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
• Controlling when handlers run
• Using variables with handlers

Handler example

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

---
- name: Verify apache installation
  hosts: webservers
  vars:
    http_port: 80
    max_clients: 200
  remote_user: root
  tasks:
  - name: Ensure apache is at the latest version
    ansible.builtin.yum:
      name: httpd
      state: latest

  - name: Write the apache config file
    ansible.builtin.template:
      src: /srv/httpd.j2
      dest: /etc/httpd.conf
    notify:
    - Restart apache

  - name: Ensure apache is running
    ansible.builtin.service:
      name: httpd
      state: started

  handlers:
    - name: Restart apache
      ansible.builtin.service:
        name: httpd
        state: restarted

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

- name: Template configuration file
  ansible.builtin.template:
    src: template.j2
    dest: /etc/foo.conf
  notify:
    - Restart memcached
    - Restart apache

  handlers:
    - name: Restart memcached
      ansible.builtin.service:
        name: memcached
        state: restarted

    - name: Restart apache
      ansible.builtin.service:
        name: apache
        state: restarted

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:

tasks:
  - name: Some tasks go here
    ansible.builtin.shell: ...

  - name: Flush handlers
    meta: flush_handlers

  - name: Some other tasks
    ansible.builtin.shell: ...

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:

handlers:
# This handler name may cause your play to fail!
- name: Restart "{{ web_service_name }}"

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:

handlers:
  - name: Restart memcached
    ansible.builtin.service:
      name: memcached
      state: restarted
    listen: "restart web services"

  - name: Restart apache
    ansible.builtin.service:
      name: apache
      state: restarted
    listen: "restart web services"

tasks:
  - name: Restart everything
    ansible.builtin.command: echo "this task will restart the web services"
    notify: "restart web services"

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).

• Handlers always run in the order they are defined, not in the order listed in the notify-statement. This is also the case for handlers using listen.

• Handler names and listen topics live in a global namespace.

• Handler names are templatable and listen topics are not.

• Use unique handler names. If you trigger more than one handler with the same name, the first one(s) get overwritten. Only the last one defined will run.

• You can notify a handler defined inside a static include.

• You cannot notify a handler defined inside a dynamic include.

When using handlers within roles, note that:

• handlers notified within pre_tasks, tasks, and post_tasks sections are automatically flushed at the end of section where they were notified.

• handlers notified within roles section are automatically flushed at the end of tasks section, but before any tasks handlers.

• handlers are play scoped and as such can be used outside of the role they are defined in.

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