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Ansible

Playbooks

Playbooks are a completely different way to use ansible and are particularly awesome. They are the basis for a really simple configuration management and multi-machine deployment system, unlike any that already exist, and one that is very well suited to deploying complex applications.

Playbooks can declare configurations, but they can also orchestrate steps of any manual ordered process, even as different steps must bounce back and forth between sets of machines in particular orders. They can launch tasks synchronously or asynchronously.

While you might run the main /usr/bin/ansible program for ad-hoc tasks, playbooks are more likely to be kept in source control and used to push out your configuration or assure the configurations of your remote systems are in spec.

Let’s dive in and see how they work. As you go, you may wish to open the github examples directory in another tab, so you can apply the theory to what things look like in practice.

Playbook Example

Playbooks are expressed in YAML format and have a minimum of syntax. Each playbook is composed of one or more ‘plays’ in a list.

By composing a playbook of multiple ‘plays’, it is possible to orchestrate multi-machine deployments, running certain steps on all machines in the webservers group, then certain steps on the database server group, then more commands back on the webservers group, etc.

For starters, here’s a playbook that contains just one play:

---
- hosts: webservers
  vars:
    http_port: 80
    max_clients: 200
  user: root
  tasks:
  - name: ensure apache is at the latest version
    action: yum pkg=httpd state=latest
  - name: write the apache config file
    action: template src=/srv/httpd.j2 dest=/etc/httpd.conf
    notify:
    - restart apache
  - name: ensure apache is running
    action: service name=httpd state=started
  handlers:
    - name: restart apache
      action: service name=apache state=restarted

Below, we’ll break down what the various features of the playbook language are.

Basics

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 also available:

---
- hosts: webservers
  user: yourname
  sudo: True

If you need to specify a password to sudo, run ansible-playbook with –ask-sudo-pass (-K). If you run a sudo playbook and the playbook seems to hang, it’s probably stuck at the sudo prompt. Just Control-C to kill it and run it again with -K.

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 (ansible provides some of these, plus we include ones taken from facter or ohai if 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_. Ansible variables (0.3 and later) are not surprisingly prefixed with ansible_. 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.

Running Operations On Change

As we’ve mentioned, nearly all modules are written to be ‘idempotent’ and can relay when they have affected a change on the remote system. Playbooks recognize this and have a basic event system that can be used to respond to change.

These ‘notify’ actions are triggered at the end of each ‘play’ in a playbook, and trigger only once each. For instance, multiple resources may indicate that apache needs to be restarted, but apache will only be bounced once.

Here’s an example of restarting two services when the contents of a file change, but only if the file changes:

- name: template configuration file
  action: template src=template.j2 dest=/etc/foo.conf
  notify:
     - restart memcached
     - restart apache

The things listed in the ‘notify’ section of a task are called handlers.

Handlers are lists of tasks, not really any different from regular tasks, that are referenced by name. Handlers are what notifiers notify. If nothing notifies a handler, it will not run. Regardless of how many things notify a handler, it will run only once, after all of the tasks complete in a particular play.

Here’s an example handlers section:

handlers:
    - name: restart memcached
      action: service name=memcached state=restarted
    - name: restart apache
      action: service name=apache state=restarted

Handlers are best used to restart services and trigger reboots. You probably won’t need them for much else.

Note

Notify handlers are always run in the order written.

Power Tricks

Now that you have the basics down, let’s learn some more advanced things you can do with playbooks.

Local Playbooks

It may be useful to use a playbook locally, rather than by connecting over SSH. This can be useful for assuring the configuration of a system by putting a playbook on a crontab. This may also be used to run a playbook inside a OS installer, such as an Anaconda kickstart.

To run an entire playbook locally, just set the “hosts:” line to “hosts:127.0.0.1” and then run the playbook like so:

playbook playbook.yml --connection=local

Alternatively, a local connection can be used in a single playbook play, even if other plays in the playbook use the default remote connection type:

hosts: 127.0.0.1
connection: local

Variables From Other Hosts

If your database server wants to check the value of a ‘fact’ from another node, it’s easy to do so within a template or even an action line:

{{ hostvars.get('name_of_host').get('name_of_fact') }}

NOTE: No database or other complex system is required to exchange data between hosts. The hosts that you want to reference data from must be included in either the current play or any previous play.

External Variables And Prompted or 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. Similarly, sometimes you may just want to keep certain information in different files, away from the main playbook.

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

Alternatively, you may wish to prompt the user for certain input, and can do so with the similarly named ‘vars_prompt’ section. This has uses beyond security, for instance, you may use the same playbook for all software releases and would prompt for a particular release version in a push-script:

---
- hosts: all
  user: root
  vars:
    from: "camelot"
  vars_prompt:
    name: "what is your name?"
    quest: "what is your quest?"
    favcolor: "what is your favorite color?"

There are full examples of both of these items in the github examples/playbooks directory.

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:
  favcolor: blue
  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, or you can use variables that bubble up from ansible (0.3 and later). As a reminder, these variables are prefixed, so it’s $facter_operatingsystem, not $operatingsystem. Ansible’s built in variables are prefixed with ansible_. 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.

Note

Include statements are only usable from the top level playbook file. This means includes can not include other includes.

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.

Note

Playbooks do not always have to be declarative; you can do something similar to model a push process for a multi-tier web application. This is actually one of the things playbooks were invented to do.

Loop Shorthand

To save some typing, repeated tasks can be written in short-hand like so:

- name: add user $item
  action: user name=$item state=present groups=wheel
  with_items:
     - testuser1
     - testuser2

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 its 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

Note

There is no default for the async time limit. If you leave off the ‘async’ keyword, the task runs synchronously, which is Ansible’s default.

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

Note

You shouldn’t “fire and forget” with operations that require exclusive locks, such as yum transactions, if you expect to run other commands later in the playbook against those same resources.

Note

Using a higher value for –forks will result in kicking off asynchronous tasks even faster. This also increases the efficiency of polling.

Executing A Playbook

Now that you’ve learned playbook syntax, how do you run a playbook? It’s simple. Let’s run a playbook using a parallelism level of 10:

ansible-playbook playbook.yml -f 10

See also

YAML Syntax
Learn about YAML syntax
Ansible Modules
Learn about available modules
Module Development Guide
Learn how to extend Ansible by writing your own modules
The Inventory File, Patterns, and Groups
Learn about how to select hosts
Github examples directory
Complete playbook files from the github project source
Mailing List
Questions? Help? Ideas? Stop by the list on Google Groups

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