What is an Ansible inventory, and what can it contain?

The inventory is the list of hosts Ansible manages, organised into groups. Groups can contain child groups, a host can be in several groups, and the inventory can also set variables for hosts and groups.

Every Ansible command needs to know which machines it acts on; the inventory answers that. Its building blocks:

• Hosts: hostnames or IP addresses of the managed nodes.
• Groups: named collections of hosts that are managed together, for example webservers or production. A command or play then targets the group name instead of listing machines.
• Nested groups: a group can have child groups, so us can consist of raleigh and mountainview.
• Multiple membership: one host can be in many groups at once (by role, by location, by environment), so you can slice the same fleet different ways.
• Variables: values that apply only to certain hosts or groups, used later by playbooks.

There are two ways to define it: a static inventory is a text file you maintain; a dynamic inventory is generated on demand by a script or plugin that asks an external source (a cloud account, Red Hat Satellite, a CMDB) which hosts currently exist.

Go deeper:

• How to build your inventory (official docs) — hosts, groups, child groups and variables in INI and YAML.
• Ansible 101 - Episode 2 - Ad-hoc tasks and Inventory (Jeff Geerling) — builds an inventory with groups live.