Dynamic Inventory

Inventory plugins for AWS, Azure, and GCP with auto-discovery.

7 min readAnsible AutomationProvision

Why Dynamic Inventory?

Static inventory files become outdated as cloud instances are created and destroyed. Dynamic inventory plugins query cloud providers or APIs to discover hosts in real time. Ansible always knows what exists.

AWS EC2 Inventory

aws_ec2.yml


---
plugin: amazon.aws.aws_ec2
regions:
  - us-east-1
  - us-west-2

filters:
  tag:Environment: production
  instance-state-name: running

keyed_groups:
  - key: tags.Role
    prefix: role
    separator: "_"
  - key: placement.availability_zone
    prefix: az
  - key: instance_type
    prefix: type

compose:
  ansible_host: private_ip_address
  ansible_user: "'ec2-user'"

hostnames:
  - private-ip-address
  - tag:Name

The aws_ec2 plugin discovers EC2 instances and groups them by tags, AZ, or instance type. Compose maps instance attributes to Ansible variables.

Azure Inventory

azure.yml


---
plugin: azure.azcollection.azure_rm

source:
  subscription_id: "{{ azure_subscription_id }}"
  tenant: "{{ azure_tenant_id }}"

resource_groups:
  - production-webservers
  - production-dbservers

plain_host_names: yes

keyed_groups:
  - key: tags.Role
    prefix: role
  - key: location
    prefix: location

compose:
  ansible_host: private_ip
  ansible_user: "'adminuser'"

Azure inventory plugin discovers VMs by resource group. Tags define host groups automatically.

Custom Inventory Scripts

dynamic_inventory.py


#!/usr/bin/env python3
import json
import sys
import argparse

def get_inventory():
    inventory = {
        "_meta": {"hostvars": {}},
        "webservers": {
            "hosts": ["web1.example.com", "web2.example.com"],
            "vars": {"http_port": 80}
        },
        "dbservers": {
            "hosts": ["db1.example.com"],
            "vars": {"db_port": 5432}
        }
    }
    return json.dumps(inventory)

def get_host(hostname):
    hostvars = {
        "web1.example.com": {"ansible_host": "10.0.1.10"},
        "web2.example.com": {"ansible_host": "10.0.1.11"},
        "db1.example.com": {"ansible_host": "10.0.2.10"}
    }
    return json.dumps(hostvars.get(hostname, {}))

if __name__ == "__main__":
    parser = argparse.ArgumentParser()
    parser.add_argument("--list", action="store_true")
    parser.add_argument("--host", type=str)
    args = parser.parse_args()

    if args.list:
        print(get_inventory())
    elif args.host:
        print(get_host(args.host))

Inventory scripts must support --list (all hosts) and --host (single host). Return JSON with _meta for host variables.

Testing Inventory Plugins

Debug dynamic inventory


# List all hosts
ansible-inventory -i aws_ec2.yml --list

# Show host graph
ansible-inventory -i aws_ec2.yml --graph

# Show variables for a host
ansible-inventory -i aws_ec2.yml --host web1.example.com

# Test plugin directly
ansible-inventory -i aws_ec2.yml --yaml

Use ansible-inventory to verify your dynamic inventory plugin returns the expected hosts and groups before running playbooks.

Cache dynamic inventory

Dynamic inventory queries can be slow. Enable caching in ansible.cfg with inventory_cache_refresh to avoid repeated API calls during development.

Mark this lesson complete to store local progress and unlock a cleaner resume path the next time you visit.

Stage 4 · Provision

Dynamic Inventory & Scale