Trigger Actions with Handlers

Ansible · 12 min · Practical

Restarting a service every time a playbook runs is inefficient and causes unnecessary downtime. Handlers solve this: they are special tasks that only execute when another task reports a "changed" state.

Why Use Handlers?

If you update an Apache configuration file or replace an index.html, the Apache service needs to be restarted to apply the changes. But if the file hasn't changed, restarting the service is a waste of time and drops active connections.

How it Works

You add the notify directive to any standard task. If that task results in a changed state (e.g., a file was actually modified or a package was newly installed), it sends a signal to a corresponding handler by name.

The Apache Handler Playbook

deploy_apache.yml

---
- name: install apache using handler
  hosts: localhost
  become: yes
  tasks:
    - name: install apache
      apt:
        name: apache2
        state: present
      notify: Restart apache

    - name: copy file
      copy:
        src: index.html
        dest: /var/www/html
      notify: Restart apache

    - name: restart service
      service:
        name: apache2
        state: started
        enabled: true

  handlers:
    - name: Restart apache
      service:
        name: apache2
        state: restarted

The Execution Flow

What happens when you run this?

1. Install Ansible ensures apache2 is installed. If it has to download it, it registers a "changed" state and queues the Restart apache handler.
2. Copy Ansible copies index.html. If the file is different from what's on the server, it queues the handler again.
3. Start Ansible simply ensures the service is currently running and enabled on boot.
4. Handlers Flush At the very end of the play, Ansible looks at the queue. It sees the Restart apache handler was notified, and executes it exactly once.

terminal output

$ ansible-playbook deploy_apache.yml

TASK [install apache] *************************************
ok: [localhost]

TASK [copy file] ******************************************
changed: [localhost]

TASK [restart service] ************************************
ok: [localhost]

RUNNING HANDLER [Restart apache] **************************
changed: [localhost]