Ansible Playbooks vs Roles -

according ansible docs, playbook is:

...the basis simple configuration management , multi-machine deployment system, unlike exist, , 1 suited deploying complex applications.

and, again, according same docs, roles are:

...ways of automatically loading vars_files, tasks, , handlers based on known file structure. grouping content roles allows easy sharing of roles other users.

however distinction between these , different use cases not obvious me. instance, if configure /etc/ansible/hosts file like:

[databases] mydb01.example.org mydb02.example.org  [mail_servers] mymail01.example.org mymail_dr.example.org 

...then "[databases]" entry...a role? or name of playbook yaml file somewhere? or else?!?

if explain me differences on these, understanding of ansible enhance!

• playbook vs role vs [databases] , similar entries in /etc/ansible/hosts
• if playbooks defined inside of yaml files, roles defined?
• aside ansible.cfg living on ansible server, how add/configure ansible available playbooks/roles? instance, when run ansible-playbook someplaybook.yaml, how ansible know find playbook?

playbook vs role vs [databases] , similar entries in /etc/ansible/hosts

[databases] single name group of hosts. allows reference multiple hosts single name.

role set of tasks , additional files configure host serve role.

playbook mapping between hosts , roles.

example documentation describes example project. contains 2 things:

• playbooks. site.yml, webservers.yml, fooservers.yml playbooks.
• roles: roles/common/ , roles/webservers/ contain definitions of common , webservers roles accordingly.

inside playbook (webservers.yml) have like:

--- - hosts: webservers <- group of hosts defined in /etc/ansible/hosts, databases , mail_servers in example question   roles: <- list of roles assign these hosts      - common      - webservers 

if playbooks defined inside of yaml files, roles defined?

they defined inside roles/* directories. roles defined using yaml files, can contain resources of types (files/, templates/). according documentation role definition structured way:

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

the important file roles/x/tasks/main.yml, here define tasks, executed, when role executed.

aside ansible.cfg living on ansible server, how add/configure ansible available playbooks/roles? instance, when run ansible-playbook someplaybook.yaml, how ansible know find playbook?

$ ansible-playbook someplaybook.yaml 

will playbook inside current directory.

$ ansible-playbook somedir/somedir/someplaybook.yaml 

will playbook inside somedir/somedir/ directory.

it's responsibility put project playbooks , roles on server. ansible has nothing that.