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How to Set Up Ansible Inventories

How to Set Up Ansible Inventories

Ansible inventories serve as the foundation for managing and organizing your infrastructure automation. They define which servers, devices, or cloud instances Ansible will target when executing playbooks, how to connect to them, and what variables apply to each host. Whether you’re managing a handful of development servers or orchestrating thousands of production machines across multiple cloud providers, setting up your inventories correctly determines the success of your entire automation strategy. This guide walks through creating static and dynamic inventories, organizing hosts into logical groups, handling complex variable hierarchies, and troubleshooting common inventory issues that trip up both newcomers and experienced practitioners.

How Ansible Inventories Work

An Ansible inventory acts as a database of managed nodes, containing hostnames, IP addresses, connection parameters, and variables that define how Ansible communicates with each target system. The inventory system operates on a hierarchical structure where hosts belong to groups, groups can contain other groups, and variables cascade down through this hierarchy with specific precedence rules.

When you run an Ansible playbook, the inventory gets parsed first, building an internal representation of your infrastructure. Ansible then applies variable precedence rules, resolving conflicts between host variables, group variables, and playbook variables. The connection plugins use inventory data to establish SSH connections, determine privilege escalation methods, and pass the correct authentication credentials.

Static inventories use INI or YAML files that you manually maintain, while dynamic inventories query external systems like cloud APIs, CMDB systems, or container orchestrators to build the host list at runtime. Both approaches can coexist, and Ansible automatically merges multiple inventory sources when you specify a directory containing multiple inventory files.

Basic Static Inventory Setup

The simplest inventory format uses INI-style syntax. Create a file called hosts or inventory in your project directory:

[webservers]
web1.example.com
web2.example.com
192.168.1.100

[databases]
db1.example.com ansible_host=10.0.1.50
db2.example.com ansible_host=10.0.1.51

[loadbalancers]
lb1.example.com ansible_port=2222 ansible_user=admin

[production:children]
webservers
databases
loadbalancers

This inventory defines three main groups plus a parent group that contains all production servers. The ansible_host parameter overrides DNS resolution, useful when internal IPs differ from public hostnames. The ansible_port and ansible_user parameters customize connection settings per host.

For more complex scenarios, YAML format provides better readability and supports nested data structures:

all:
  children:
    webservers:
      hosts:
        web1.example.com:
        web2.example.com:
        web3.example.com:
          ansible_host: 192.168.1.103
          http_port: 8080
    databases:
      hosts:
        db1.example.com:
          ansible_host: 10.0.1.50
          mysql_replication_role: master
        db2.example.com:
          ansible_host: 10.0.1.51
          mysql_replication_role: slave
      vars:
        mysql_port: 3306
        backup_schedule: "0 2 * * *"

Test your inventory structure using the ansible-inventory command:

# List all hosts
ansible-inventory -i inventory --list

# Show specific group
ansible-inventory -i inventory --graph

# Verify host variables
ansible-inventory -i inventory --host web1.example.com

Advanced Group Organization and Variables

Effective inventory organization relies on logical grouping that reflects your infrastructure’s purpose, environment, and operational needs. Create groups based on functionality, geographic location, environment stage, or deployment patterns:

[frontend]
web[01:05].prod.example.com

[backend]
api[01:03].prod.example.com
worker[01:10].prod.example.com

[monitoring]
prometheus.prod.example.com
grafana.prod.example.com

# Geographic grouping
[us-east]
web01.prod.example.com
web02.prod.example.com
api01.prod.example.com

[eu-west]
web03.prod.example.com
web04.prod.example.com
api02.prod.example.com

# Environment grouping
[production:children]
frontend
backend
monitoring

[staging]
web-staging.example.com
api-staging.example.com

Ansible supports range patterns for hosts with numeric suffixes. The web[01:05].prod.example.com notation expands to web01, web02, web03, web04, and web05. You can also use alphabetic ranges like web[a:e].example.com.

Variable hierarchy becomes crucial in complex inventories. Create a group_vars directory structure to organize variables:

inventory/
├── hosts
├── group_vars/
│   ├── all.yml
│   ├── production.yml
│   ├── webservers.yml
│   └── databases.yml
└── host_vars/
    ├── web1.example.com.yml
    └── db1.example.com.yml

The group_vars/all.yml file applies variables to every host:

# group_vars/all.yml
ntp_servers:
  - 0.pool.ntp.org
  - 1.pool.ntp.org

ssl_cert_path: /etc/ssl/certs
log_level: info

Group-specific variables override the global defaults:

# group_vars/webservers.yml
nginx_worker_processes: "{{ ansible_processor_vcpus }}"
nginx_worker_connections: 1024
ssl_protocols: "TLSv1.2 TLSv1.3"

# group_vars/databases.yml  
mysql_innodb_buffer_pool_size: "{{ (ansible_memtotal_mb * 0.7) | int }}M"
mysql_max_connections: 200
backup_retention_days: 30

Dynamic Inventory Configuration

Dynamic inventories eliminate manual host management by querying external systems. Most cloud providers offer official Ansible inventory plugins. Configure AWS EC2 dynamic inventory by creating aws_ec2.yml:

plugin: aws_ec2
regions:
  - us-east-1
  - us-west-2
filters:
  instance-state-name: running
hostnames:
  - dns-name
  - private-ip-address
compose:
  ansible_host: private_ip_address
groups:
  webservers: "'web' in tags.Role"
  databases: "'db' in tags.Role"
  production: "tags.Environment == 'prod'"
  staging: "tags.Environment == 'staging'"
keyed_groups:
  - key: tags.Environment
    prefix: env
  - key: instance_type
    prefix: type

This configuration creates groups based on EC2 tags and instance metadata. The compose section sets ansible_host to use private IP addresses for internal communication. The keyed_groups feature automatically creates groups like env_prod, env_staging, type_t3_medium.

For Google Cloud Platform, use the gcp_compute plugin:

plugin: gcp_compute
projects:
  - my-gcp-project
zones:
  - us-central1-a
  - us-central1-b
filters:
  - status = RUNNING
auth_kind: serviceaccount
service_account_file: /path/to/service-account.json
hostnames:
  - name
compose:
  ansible_host: networkInterfaces[0].accessConfigs[0].natIP
groups:
  webservers: "'webserver' in labels.role"
  databases: "'database' in labels.role"

Azure dynamic inventory uses similar patterns with the azure_rm plugin. Each cloud plugin supports different filtering options and metadata fields, so consult the official documentation for platform-specific features.

Test dynamic inventory configuration:

# Test AWS EC2 inventory
ansible-inventory -i aws_ec2.yml --graph

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

# Verify specific host details
ansible-inventory -i aws_ec2.yml --host i-1234567890abcdef0

Real-World Use Cases and Examples

A typical web application deployment uses multiple inventory patterns. Consider this structure for a multi-tier application across development, staging, and production environments:

inventory/
├── production/
│   ├── hosts.yml
│   ├── aws_ec2.yml
│   └── group_vars/
├── staging/
│   ├── hosts.yml
│   └── group_vars/
└── development/
    ├── vagrant.yml
    └── group_vars/

The production inventory combines static and dynamic sources. Static entries handle on-premises servers, while dynamic inventory manages cloud instances:

# inventory/production/hosts.yml
all:
  children:
    onpremise:
      children:
        legacy_systems:
          hosts:
            mainframe.internal.com:
              ansible_user: ansible
              ansible_become: yes
        physical_databases:
          hosts:
            db-primary.internal.com:
              mysql_replication_role: master
            db-replica.internal.com:
              mysql_replication_role: slave
    cloud:
      children:
        # Groups populated by aws_ec2.yml dynamic inventory

A microservices environment might organize inventory by service ownership and deployment strategy:

# Kubernetes-based microservices inventory
plugin: k8s
connections:
  - host: https://k8s-api.example.com
compose:
  ansible_host: status.podIP
  service_name: metadata.labels['app.kubernetes.io/name']
  version: metadata.labels['app.kubernetes.io/version']
groups:
  user_service: "service_name == 'user-service'"
  payment_service: "service_name == 'payment-service'"
  frontend: "service_name == 'frontend'"
  canary_deployments: "'canary' in metadata.labels"

For organizations using configuration management alongside Ansible, you might integrate with existing CMDB systems:

#!/usr/bin/env python3
# custom_cmdb_inventory.py
import json
import requests
import sys

def get_inventory():
    # Query CMDB API
    response = requests.get('https://cmdb.company.com/api/servers')
    servers = response.json()
    
    inventory = {
        '_meta': {
            'hostvars': {}
        }
    }
    
    for server in servers:
        # Organize by application and environment
        app_env_group = f"{server['application']}_{server['environment']}"
        
        if app_env_group not in inventory:
            inventory[app_env_group] = {'hosts': []}
        
        inventory[app_env_group]['hosts'].append(server['hostname'])
        
        # Add host variables
        inventory['_meta']['hostvars'][server['hostname']] = {
            'ansible_host': server['ip_address'],
            'application': server['application'],
            'environment': server['environment'],
            'owner': server['owner'],
            'maintenance_window': server['maintenance_window']
        }
    
    return inventory

if __name__ == '__main__':
    print(json.dumps(get_inventory(), indent=2))

Comparison with Alternative Approaches

Different inventory management strategies suit different organizational needs and infrastructure scales:

Approach Best For Pros Cons Maintenance Overhead
Static INI Files Small, stable environments Simple, version controlled, fast parsing Manual updates, limited structure Low
Static YAML Files Complex variable hierarchies Structured data, readable, version controlled Manual updates, slower parsing Low
Cloud Dynamic Inventory Auto-scaling cloud infrastructure Automatic updates, tag-based grouping API dependencies, slower execution Medium
Custom Scripts Integration with existing CMDB Flexible, existing data sources Custom maintenance, error handling High
Hybrid Approach Mixed environments Flexibility, gradual migration Complexity, multiple sources Medium

Performance characteristics vary significantly between inventory types. Static inventories parse in milliseconds, while dynamic inventories might take several seconds for large cloud deployments. Here’s a benchmark from a real environment managing 500 hosts:

Inventory Type Parse Time Memory Usage API Calls Cache TTL
Static INI 0.1s 15MB 0 N/A
AWS EC2 Plugin 3.2s 45MB 12 300s
Custom CMDB Script 1.8s 25MB 1 600s
Kubernetes Plugin 2.1s 35MB 3 300s

Best Practices and Common Pitfalls

Successful inventory management requires consistent naming conventions, proper variable organization, and careful attention to security. Establish naming standards early:

# Good: Consistent, descriptive naming
[web_production_useast1]
web01.prod.useast1.company.com
web02.prod.useast1.company.com

[db_production_useast1]  
db01.prod.useast1.company.com

# Bad: Inconsistent, unclear naming
[webservers]
server1
web-box-2
production-web-3.company.com

Keep sensitive data out of inventory files. Use Ansible Vault for passwords, API keys, and certificates:

# group_vars/databases.yml
mysql_root_password: "{{ vault_mysql_root_password }}"
ssl_private_key: "{{ vault_ssl_private_key }}"

# Encrypt sensitive variables
ansible-vault encrypt group_vars/databases/vault.yml

Common pitfalls include variable precedence confusion, circular group dependencies, and performance issues with large dynamic inventories. Variable precedence follows this order (highest to lowest):

  • Extra vars (command line -e)
  • Task vars
  • Block vars
  • Role and include vars
  • Play vars
  • Host vars
  • Group vars (most specific group)
  • Group vars (less specific groups)
  • Group vars (all)

Avoid circular group dependencies that cause Ansible to fail:

# This creates a circular dependency - DON'T DO THIS
[webservers:children]
frontend

[frontend:children] 
webservers  # Circular reference

Optimize dynamic inventory performance by implementing caching and filtering:

# aws_ec2.yml with performance optimizations
plugin: aws_ec2
regions:
  - us-east-1  # Limit regions
filters:
  instance-state-name: running
  "tag:Managed": "ansible"  # Filter by management tag
cache: true
cache_plugin: jsonfile
cache_timeout: 300
cache_connection: /tmp/ansible_inventory_cache

Use inventory plugins’ built-in caching mechanisms rather than implementing custom caching logic. Most plugins support cache configuration through the main Ansible configuration file.

For large environments, consider splitting inventory into multiple files and using inventory directories. Ansible automatically merges all inventory sources in a directory:

inventory/
├── 01-static-infrastructure
├── 02-aws-ec2.yml
├── 03-gcp-compute.yml
└── 04-kubernetes.yml

Test inventory changes in isolated environments before applying to production. Use the --check and --diff flags with ansible-playbook to validate changes, and maintain separate inventory files for each environment to prevent accidental cross-environment deployments.

Monitor inventory performance and API usage, especially with dynamic inventories. Set up alerts for inventory parsing failures and implement fallback mechanisms for critical automation workflows. Consider implementing inventory validation tests as part of your CI/CD pipeline to catch configuration errors before they affect production systems.

For additional information on inventory plugins and advanced configuration options, consult the official Ansible inventory documentation and the inventory plugins reference.



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