Ansible roles. Default values

By the sound of this documentation;

http://docs.ansible.com/ansible/playbooks_roles.html#roles

Roles are just automation around ‘include’ directives as described above, and really don’t contain much additional magic beyond some improvements to search path handling for referenced files. However, that can be a big thing!

This designates the following behaviors, for each role ‘x’:

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/defaults/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, script, template or include tasks (in the role) can reference files in roles/x/{files,templates,tasks}/ (dir depends on task) without having to path them relatively or absolutely

Q: I mean, is main.yml the only valid name for a file with default values?
A: Sounds like it is only if you are wanting ansible to automatically load certain var_files for that role and auto include the task file.

Q: Is it possible to name files
A: Yes, it just means in the playbook you have to use the include directive and explicitly point it to the unique task file name.

Example:

- name: Do role
  hosts: localhost
  tasks:
  - include: roles/operations/tasks/installProgram.yml

As you tell in the comments, it's possible to call a file anything else under the roles/my_role/defaults/. The issue is that files like roles/my_role/defaults/anything_else.yml will never be loaded as default. Even you will be able to load it with include_vars, it will gain the high precedence of included vars (18/22).

Nevertheless, since Ansible 2.6, your are able to use a directory main in place of a main.yml file!

By doing so, you can reach your goal by splitting your variables into roles/my_role/defaults/main/something.yml, roles/my_role/defaults/main/anything_else.yml, …

More details in this StackOverflow answer.

But indeed this feature is not yet documented (I just created an issue about that).

Ansible will look in the roles/ directory for tasks, variables, handlers, etc by default. roles/default is simply an example provided in their docs. You can structure your roles differently to suit your needs. For example, I have mine set up as such;

roles/windows/<somerole>/tasks/main.yml
roles/linux/<somerole>/tasks/main.yml

main.yml is the only file that will be loaded automatically (by "default") when running playbooks. If you want to specify another file name for whatever reason you need to use the include function and point to your file as described above.

The "Best Practices" guide offers some good insight into directory structuring.