A Beginners Guide To Ansible Inventory File System 

The Ansible inventory file system is a foundational component that defines the hosts and groups of hosts on which your automation tasks will run. It is essentially the list of machines that Ansible will manage. 

Whether you are deploying apps, configuring servers, or running updates, Ansible needs to know which machines to target, and this is where the inventory file comes in. 

In this guide, we shall walk through the basics of Ansible inventory file, covering the types, formats, and best practices. 

Types of Ansible Inventories

Ansible supports two main types of inventories; static and dynamic. The type you choose depends on the size, complexity, and the flexibility needed in your infrastructure. 

1. Static Inventory

A static Ansible inventory file is a manually maintained file that is typically written in INI or YAML format and lists all the hosts and host groups. 

Example (INI format):

[webservers]

web1.example.com

web2.example.com

[databases]

db1.example.com ansible_user=admin ansible_port=2222

Use Cases:

• Small to medium-sized environments
• Simple testing or development setups
• Scenarios where infrastructure doesn’t change frequently

Advantages:

• Easy to create and understand
• No dependencies on external services

Limitations:

• Manual updates are required as infrastructure changes
• Not ideal for dynamic or cloud-based environments

2. Dynamic Inventory

A dynamic Ansible inventory file is generated automatically by querying external systems, such as cloud providers like AWS, Azure, GCP, databases, or APIs. Ansible retrieves a real-time list of hosts when the playbook runs. 

Example: AWS EC2 Dynamic Inventory:

ansible-inventory -i aws_ec2.yml –graph

Use Cases:

• Cloud-native or auto-scaling environments
• Large infrastructures with frequent changes
• Integration with tagging or metadata systems

Advantages:

• No need to manually maintain inventory
• Automatically reflects changes in infrastructure
• Supports filtering, grouping, and metadata

Limitations:

• Requires initial setup and configuration
• Can be more complex to debug than static inventories

Related Article: What Are Ansible Roles & Why Are They Important

Basic Ansible Inventory File Format With Examples 

There are two primary types of Ansible inventory file format; INI and YAML. Here is an in depth comparison of both to understand them clearly. 

Feature	INI Format	YAML Format (Inventory Plugin)
Syntax Style	Flat, section-based	Structured, hierarchical
Readability	Simple and compact	More readable for complex configurations
Built-in Support	Default format in Ansible	Requires enabling inventory plugins
Host Variables	Declared inline or using host_vars/	Declared under vars: section
Group Nesting	Supported via [group:children]	Native YAML nesting
Best for	Small, static setups	Complex, dynamic, or nested environments
Flexibility	Limited with advanced grouping or plugins	Highly flexible with metadata and plugins

INI Format Example

[web]

web1.example.com

web2.example.com ansible_user=admin

[db]

db1.example.com ansible_port=2222

[app:children]

web

db

YAML Format Example (Using Inventory Plugin)

all:

  children:

    web:

      hosts:

        web1.example.com:

        web2.example.com:

          ansible_user: admin

    db:

      hosts:

        db1.example.com:

          ansible_port: 2222

How to Add Hosts to an Ansible Inventory File

Adding hosts to an Ansible inventory file is pretty streamlined and also depends on the file format that you are using. You can define individual hosts, group multiple hosts, and even assign host-specific variables like SSH port, user, or IP. 

1.  Adding Single Hosts

INI Format:

web1.example.com

YAML Format:

all:

  hosts:

    web1.example.com:

You can simply list the hostname or IP. Ansible will try to connect using the default SSH user unless specified otherwise.

2. Adding Groups of Hosts

Grouping hosts will help you organize them by function, environment, or role (e.g. web servers, database servers).

INI Format:

[web]

web1.example.com

web2.example.com

[db]

db1.example.com

YAML Format:

all:

  children:

    web:

      hosts:

        web1.example.com:

        web2.example.com:

    db:

      hosts:

        db1.example.com:

You can then run a playbook on a group using -l web or define a playbook target as hosts: web.

3. Using Host Variables

You can specify variables like SSH user, custom ports, or other parameters per host. 

INI Format:

[web]

web1.example.com ansible_user=admin ansible_port=2222

web2.example.com ansible_host=192.168.1.15

[db]

db1.example.com ansible_connection=ssh ansible_user=ubuntu

YAML Format:

all:

  children:

    web:

      hosts:

        web1.example.com:

          ansible_user: admin

          ansible_port: 2222

        web2.example.com:

          ansible_host: 192.168.1.15

    db:

      hosts:

        db1.example.com:

          ansible_user: ubuntu

          ansible_connection: ssh

You can also keep variables in separate host_vars/ or group_vars/ directories if you prefer to keep the inventory file clean.

Working with Groups and Nested Groups

Ansible allows you to host groups logically and even create nested groups for more structured environments. 

Grouping Hosts

You can define groups like web, db, or loadbalancer to run playbooks on selected subsets. 

INI Format:

[web]

web1.example.com

web2.example.com

[db]

db1.example.com

db2.example.com

Nested Groups

Nested groups allow you to create higher level abstractions like production or staging that includes groups. 

INI Format:

[production:children]

web

db

YAML Format:

all:

  children:

    production:

      children:

        web:

        db:

Nested groups are useful when you want to target multiple groups together in a playbook (e.g., hosts: production).

Using Inventory Variables

Inventory variables allow you to assign configuration details like SSH credentials, service ports, or environment specific settings to hosts or groups. 

Host Variables Example (INI):

web1.example.com ansible_user=admin ansible_port=2222

Group Variables Example (INI):

[web:vars]

ansible_user=webadmin

ansible_port=2200

YAML Format:

all:

  children:

    web:

      hosts:

        web1.example.com:

        web2.example.com:

      vars:

        ansible_user: webadmin

        ansible_port: 2200

Best Practices for Managing Inventory Files

To make the most use of the Ansible inventory file system, you should follow these best practices. 

• Since it is easier to manage nested groups and metadata, use YAML for complex setups. 
• Use group_vars/ and host_vars/ for variable management to organize directories. 
• Avoid hardcoding data for greater useability. 
• Use dynamic inventories for cloud environments to automate host discovery. 
• Include comments and make separate documents with proper structure. 

Conclusion

The Ansible inventory file is the foundation of your automation strategy. Whether you are using a static file or a dynamic source from the cloud, understanding how to structure hosts and other elements ensure that your playbooks run effectively.