Intro to Playbooks

Ansible Playbooks offer a repeatable, re-usable, simple configuration management and multi-machine deployment system, one that is well suited to deploying complex applications. If you need to execute a task with Ansible more than once, write a playbook and put it under source control. Then you can use the playbook to push out new configuration or confirm the configuration of remote systems. The playbooks in the ansible-examples repository illustrate many useful techniques. You may want to look at these in another tab as you read the documentation.

Playbooks can:

  • declare configurations
  • orchestrate steps of any manual ordered process, on multiple sets of machines, in a defined order
  • launch tasks synchronously or asynchronously

Playbook syntax

Playbooks are expressed in YAML format with a minimum of syntax. If you are not familiar with YAML, look at our overview of YAML Syntax and consider installing an add-on for your text editor (see Other Tools And Programs) to help you write clean YAML syntax in your playbooks.

A playbook is composed of one or more ‘plays’ in an ordered list. The terms ‘playbook’ and ‘play’ are sports analogies. Each play executes part of the overall goal of the playbook, running one or more tasks. Each task calls an Ansible module.

Playbook execution

A playbook runs in order from top to bottom. Within each play, tasks also run in order from top to bottom. Playbooks with multiple ‘plays’ can orchestrate multi-machine deployments, running one play on your webservers, then another play on your database servers, then a third play on your network infrastructure, and so on. At a minimum, each play defines two things:

  • the managed nodes to target, using a pattern
  • at least one task to execute

In this example, the first play targets the web servers; the second play targets the database servers:

---
- name: update web servers
  hosts: webservers
  remote_user: root

  tasks:
  - name: ensure apache is at the latest version
    yum:
      name: httpd
      state: latest
  - name: write the apache config file
    template:
      src: /srv/httpd.j2
      dest: /etc/httpd.conf

- name: update db servers
  hosts: databases
  remote_user: root

  tasks:
  - name: ensure postgresql is at the latest version
    yum:
      name: postgresql
      state: latest
  - name: ensure that postgresql is started
    service:
      name: postgresql
      state: started

Your playbook can include more than just a hosts line and tasks. For example, the playbook above sets a remote_user for each play. This is the user account for the SSH connection. You can add other Playbook Keywords at the playbook, play, or task level to influence how Ansible behaves. Playbook keywords can control the connection plugin, whether to use privilege escalation, how to handle errors, and more. To support a variety of environments, Ansible lets you set many of these parameters as command-line flags, in your Ansible configuration, or in your inventory. Learning the precedence rules for these sources of data will help you as you expand your Ansible ecosystem.

Task execution

By default, Ansible executes each task in order, one at a time, against all machines matched by the host pattern. Each task executes a module with specific arguments. When a task has executed on all target machines, Ansible moves on to the next task. You can use strategies to change this default behavior. Within each play, Ansible applies the same task directives to all hosts. If a task fails on a host, Ansible takes that host out of the rotation for the rest of the playbook.

When you run a playbook, Ansible returns information about connections, the name lines of all your plays and tasks, whether each task has succeeded or failed on each machine, and whether each task has made a change on each machine. At the bottom of the playbook execution, Ansible provides a summary of the nodes that were targeted and how they performed. General failures and fatal “unreachable” communication attempts are kept separate in the counts.

Desired state and ‘idempotency’

Most Ansible modules check whether the desired final state has already been achieved, and exit without performing any actions if that state has been achieved, so that repeating the task does not change the final state. Modules that behave this way are often called ‘idempotent.’ Whether you run a playbook once, or multiple times, the outcome should be the same. However, not all playbooks and not all modules behave this way. If you are unsure, test your playbooks in a sandbox environment before running them multiple times in production.

Running playbooks

To run your playbook, use the ansible-playbook command:

ansible-playbook playbook.yml -f 10

Use the --verbose flag when running your playbook to see detailed output from successful modules as well as unsuccessful ones.

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.

This playbook, verify-apache.yml, contains a single play with variables, the remote user, and 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
    yum:
      name: httpd
      state: latest
  - name: write the apache config file
    template:
      src: /srv/httpd.j2
      dest: /etc/httpd.conf
    notify:
    - restart apache
  - name: ensure apache is running
    service:
      name: httpd
      state: started
  handlers:
    - name: restart apache
      service:
        name: httpd
        state: restarted

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

- name: template configuration file
  template:
    src: template.j2
    dest: /etc/foo.conf
  notify:
     - restart memcached
     - restart apache
 handlers:
     - name: restart memcached
       service:
         name: memcached
         state: restarted
     - name: restart apache
       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:
   - shell: some tasks go here
   - meta: flush_handlers
   - shell: some other tasks

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
    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
      service:
        name: memcached
        state: restarted
      listen: "restart web services"
    - name: restart apache
      service:
        name: apache
        state: restarted
      listen: "restart web services"

tasks:
    - name: restart everything
      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).

Note

  • 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 in the end of section where they were notified.
  • handlers notified within roles section are automatically flushed in 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.

Ansible-Pull

Should you want to invert the architecture of Ansible, so that nodes check in to a central location, instead of pushing configuration out to them, you can.

The ansible-pull is a small script that will checkout a repo of configuration instructions from git, and then run ansible-playbook against that content.

Assuming you load balance your checkout location, ansible-pull scales essentially infinitely.

Run ansible-pull --help for details.

There’s also a clever playbook available to configure ansible-pull via a crontab from push mode.

Verifying playbooks

You may want to verify your playbooks to catch syntax errors and other problems before you run them. The ansible-playbook command offers several options for verification, including --check, --diff, --list-hosts, list-tasks, and --syntax-check. The Tools for Validating Playbooks describes other tools for validating and testing playbooks.

ansible-lint

You can use ansible-lint for detailed, Ansible-specific feedback on your playbooks before you execute them. For example, if you run ansible-lint on the playbook called verify-apache.yml near the top of this page, you should get the following results:

$ ansible-lint verify-apache.yml
[403] Package installs should not use latest
verify-apache.yml:8
Task/Handler: ensure apache is at the latest version

The ansible-lint default rules page describes each error. For [403], the recommended fix is to change state: latest to state: present in the playbook.

See also

ansible-lint
Learn how to test Ansible Playbooks syntax
YAML Syntax
Learn about YAML syntax
Tips and tricks
Tips for managing playbooks in the real world
All modules
Learn about available modules
Should you develop a module?
Learn to extend Ansible by writing your own modules
Patterns: targeting hosts and groups
Learn about how to select hosts
GitHub examples directory
Complete end-to-end playbook examples
Mailing List
Questions? Help? Ideas? Stop by the list on Google Groups