Understanding Ansible Playbooks

Introduction

Ansible is a powerful IT automation tool that simplifies the management of complex infrastructures. One of the core components of Ansible is the Playbook a simple and powerful configuration management and multi-machine deployment system. In this blog we'll dive deep into understanding Ansible Playbooks, their structure and how to write and run a basic Playbook to install a package. By the end of this guide you'll be well-equipped to craft your first Ansible Playbook.

What is an Ansible Playbook?

Ansible Playbooks are YAML files that describe a series of tasks to be executed on a set of hosts. They are human-readable and can be versioned and shared, making them an excellent tool for automating repetitive tasks, ensuring consistency and managing complex deployments.

Playbooks are designed to be simple and reusable. They provide a clear way to manage configurations, deployment and orchestration. Each Playbook can run multiple Ansible tasks and you can chain them together to accomplish complex workflows.

Structure of a Playbook

An Ansible Playbook is structured into several key sections :-

1. Hosts

The hosts section specifies the target machines where the Playbook tasks will run. Hosts can be grouped into different categories and you can specify individual hosts or groups.

2. Tasks

Tasks are the basic units of action in a Playbook. Each task performs a specific action such as installing a package, creating a file or restarting a service. Tasks are executed sequentially.

3. Variables

Variables allow you to store values that can be reused throughout the Playbook. They help make your Playbooks more flexible and reusable.

4. Handlers

Handlers are similar to tasks but are only run when notified by other tasks. They are typically used for operations that need to occur only when a change has been made such as restarting a service after a configuration file has been modified.

Writing a Simple Playbook to Install a Package

Let's write a simple Ansible Playbook to install the Apache web server on a group of web servers. We'll cover each part of the Playbook in detail.

Example Playbook : Install Apache

---
- name: Install Apache on web servers
  hosts: webservers
  become: yes
  tasks:
    - name: Install Apache
      apt:
        name: apache2
        state: present

Explanation :-

1. Playbook Header (---) :- The --- line indicates the beginning of a YAML file.

2. Name (- name: Install Apache on web servers) :- The name field provides a description of the Playbook's purpose.

3. Hosts (hosts: webservers) :- The hosts field specifies that the Playbook should run on the group of hosts named webservers. This group should be defined in your inventory file.

4. Privilege Escalation (become: yes) :- The become field allows the tasks to be executed with elevated privileges (e.g., using sudo).

5. Tasks :-

   • Task Name (- name: Install Apache) :- Each task begins with a name, describing what the task does.

   • Task Action (apt) :- The apt module is used to manage packages on Debian-based systems.

   • Module Parameters :-

     • name: apache2 specifies the package to be installed.

     • state: present ensures the package is installed.

Inventory File

Before running the Playbook, you need to define an inventory file that lists the target hosts. Here's a simple example :-

# inventory.ini
[webservers]
webserver1 ansible_host=192.168.1.10
webserver2 ansible_host=192.168.1.11

Running the Playbook

To execute the Playbook, use the ansible-playbook command, specifying the inventory file and the Playbook file.

ansible-playbook -i inventory.ini install_apache.yml

Output

When you run the Playbook, Ansible will connect to the hosts defined in the webservers group, elevate privileges and execute the tasks to install Apache. The output will show the status of each task indicating whether changes were made or if the system was already in the desired state.

PLAY [Install Apache on web servers] *************************************

TASK [Gathering Facts] ***************************************************
ok: [webserver1]
ok: [webserver2]

TASK [Install Apache] ****************************************************
changed: [webserver1]
changed: [webserver2]

PLAY RECAP ***************************************************************
webserver1                 : ok=2    changed=1    unreachable=0    failed=0
webserver2                 : ok=2    changed=1    unreachable=0    failed=0

In this example, changed=1 indicates that the Apache package was installed successfully on both webserver1 and webserver2.

Adding Variables and Handlers

Let's enhance our Playbook by adding variables and handlers.

Example Playbook with Variables and Handlers

---
- name: Install and configure Apache on web servers
  hosts: webservers
  become: yes
  vars:
    apache_package: apache2
    apache_service: apache2
  tasks:
    - name: Install Apache
      apt:
        name: "{{ apache_package }}"
        state: present

    - name: Ensure Apache is running
      service:
        name: "{{ apache_service }}"
        state: started
        enabled: yes

    - name: Create a simple HTML file
      copy:
        content: "<h1>Hello from Ansible</h1>"
        dest: /var/www/html/index.html
      notify:
        - Restart Apache

  handlers:
    - name: Restart Apache
      service:
        name: "{{ apache_service }}"
        state: restarted

Explanation :-

1. Variables (vars) :-

   • apache_package and apache_service variables store the package and service names.

2. Tasks :-

   • Install Apache :- Uses the apt module to install the Apache package referencing the apache_package variable.

   • Ensure Apache is Running :- Uses the service module to start and enable the Apache service.

   • Create a Simple HTML File :- Uses the copy module to create an HTML file and triggers a handler to restart Apache if the file is changed.

3. Handlers :-

   • Restart Apache :- This handler restarts the Apache service when notified by the Create a Simple HTML File task.

Running the Enhanced Playbook

ansible-playbook -i inventory.ini install_and_configure_apache.yml

Output

PLAY [Install and configure Apache on web servers] ***********************

TASK [Gathering Facts] ***************************************************
ok: [webserver1]
ok: [webserver2]

TASK [Install Apache] ****************************************************
changed: [webserver1]
changed: [webserver2]

TASK [Ensure Apache is running] ******************************************
ok: [webserver1]
ok: [webserver2]

TASK [Create a simple HTML file] *****************************************
changed: [webserver1]
changed: [webserver2]

RUNNING HANDLER [Restart Apache] *****************************************
changed: [webserver1]
changed: [webserver2]

PLAY RECAP ***************************************************************
webserver1                 : ok=5    changed=3    unreachable=0    failed=0
webserver2                 : ok=5    changed=3    unreachable=0    failed=0

Conclusion

In this guide we've covered the basics of Ansible Playbooks including their structure and key components. We wrote a simple Playbook to install the Apache web server and demonstrated how to run it. We then enhanced the Playbook by adding variables and handlers, showcasing how to make your Playbooks more flexible and reusable.

By mastering Ansible Playbooks you can automate complex tasks, ensure consistency across your infrastructure and save valuable time.

Let's continue exploring the capabilities of Ansible in future posts diving deeper into advanced topics and real-world use cases to further enhance your automation skills.

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Sudha Yadav