What are handlers?

Handlers in the context of DevOps, especially in tools like Ansible, are tasks or blocks of code that execute only under certain conditions — when they are explicitly "notified" by another task. They are used to restart services, reboot servers, or perform other actions that should only occur after significant configuration changes.

Key features:

• Executed upon notification: A handler runs only after one or more tasks explicitly "notify" it (for example, using notify in Ansible).
• Run once: Even if multiple tasks notify the same handler, it will only execute once at the end of the playbook or within the include_tasks/import_tasks section where it is defined. This prevents multiple restarts of services.
• Declarative approach: They allow you to declaratively specify what actions should be taken when the system's state changes, without worrying about manually triggering these actions.

Example of notifying a handler in an Ansible task:

- name: Configure web server  # Task that may change configuration
  template:
    src: nginx.conf.j2
    dest: /etc/nginx/nginx.conf
  notify:  # Notify the handler
    - restart nginx

Definition of a handler:

handlers:
  - name: restart nginx  # Handler name, matches the name in notify
    service:
      name: nginx
      state: restarted