Every task has a module reference. This tells Ansible which bit of work to carry out. Ansible is designed to easily allow for custom modules to live alongside a playbook. These custom modules can be wholly new functionality, or they can replace modules shipped with Ansible itself. When Ansible parses a task and discovers the name of the module to use for a task, it looks into a series of locations in order to find the module requested. Where it looks also depends on where the task lives, whether in a role or not.

If a task is in a role, Ansible will first look for the module within a directory tree named library within the role the task resides in. If the module is not found there, Ansible looks for a directory named library at the same level as the main playbook (the one referenced by the ansible-playbook execution). If the module is not found there, Ansible will finally look in the configured library path, which defaults to /usr/share/ansible/. This library path can be configured in an Ansible config file, or by way of the ANSIBLE_LIBRARY environment variable.

This design, allowing modules to be bundled with roles and playbooks, allows for the addition of functionality or the reparation of problems quickly and easily.