# Handlers: Act Only on Change — Ansible: Automate Servers with Playbooks

Source: https://www.geekswithgeeks.com/en/ansible/flow-handlers

> Use notify and handlers to restart or reload services only when a configuration actually changed.

## Restart only when needed

A **handler** is a task that runs only when another task **notifies** it, and only if that task reported **changed**. Handlers run once at the end of the play, even if notified many times. This is the standard way to reload a service after a config change without restarting it needlessly on every run.

## The handler in action

Look back at the first and second runs of `site.yml`. In the first run the template changed, so `Reload app` ran for each host. In the second run nothing changed, so the handler did not run at all. Real handlers would call `ansible.builtin.service` or `systemd` instead of `debug`.

```yaml
  tasks:
    - name: Deploy nginx config
      ansible.builtin.template:
        src: nginx.conf.j2
        dest: /etc/nginx/nginx.conf
      notify: Reload nginx

  handlers:
    - name: Reload nginx
      ansible.builtin.service:
        name: nginx
        state: reloaded
```

## Flush when order matters

If a later task needs the service reloaded first, add `- meta: flush_handlers` to run pending handlers at that point instead of waiting for the end.

**Quiz:** When does a handler run?

- [ ] Never
- [ ] On every run, always
- [ ] Before the first task
- [x] When notified by a task that reported changed

*Answer:* When notified by a task that reported changed. Handlers fire only on change, once at the end of the play.