Ansible Inventory Management

Introduction to Ansible Inventories

  • What is an Inventory?

    • An inventory in Ansible is a list of managed nodes (hosts) that you want to automate.

    • It defines which systems Ansible interacts with.

    • Inventories can be simple flat files or dynamically generated.

  • Inventory Basics:

    • Default location: /etc/ansible/hosts.

    • You can create your own inventory file.

    • Formats: INI (default) or YAML.

    • Groups: Organize hosts into logical groups.

    • Host variables: Customize settings for individual hosts.

    • Group variables: Apply settings to entire groups.

Inventory Management

  • Creating an Inventory File (INI Format):

    • Define hosts and groups.

    • Example:

      [webservers]
webserver1 ansible_host=192.168.1.101
webserver2 ansible_host=192.168.1.102

[dbservers]
dbserver1 ansible_host=192.168.1.201

  • Using Patterns to Select Hosts:

    • Patterns allow you to target specific hosts or groups.

    • Examples:

      • $ ansible webservers -m ping (Ping all webservers)

      • $ ansible dbservers -a "uptime" (Check uptime on DB servers)

  • Dynamic Inventories:

    • Pull inventory dynamically (e.g., from cloud providers).

    • Use dynamic inventory plugins.

    • Example: AWS EC2 instances as inventory.

  • Organizing Inventory:

    • Create a directory with multiple inventory files.

    • Use different formats (YAML, INI, etc.).

    • Organize by environment, function, or location.

1. Using ansible-inventory Command

The ansible-inventory command allows you to view and manipulate your Ansible inventory. It provides information about hosts, groups, and variables defined in your inventory files.

  • To list all hosts and their details:

    ansible-inventory -i inventory.ini --list

2. Host Variables

Host variables allow you to define specific settings for individual hosts. These variables override group variables for that particular host. Here is an example:

Suppose you have an inventory file (inventory.ini) with the following content:

[webservers]
webserver1 ansible_host=192.168.1.101
webserver2 ansible_host=192.168.1.102

You can define host-specific variables in a separate file (e.g., host_vars/webserver1.yml):

# host_vars/webserver1.yml
ntp_server: ntp.example.com

Now, webserver1 will use ntp.example.com as its NTP server, while other hosts in the group won't be affected.

3. Group Variables

Group variables apply to all hosts within a specific group. For example, if all machines in the [webservers] group use the same NTP server, set a group variable:

# group_vars/webservers.yml
ntp_server: ntp.example.com

This way, all hosts in the [webservers] group will use the specified NTP server.

Use Cases

  • Host Variables:

    • Setting unique SSH keys for specific hosts.

    • Customizing configurations based on individual requirements (e.g., different database credentials).

  • Group Variables:

    • Defining common settings for all hosts within a group (e.g., NTP servers, DNS servers).

    • Applying security policies consistently across a group of servers.

Reference

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