Handlers are special tasks that mostly work like normal tasks (that is to say, they run modules), but are not necessarily run in every playbook run on every single host. Why? They are triggered by conditions occuring from other modules changing something.
The most common use of handlers is to restart a service only when the configuration file for that service changes, for example, if the configuration file is replaced by a new version of a template, we may want to restart the service using that configuration file. Many users will use handlers for no other reason than this.
To phrase this differently, handlers are run only when notified by some other task. These notifications happen only when changes occur on that triggering task. If the triggering task found that nothing needed to be changed or executed, the handler would not need to run.
Overflow Of Handler Logic
notify: restart foo
subscribe: restart foo
The name of the handler must match the name of the "notify" signal string for the handler to run.
subscribe vs name
The names of tasks in Jet are just optional comments, the "subscribe" parameter is how the handlers are identified. It is also possible for multiple handlers to subscribe to the same event messages!