Ansible best practice do not repeat common role
On the Ansible best practices page: http://docs.ansible.com/ansible/playbooks_best_practices.html#top-level-playbooks-are-separated-by-role it shows an example where the master playbook site.yml includes a couple of other top-level playbooks webservers.yml and dbservers.yml. Within those playbooks they each include the common role. Some inventory files I have all my groups run on one single host. Another inventory file I have a host per group. For the case where ever group is on one host, if I run site.yml you can see that the common role is getting played twice, one for webservers.yml and one for dbservers.yml.
What is a solution to avoid this? I guess you can take out the common role from webservers.yml and dbservers.yml and instead within site.yml have a task that targets both with the common role. But then I can not individually provision a webserver or dbserver with common.
Answer
I determine role dependencies using meta files in my role directory. Role dependencies allow you to automatically pull in other roles when using a role. Role dependencies are stored in the meta/main.yml file contained within the role directory.
Roles dependencies are always executed before the role that includes them, and are recursive. By default, roles can also only be added as a dependency once - if another role also lists it as a dependency it will not be run again. This behavior can be overridden by adding allow_duplicates: yes to the meta/main.yml file.