How to Use Handlers in Ansible Playbook

In Ansible, a handler is just like any other task but only runs when called or notified. It takes action when a change has been made on the managed host. Handlers are used in initiating a secondary change such as starting or restarting a service after installation or even reloading a service after some modifications have been made in the configuration files.  In this guide, we will shed more light on Ansible handlers. We will learn how to use handlers in ansible playbook.

Ansible playbook file with a handler

To better understand how Handlers work, we will take an example of a playbook file – install_apache.yml – that installs the Apache webserver and later restarts the Apache service. In the example below, the handler is notified to restart the Apache service soon after installation. This is achieved using the notify module as shown. Note that the ‘notify’ name should coincide with the handler name as pointed out, otherwise you will encounter errors in your playbook file.

---
- hosts: staging
  name: Install
  become: yes
  tasks:
          - name: Install Apache2 on  Ubuntu server
            apt:
                    name: apache2
                    state: present
                    update_cache: yes
            notify:
                    - Restart apache2

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

Now let’s run the playbook file.

$ ansible-playbook /etc/ansible/install_apache.yml -K

From the output, you can see the Handler being executed right after the task.

Multiple tasks with multiple handlers

Additionally, we can have several tasks calling multiple handlers. Consider the playbook file below.

Here are have 2 tasks to run:

• Installing Apache webserver
• Allowing HTTP traffic on the UFW firewall.

After the tasks are successfully executed, I have called each of the handlers with the ‘notify’ module as shown below. The first handler restarts Apache and the second one reloads the UFW firewall.

---
- hosts: staging
  name: Install
  become: yes
  tasks:
         - name: Install Apache2 on  Ubuntu server
           apt:
                   name: apache2
                   state: present
                   update_cache: yes

         - name: Allow HTTP traffic on UFW firewall
           ufw:
                   rule: allow
                   port: http
                   proto: tcp

           notify:
                   - Restart apache2
                   - Reload ufw firewall
  handlers:
          - name: Restart apache2
            service:
                    name:  apache2
                    state: restarted

          - name: Reload ufw firewall
            ufw:
                    state: enabled

When the playbook file is executed,  both handlers are executed by Ansible right after Apache is installed and HTTP traffic is allowed on the firewall.

The secondary actions executed by the handlers here are:

• Restarting Apache
• Enabling and reloading the firewall for the changes made to be effected.

Conclusion

As you have seen, handlers are just like regular tasks, only that they are referenced using a globally unique module called ‘notify’. If a handler is no notified, it fails to run. Remember that all handlers run after all the tasks have been completed.

Also Read : How to Create Ansible Roles and Use them in Playbook