In this blog, we are talking about how to use handlers in ansible playbook.
Introduction to Ansible Handlers
Handlers are commonly used in Ansible to start, reload, restart, and stop services. If your playbook calls for changing configuration files, you’ll almost certainly need to restart a service to see the changes take effect. In this case, you must define a handler for that service and include the notify directive in any tasks that require it.
Handlers are executed last by default, regardless of where they are in the playbook. In a playbook, you can define and call one or more handlers based on the tasks to be completed.
Using Ansible Handlers
Handlers, as previously stated, are similar to other tasks in a playbook, with the exception that they are triggered using the notify directive and are only executed when the state changes.
Some points to keep in mind while using handlers.
- Within the playbook, a handler should have a globally unique name.
- Only the first handler will be called if more than one with the same name are defined. All other handlers will be disregarded.
- Handlers are always executed in the order they are defined in the handler’s section, not in the notify section.
- A handler task only runs if the state changes; otherwise, it does not.
- The notify and handlers directives are used to define a handler. The notify directive causes the task(s) specified in the handler's section to be executed.
Handler Example
To start with the hands-on lab for handlers we need to have the below set in AWS console.
Prerequisites
1. server.cnl.com — 1 CPU — 1GB RAM (Python 2.7) — Ansible Server
2. node1.cnl.com — 1 CPU — 1GB RAM (python 2.6 and above) — Ansible Client 1
3. node2.cnl.com — 1 CPU — 1GB RAM (python 2.6 and above) — Ansible Client 2
From ansible server execute below command
ansible all -m pingping command should return with ping / pong green colour. Now create a yaml file with below content added in it.
---
- name: Install Apache on RHEL server
hosts: all
tasks:
- name: Install latest version of Apache
yum:
name: httpd
state: latest
notify:
- start apache
handlers:
- name: start apache
service:
name: httpd
state: restarted
In the preceding example, we install the most recent Apache package and then restart the service. However, if the most recent package is already present, the service will not be restarted. To accomplish this, we use handlers.
When the above playbook is executed, the following output is obtained.
The handlers are not executed because the Apache is already present. So, we’ve removed Apache from node 1 and attempted to run the above playbook. Because Apache is not present in node1, it installs the most recent package of Apache and then executes the handler.
Ansible only bounces Apache once to avoid unnecessary restarts when multiple tasks update a configuration file and notify a handler to restart Apache.
Using the ‘listen’ Directive to Group Handlers
You can use the listen keyword to group handlers and call them all with a single notify statement. The notify directive is set to restart services in the following playbook, which triggers all handler tasks with the listen directive.
Determine When Handlers Should Run
If you want handlers to run before the end of the play or between tasks, use the meta module to add a meta task to flush them. The following is the definition of the meta task.
- name: Flush handlers
meta: flush_handlersThank you for visiting the blog.
