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Ansible tutorial

Migrating to roles!

Now that our playbook is done, let's refactor everything! We'll replace our plays with roles. Roles are just a new way of organizing files but bring interesting features. I won't go into great lengths here, since they're listed in Ansible's documentation, but my favorite is probably roles dependencies: role B can depend on another role A. Thus, when applying role B, role A will automatically be applied too. We'll see this in the next chapter, but for now, let's refactor our playbook to use roles.

Roles structures

Roles add a bit of "magic" to Ansible: they assume a specific file organization. While there is a suggested layout regarding roles, you can organize things the way you want using includes. However, role's conventions help building modular playbooks, and housekeeping will be much simpler. Rubyists would call this "convention over configuration".

The file layout for roles looks like this:

       |   |
       |   |_main.yml
       |   |_...
       |   |
       |   |_file1
       |   |_...
       |   |
       |   |_main.yml
       |   |_some_other_file.yml
       |   |_ ...
       |   |
       |   |_main.yml
       |   |_some_other_file.yml
       |   |_ ...
       |   |
       |   |_main.yml
       |   |_some_other_file.yml
       |   |_ ...
       |   |
       |   |_template1.j2
       |   |_...
           |_ ...

The files named main.yml are not mandatory. However, when they exist, roles will add them to the play automatically. You can use this file to include other tasks, handlers, ... in the play. We'll see that in a minute.

Note that there is also a vars and a meta directory. vars is used when you want to put a bunch of variables regarding the roles. However, I don't like setting vars in roles (or plays) directly. I think variables belong to configuration, while plays are the structure. In other words, I see plays and roles as a factory, and data as inputs to this factory. So I really prefer to have "data" (e.g. variables) outside roles and play. This way, I can share my roles more easily, without worrying about exposing too much about my servers. But that's just a personal preference. Ansible just lets you do it the way you want.

But you have some vars that you hardly want to change. For instance, if you have a role for nginx that pulls the .deb package from a PPA, you might want to add the PPA address in vars/main.yml. It is something that you can configure, but that will be mostly static 99% of the time. Using vars will let you pull out this information out of your role, making it more generic. But really, this is a matter of taste.

However, for real vars (e.g. things you would like to use in a configuration file generated by a template), you can set defaults for roles, and this is a recommended practice. Using sane defaults ensures your role always work. For instance, you could set the number of pre-forked servers for your apache server. The best place to put the defaults is... you guessed it, the defaults directory.

The meta directory is where you can add dependencies, and it's really a neat feature. We'll see that later.

Note that roles sit in the roles directory, which is also cool since it will reduce top level ansible playbook clutter. But you can configure Ansible to use an alternate directory to store role (see roles_path variable in ansible.cfg). This way you can setup a 'central place ' for all your roles, and use them in all your playbooks.

Creating the Apache role

Ok, now that we know the required layout, we can create our apache role from our apache playbook.

The steps required are really simple:

  • create the roles directory and apache role layout
  • extract the apache handler into roles/apache/handlers/main.yml
  • move the apache configuration file awesome-app into roles/apache/files/
  • create a role playbook

Creating the role layout

This is what has been done to convert step-11 apache files into a role:

mkdir -p step-12/roles/apache/{tasks,handlers,files}

Now we need to copy the tasks from apache.yml to main.yml, so this file looks like this:

- name: Updates apt cache
    update_cache: true

- name: Installs necessary packages
    pkg: "{{ item }}"
    state: latest
    - apache2
    - libapache2-mod-php5
    - git


- name: Deactivates the default ssl virtualhost
  command: a2dissite default-ssl
    - restart apache

The file is not fully reproduced, but it is exactly the content of apache.yml between tasks: and handlers:.

Note that we also have to remove references to files/ and templates/ directories in tasks. Since we're using the roles structure, Ansible will look for them in the right directories.

Extracting the handler

We can extract the handlers part and create step-12/roles/apache/handlers/main.yml:

- name: restart apache
    name: apache2
    state: restarted

Moving the configuration file

As simple as:

cp step-11/files/awesome-app step-12/roles/apache/files/

At this point, the apache role is fully working, but we need a way to invoke it.

Create a role playbook

Let's create a top level playbook that we'll use to map hosts and host groups to roles. We'll call it site.yml, since our goal is to have our site-wide configuration in it. While we're at it, we'll include haproxy in it too:

- hosts: web
    - apache

- hosts: haproxy
    - haproxy

That wasn't too hard.

Now let's create the haproxy role:

mkdir -p step-12/roles/haproxy/{tasks,handlers,templates}
cp step-11/templates/haproxy.cfg.j2 step-12/roles/haproxy/templates/

Instead of creating the haproxy role directory structure using mkdir, it's possible to use ansible-galaxy:

ansible-galaxy --offline init step-12/roles/haproxy
cp step-11/templates/haproxy.cfg.j2 step-12/roles/haproxy/templates/

then extract the handler, and remove reference to templates/.

We can try out our new playbook with:

ansible-playbook -i step-12/hosts step-12/site.yml

If eveything goes well, we should end up with a happy "PLAY RECAP" like this one:

host0.example.org          : ok=5    changed=2    unreachable=0 failed=0
host1.example.org          : ok=10   changed=5    unreachable=0 failed=0
host2.example.org          : ok=10   changed=5    unreachable=0 failed=0

You may have noticed that running all roles in site.yml can take a long time. What if you only wanted to push changes to web? This is also easy, with the limit flag:

ansible-playbook -i step-12/hosts -l web step-12/site.yml

This concludes our migration to roles. It was quite easy, and adds a bunch of features to our playbook that we'll use in a future step.

In step-13, we will see how we can use tags to select which parts of our playbook we want to run.