Sometimes 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. Each handler should have a globally unique name.
Ansible provides feature named handlers, which is like a task but will only run when called by a notifier in another task. This feature is important because your requirements for running a task may depend on the state of a service, existence of a file or a follow up tasks when state changed. We can make use of variables in notify and handlers which makes it more flexible. Ansible Handlers are used extensively in production and live environments as you always depend on something else to decide whether a task should run or not.