Handlers in Ansible

What You’ll Learn 📚

• Understanding what handlers are in Ansible
• Key terminology and definitions
• Simple to complex examples of handlers
• Common questions and answers
• Troubleshooting common issues

Introduction to Handlers

In Ansible, handlers are special tasks that are triggered by other tasks. They are typically used to restart services after a configuration change. Think of handlers as a way to say, “Hey, if this task changes something, make sure to do this other thing too!”

Lightbulb moment: Handlers are like a safety net, ensuring that necessary actions are taken when changes occur.

Key Terminology

• Task: A single unit of work in Ansible, like installing a package or copying a file.
• Handler: A task that is run only when notified by another task.
• Notify: A directive used in a task to trigger a handler.

Simple Example: Restarting a Service

---
- name: Simple playbook with a handler
  hosts: localhost
  tasks:
    - name: Install Apache
      apt:
        name: apache2
        state: present
      notify: Restart Apache
  handlers:
    - name: Restart Apache
      service:
        name: apache2
        state: restarted

Progressively Complex Examples

Example 1: Multiple Handlers

---
- name: Playbook with multiple handlers
  hosts: localhost
  tasks:
    - name: Update package cache
      apt:
        update_cache: yes
      notify: Update Completed
    - name: Install MySQL
      apt:
        name: mysql-server
        state: present
      notify: Restart MySQL
  handlers:
    - name: Update Completed
      debug:
        msg: "Package cache updated."
    - name: Restart MySQL
      service:
        name: mysql
        state: restarted

Example 2: Conditional Handlers

---
- name: Conditional handler example
  hosts: localhost
  tasks:
    - name: Create a file
      file:
        path: /tmp/testfile
        state: touch
      notify: Clean Up
  handlers:
    - name: Clean Up
      command: rm /tmp/testfile
      when: ansible_os_family == 'Debian'

Example 3: Handlers with Loops

---
- name: Handlers with loops
  hosts: localhost
  tasks:
    - name: Install multiple packages
      apt:
        name: "{{ item }}"
        state: present
      loop:
        - apache2
        - mysql-server
      notify: Restart Services
  handlers:
    - name: Restart Services
      service:
        name: "{{ item }}"
        state: restarted
      loop:
        - apache2
        - mysql

Common Questions and Answers

1. What happens if a task doesn’t notify a handler?

   The handler won’t run. Handlers only execute when explicitly notified by a task.

2. Can a handler notify another handler?

   No, handlers cannot notify other handlers directly. They are only triggered by tasks.

3. Do handlers run every time a task runs?

   No, handlers only run if the task results in a change and explicitly notifies the handler.

4. Can I use handlers with any module?

   Yes, handlers can be used with any module that supports the notify directive.

5. What if multiple tasks notify the same handler?

   The handler will only run once, at the end of the playbook, regardless of how many tasks notify it.

Troubleshooting Common Issues

• Handler not running: Ensure the task that should notify the handler results in a change and has the correct notify directive.
• Handler runs unexpectedly: Check if other tasks are notifying the handler or if there are conditions affecting its execution.
• Syntax errors: Double-check your YAML syntax, especially indentation and spacing.

Remember: YAML is sensitive to indentation. Make sure your handlers are properly aligned with tasks!

Practice Exercises

• Create a playbook that installs a package and uses a handler to start a service only if the package is installed.
• Modify an existing playbook to add a handler that sends a notification email when a task changes.
• Experiment with conditional handlers based on different operating systems or environments.

Don’t worry if this seems complex at first. With practice, you’ll master handlers in no time! Keep experimenting and learning. You’ve got this! 💪

Further Reading and Resources

• Ansible Handlers Documentation
• Ansible Resources