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Ansible Playbooks vs Roles

According to the Ansible docs, a Playbook is:

...the basis for a really simple configuration management and multi-machine deployment system, unlike any that already exist, and one that is very well suited to deploying complex applications.

And, again, according to those same docs, a Role are:

...ways of automatically loading certain vars_files, tasks, and handlers based on a known file structure. Grouping content by roles also allows easy sharing of roles with other users.

However the distinction between these and their different use cases is not immediately obvious to me. For instance, if I configure my /etc/ansible/hosts file to look like:

[databases]
mydb01.example.org
mydb02.example.org

[mail_servers]
mymail01.example.org
mymail_dr.example.org

...then what is this "[databases]" entry...a role? Or the name of a playbook YAML file somewhere? Or something else?!?

If someone could explain to me the differences on these, my understanding of Ansible would be greatly enhance!

Solution

  • Playbook vs Role vs [databases] and similar entries in /etc/ansible/hosts

    [databases] is a single name for a group of hosts. It allows you to reference multiple hosts by a single name.

    Role is a set of tasks and additional files to configure host to serve for a certain role.

    Playbook is a mapping between hosts and roles.

    Example from documentation describes example project. It contains two things:

    Inside playbook (webservers.yml) you have something like:

    ---
- hosts: webservers <- this group of hosts defined in /etc/ansible/hosts, databases and mail_servers in example from your question
  roles: <- this is list of roles to assign to these hosts
    - common
    - webservers

    If Playbooks are defined inside of YAML files, then where are Roles defined?

    They are defined inside roles/* directories. Roles are defined mostly using YAML files, but can also contain resources of any types (files/, templates/). According to documentation role definition is structured this way:

    • If roles/x/tasks/main.yml exists, tasks listed therein will be added to the play
    • If roles/x/handlers/main.yml exists, handlers listed therein will be added to the play
    • If roles/x/vars/main.yml exists, variables listed therein will be added to the play
    • If roles/x/meta/main.yml exists, any role dependencies listed therein will be added to the list of roles (1.3 and later)
    • Any copy tasks can reference files in roles/x/files/ without having to path them relatively or absolutely
    • Any script tasks can reference scripts in roles/x/files/ without having to path them relatively or absolutely
    • Any template tasks can reference files in roles/x/templates/ without having to path them relatively or absolutely
    • Any include tasks can reference files in roles/x/tasks/ without having to path them relatively or absolutely

    The most important file is roles/x/tasks/main.yml, here you define tasks, which will be executed, when role is executed.

    Aside from the ansible.cfg living on the Ansible server, how do I add/configure Ansible with available Playbooks/Roles? For instance, when I run ansible-playbook someplaybook.yaml, how does Ansible know where to find that playbook?

    $ ansible-playbook someplaybook.yaml

    Will look for a playbook inside current directory.

    $ ansible-playbook somedir/somedir/someplaybook.yaml

    Will look for a playbook inside somedir/somedir/ directory.

    It's your responsibility to put your project with all playbooks and roles on server. Ansible has nothing to do with that.