Inventory¶

What You'll Learn¶

• Static inventory in INI and YAML form
• Groups, children, and how patterns select hosts
• When a static file stops being enough

Why This Matters¶

Every run starts by resolving hosts: against an inventory. Get the inventory wrong and a playbook can run flawlessly — against the wrong machines, or none at all.

Minimal Example¶

web01 ansible_host=10.0.1.10
web02 ansible_host=10.0.1.11

Practical Example — Groups¶

[web]
web01 ansible_host=10.0.1.10
web02 ansible_host=10.0.1.11

[db]
db01 ansible_host=10.0.1.20

[production:children]
web
db

[web:vars]
http_port=80

hosts: web now targets both web servers. [production:children] groups groups — hosts: production targets all four... two, in this case (web + db). [web:vars] sets a variable for every host in the group.

The Same Inventory in YAML¶

all:
  children:
    web:
      hosts:
        web01:
          ansible_host: 10.0.1.10
        web02:
          ansible_host: 10.0.1.11
      vars:
        http_port: 80
    db:
      hosts:
        db01:
          ansible_host: 10.0.1.20

Functionally identical to the INI version above. YAML tends to win once nested group variables get complex; INI stays more compact for small, flat inventories.

Host Patterns¶

ansible-inventory --graph

is the fastest way to check your mental model of the inventory matches Ansible's — run it whenever a pattern isn't matching what you expect.

Common Mistakes¶

• Assuming web:db intersects instead of unions — it's the opposite of &.
• Stale static inventory that no longer matches real infrastructure — the moment servers autoscale, a static file starts lying.
• Overlapping group variable definitions producing confusing precedence results — see Variable Precedence.

Beyond Static Files: Dynamic Inventory¶

A hand-maintained file doesn't survive autoscaling or a team bigger than one person. Dynamic inventory plugins (amazon.aws.aws_ec2, azure.azcollection.azure_rm, kubernetes.core.k8s) query a live API at run time and build the host list automatically, so the inventory can never drift from what's actually running. Covered in full in Dynamic Inventory.

Interview Questions¶

• What's the difference between static and dynamic inventory, and when do you reach for each?
• How does the web:!staging pattern work?
• How would you source inventory automatically from AWS EC2?

Next¶

Continue to Ad-Hoc Commands.