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Ansible

2 min read

Core Concepts

Ansible operates on a simple model: a control node (where Ansible is installed) orchestrates managed nodes (the target servers) without requiring any agents on them. This simplifies setup and reduces the security footprint.

Inventory

The inventory is the foundational list of your managed nodes. It’s a text file (INI or YAML) that groups hosts, allowing you to target specific machines for specific tasks.

# A sample inventory file
[web_servers]
host1.example.com
host2.example.com
 
[db_servers]
db1.example.com

Playbooks

Playbooks are YAML files that describe the desired state of your systems. They contain a series of plays, and each play maps a group of hosts to a set of tasks. Tasks are executed sequentially by calling Ansible modules.

Modules

A module is a script that performs a specific action, such as managing packages (apt), creating directories (file), or starting services (service). Ansible has thousands of built-in modules, and you can also write your own.

Idempotency

This is a critical principle in Ansible. An operation is idempotent if running it once has the same effect as running it multiple times. Ansible modules are designed to be idempotent; they check the current state of the system and only make changes if necessary to achieve the desired state defined in the playbook.

Code Examples

Playbook with Variables

This playbook installs the tree utility on all servers in the web_servers group. It uses a variable to define the package name, making the playbook more reusable.

# content of setup_tools.yml
---
- name: Install common utilities on web servers
  hosts: web_servers
  become: yes # Use 'sudo' to gain root privileges
  vars:
    package_to_install: tree
 
  tasks:
    - name: "Install the {{ package_to_install }} package"
      ansible.builtin.apt:
        name: "{{ package_to_install }}"
        state: present
        update_cache: yes # Good practice to update cache before installing
 
    - name: Verify installation by running the command
      ansible.builtin.command: "{{ package_to_install }} --version"
      register: command_output
      changed_when: false # This command doesn't change state
 
    - name: Display the installed version
      ansible.builtin.debug:
        msg: "{{ command_output.stdout }}"

To run this playbook: ansible-playbook -i inventory.ini setup_tools.yml

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