Playbooks¶

Hosts and Users¶

For each play in a playbook, you get to choose which machines in your infrastructure to target and what remote user to complete the steps (called tasks) as.

The hosts line is a list of one or more groups or host patterns, separated by colons, as described in the The Inventory File, Patterns, and Groups documentation. The user is just the name of the user account:

---
- hosts: webservers
  user: root

Support for running things from sudo is pending.

Vars section¶

The vars section contains a list of variables and values that can be used in the plays, like this:

---
- hosts: webservers
  users: root
  vars:
     http_port: 80
     van_halen_port: 5150
     other: 'magic'

These variables can be used later in the playbook, or on the managed system (in templates), just like this:

{{ varname }}

Within playbooks themselves, but not within templates on the remote machines, it’s also legal to use nicer shorthand like this:

$varname

Further, if there are discovered variables about the system (say, if facter or ohai were installed) these variables bubble up back into the playbook, and can be used on each system just like explicitly set variables.

Facter variables are prefixed with facter_ and Ohai variables are prefixed with ohai_. So for instance, if I wanted to write the hostname into the /etc/motd file, I could say:

- name: write the motd
  action: template src=/srv/templates/motd.j2 dest=/etc/motd

And in /srv/templates/motd.j2:

You are logged into {{ facter_hostname }}

But we’re getting ahead of ourselves. Let’s talk about tasks.

Tasks list¶

Each play contains a list of tasks. Tasks are executed in order, one at a time, against all machines matched by the host pattern, before moving on to the next task.

Hosts with failed tasks are taken out of the rotation for the entire playbook. If things fail, simply correct the playbook file and rerun.

The goal of each task is to execute a module, with very specific arguments. Variables, as mentioned above, can be used in arguments to modules.

Modules other than command are ‘idempotent’, meaning if you run them again, they will make the changes they are told to make to bring the system to the desired state. This makes it very safe to rerun the same playbook multiple times. They won’t change things unless they have to change things.

Command will actually rerun the same command again, which is totally ok if the command is something like ‘chmod’ or ‘setsebool’, etc.

Every task must have a name, which is included in the output from running the playbook. This is output for humans, so it is nice to have reasonably good descriptions of each task step.

Here is what a basic task looks like, as with most modules, the service module takes key=value arguments:

tasks:
  - name: make sure apache is running
    action: service name=httpd state=running

The command module is the one module that just takes a list of arguments, and doesn’t use the key=value form. This makes it work just like you would expect. Simple:

tasks:
  - name: disable selinux
    action: command /sbin/setenforce 0

Variables can be used in action lines. Suppose you defined a variable called ‘vhost’ in the ‘vars’ section, you could do this:

tasks:
  - name: make a directory
    action: template src=somefile.j2 dest=/etc/httpd/conf.d/$vhost

Those same variables are usable in templates, which we’ll get to later.

External Variables And Sensitive Data¶

It’s a great idea to keep your playbooks under source control, but you may wish to make the playbook source public while keeping certain important variables private. You can do this by using an external variables file, or files, just like this:

---
- hosts: all
  user: root
  vars:
    favcolor: blue
  vars_files:
    - /vars/external_vars.yml
  tasks:
  - name: this is just a placeholder
    action: command /bin/echo foo

This removes the risk of sharing sensitive data with others when sharing your playbook source with them.

The contents of each variables file is a simple YAML dictionary, like this:

---
# in the above example, this would be vars/external_vars.yml
somevar: somevalue
password: magic

Conditional Execution¶

Sometimes you will want to skip a particular step on a particular host. This could be something as simple as not installing a certain package if the operating system is a particular version, or it could be something like performing some cleanup steps if a filesystem is getting full.

This is easy to do in Ansible, with the only_if clause. This clause can be applied to any task, and allows usage of variables from anywhere in ansible, either denoted with $dollar_sign_syntax or {{ braces_syntax }} and then evaluates them with a Python expression. Don’t panic – it’s actually pretty simple.:

vars:
  is_favcolor_blue: "'$favcolor' == 'blue'"
  is_centos: "'$facter_operatingsystem' == 'CentOS'"
tasks:
  - name: "shutdown if my favorite color is blue"
    action: command /sbin/shutdown -t now
    only_if: '$is_favcolor_blue'

Variables from tools like facter and ohai can be used here, if installed. As a reminder, these variables are prefixed, so it’s $facter_operatingsystem, not $operatingsystem. The only_if expression is actually a tiny small bit of Python, so be sure to quote variables and make something that evaluates to True or False. It is a good idea to use ‘vars_files’ instead of ‘vars’ to define all of your conditional expressions in a way that makes them very easy to reuse between plays and playbooks.

Conditional Imports¶

Sometimes you will want to do certain things differently in a playbook based on certain criteria. Having one playbook that works on multiple platforms and OS versions is a good example.

As an example, the name of the Apache package may be different between CentOS and Debian, but it is easily handled with a minimum of syntax in an Ansible Playbook:

---
- hosts: all
  user: root
  vars_files:
    - "vars/common.yml"
    - [ "vars/$facter_operatingsystem.yml", "vars/os_defaults.yml" ]
  tasks:
  - name: make sure apache is running
    action: service name=$apache state=running

As a reminder, the various YAML files contain just keys and values:

---
# for vars/CentOS.yml
apache: httpd
somethingelse: 42

How does this work? If the operating system was ‘CentOS’, the first file Ansible would try to import would be ‘vars/CentOS.yml’, followed up by ‘/vars/os_defaults.yml’ if that file did not exist. If no files in the list were found, an error would be raised. On Debian, it would instead first look towards ‘vars/Debian.yml’ instead of ‘vars/CentOS.yml’, before falling back on ‘vars/os_defaults.yml’. Pretty simple.

To use this conditional import feature, you’ll need facter or ohai installed prior to running the playbook, but you can of course push this out with Ansible if you like:

# for facter
ansible -m yum -a "pkg=facter ensure=installed"
ansible -m yum -a "pkg=ruby-json ensure=installed"

# for ohai
ansible -m yum -a "pkg=ohai ensure=installed"

Ansible’s approach to configuration – seperating variables from tasks, keeps your playbooks from turning into arbitrary code with ugly nested ifs, conditionals, and so on - and results in more streamlined & auditable configuration rules – especially because there are a minimum of decision points to track.

Include Files And Reuse¶

Suppose you want to reuse lists of tasks between plays or playbooks. You can use include files to do this.

An include file simply contains a flat list of tasks, like so:

---
# possibly saved as tasks/foo.yml
- name: placeholder foo
  action: command /bin/foo
- name: placeholder bar
  action: command /bin/bar

Include directives look like this:

- tasks:
   - include: tasks/foo.yml

Variables passed in can be used in the include files too. Assume a variable named ‘user’. Using jinja2 syntax, anywhere in the included file, you can say:

{{ user }}

I can also pass variables into includes directly. We might call this a ‘parameterized include’.

For instance, if deploying multiple wordpress instances, I could contain all of my wordpress tasks in a single wordpress.yml file, and use it like so:

- tasks:
  - include: wordpress.yml user=timmy
  - include: wordpress.yml user=alice
  - include: wordpress.yml user=bob

In addition to the explicitly passed in parameters, all variables from the vars section are also available for use here as well. Variables that bubble up from tools like facter and ohai are not usable here though – but they ARE available for use inside ‘action’ lines and in templates.

Includes can also be used in the ‘handlers’ section, for instance, if you want to define how to restart apache, you only have to do that once for all of your playbooks. You might make a notifiers.yaml that looked like:

----
# this might be in a file like handlers/handlers.yml
- name: restart apache
  action: service name=apache state=restarted

And in your main playbook file, just include it like so, at the bottom of a play:

handlers:
  - include: handlers/handlers.yml

You can mix in includes along with your regular non-included tasks and handlers.

Note that you can not conditionally path the location to an include file, like you can with ‘vars_files’. If you find yourself needing to do this, consider how you can restructure your playbook to be more class/role oriented.

Using Includes To Assign Classes of Systems¶

Include files are really powerful when used to reuse logic between playbooks. You could imagine a playbook describing your entire infrastructure like this, in a list of just a few plays:

---
- hosts: atlanta-webservers
  vars:
    datacenter: atlanta
  tasks:
  - include: tasks/base.yml
  - include: tasks/webservers.yml database=db.atlanta.com
  handlers:
    - include: handlers/common.yml
- hosts: atlanta-dbservers
  vars:
    datacenter: atlanta
  tasks:
  - include: tasks/base.yml
  - include: tasks/dbservers.yml
  handlers:
    - include: handlers/common.yml

There is one (or more) play defined for each group of systems, and each play maps each group to several includes. These includes represent ‘class definitions’, telling the systems what they are supposed to do or be. In the above example, all hosts get the base configuration first and further customize it depending on what class or nature of machines they are.

Asynchronous Actions and Polling¶

By default tasks in playbooks block, meaning the connections stay open until the task is done on each node. If executing playbooks with a small parallelism value (aka –forks), you may wish that long running operations can go faster. The easiest way to do this is to kick them off all at once and then poll until they are done.

You will also want to use asynchronous mode on very long running operations that might be subject to timeout.

To launch a task asynchronously, specify it’s maximum runtime and how frequently you would like to poll for status. The default poll value is 10 seconds if you do not specify a value for poll:

---
- hosts: all
  user: root
  tasks:
  - name: simulate long running op (15 sec), wait for up to 45, poll every 5
    action: command /bin/sleep 15
    async: 45
    poll: 5

Alternatively, if you do not need to wait on the task to complete, you may “fire and forget” by specifying a poll value of 0:

---
- hosts: all
  user: root
  tasks:
  - name: simulate long running op, allow to run for 45, fire and forget
    action: command /bin/sleep 15
    async: 45
    poll: 0