network_module.html
https://docs.ansible.com/ansible/2.3/docker_container_module.html
https://docs.ansible.com/ansible/2.4/ansible-playbook.html

Feel free to correct me if you see any typo or if something seems wrong to you.
You can send me an email or comment below.

Understand variables

File without variables

Play with variables

File with variables

Multiples variables

Prefix in Docker Compose way

Docker Compose up and down

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