Handlers

If you want a task to run only when a change is made on a machine. For example, you may want to restart a service if a task updates the configuration of that service, but not if the configuration is unchanged. Ansible uses handlers to address this use case. Handlers are tasks that only run when notified.

---
- name: install required software
  hosts: webserver
  gather_facts: yes
  become: yes

  tasks:
    - name: deploy latest html file
      ansible.builtin.template:
        src: html.j2
        dest: /var/www/html/index.html
      notify: restart httpd services

  handlers:
    - name: restart httpd services
      ansible.builtin.service:
        name: httpd
        state: started

Example Playbook: 01-deploy-code-restart-httpd.yml

Listen Keyword

Handlers can utilize the listen keyword. Using this handler keyword, handlers can listen on topics that can group multiple handlers as follows:

- name: install required software
 hosts: webserver
 gather_facts: yes
 become: yes
 
 tasks:
   - name: deploy latest html file
     ansible.builtin.template:
       src: html.j2
       dest: /var/www/html/index.html
     notify: latest_html_deployed

 handlers:
   - name: restart httpd services
     ansible.builtin.service:
       name: httpd
       state: restarted
     listen: latest_html_deployed

   - name: send email
     debug: 
       msg: "email sent."
     listen: latest_html_deployed

Example Playbook: 02-deploy-code-perform-multiple-tasks.yml

Controlling when handlers run

Handlers in Ansible are executed after all tasks within a play are finished. Notified handlers, specifically, are triggered automatically in a specific sequence: first after pre-tasks, then after roles/tasks, and finally after post-tasks. This mechanism is designed for efficiency, ensuring that a handler runs just once even if multiple tasks notify it. For instance, if several tasks modify a configuration file and notify a handler to restart Apache, Ansible ensures that Apache is only restarted once to avoid unnecessary restarts.

---
---
- name: flush handlers example
  hosts: webserver
  gather_facts: yes
  tasks:
    - name: update packages
      become:  yes
      package:
        name: httpd
        state: present
      notify: send email

    - name: running handlers which has been notified.
      meta: flush_handlers

    - name: perform patching
      copy:
        content: " {{ ansible_date_time }} "
        dest: /tmp/test.txt
      notify: send email
      
  handlers:
    - name: send email
      debug:
        msg: "email sent"

Example Playbook: 03-flush-handlers.yml