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Handlers: running operations on change

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.


Handler example

This playbook, verify-apache.yml, contains a single play with a handler:

In this example playbook, the second task notifies the handler. A single task can notify more than one handler:


Controlling when handlers run

By default, handlers run after all the tasks in a particular play have been completed. This approach is efficient, because the handler only runs once, regardless of how many tasks notify it. For example, if multiple tasks update a configuration file and notify a handler to restart Apache, Ansible only bounces Apache once to avoid unnecessary restarts.

If you need handlers to run before the end of the play, add a task to flush them using the meta module, which executes Ansible actions:

The meta: flush_handlers task triggers any handlers that have been notified at that point in the play.


Using variables with handlers

You may want your Ansible handlers to use variables. For example, if the name of a service varies slightly by distribution, you want your output to show the exact name of the restarted service for each target machine. Avoid placing variables in the name of the handler. Since handler names are templated early on, Ansible may not have a value available for a handler name like this:

If the variable used in the handler name is not available, the entire play fails. Changing that variable mid-play will not result in newly created handler.

Instead, place variables in the task parameters of your handler. You can load the values using include_vars like this:

    tasks:
      - name: Set host variables based on distribution
        include_vars: "{{ ansible_facts.distribution }}.yml"
    
    handlers:
      - name: Restart web service
        ansible.builtin.service:
          name: "{{ web_service_name | default('httpd') }}"
          state: restarted
    

Handlers can also 'listen' to generic topics, and tasks can notify those topics as follows:

This use makes it much easier to trigger multiple handlers. It also decouples handlers from their names, making it easier to share handlers among playbooks and roles (especially when using 3rd party roles from a shared source like Galaxy).

When using handlers within roles, note that:

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