Advanced Playbooks¶

Accessing Complex Variable Data¶

Some provided facts, like networking information, are made available as nested datastructures. To access them a simple ‘$foo’ is not sufficient, but it is still easy to do. Here’s how we get an IP address using Ansible 0.4 and later:

${ansible_eth0.ipv4.address}

It is also possible to access variables whose elements are arrays:

${somelist[0]}

And the array and hash reference syntaxes can be mixed.

In templates, the simple access form still holds, but they can also be accessed from Jinja2 in more Python-native ways if that is preferred:

{{ ansible_eth0["ipv4"]["address"] }}

Accessing Information About Other Hosts¶

If your database server wants to check the value of a ‘fact’ from another node, or an inventory variable assigned to another node, it’s easy to do so within a template or even an action line (note: this uses syntax available in 0.4 and later):

${hostvars.hostname.factname}

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.

Magic Variables¶

Some variables made available to hosts don’t come from definitions in a playbook, the inventory file, or discovery from the system. There are only two of these, and are used in special cases that many users won’t need.

groups is a list (array) of all the groups the current host is in. This can be used in templates using Jinja2 syntax to make template source files that vary based on the group membership (or role) of the host:

{% if 'webserver' in groups %}
   # some part of a configuration file that only applies to webservers
{% endif %}

inventory_hostname is the name of the hostname as configured in Ansible’s inventory host file. This can be useful for when you don’t want to rely on the discovered hostname ansible_hostname or for other mysterious reasons. Don’t worry about it unless you think you need it.

Variable File Seperation¶

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

Prompting For Sensitive Data¶

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.

Passing Variables On The Command Line¶

In addition to vars_prompt and vars_files, it is possible to send variables over the ansible command line. This is particularly useful when writing a generic release playbook where you may want to pass in the version of the application to deploy:

ansible-playbook release.yml --extra-vars "version=1.23.45 other_variable=foo"

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, which actually is 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

Note that a variable ($facter_operatingsystem) is being interpolated into the list of filenames being defined for vars_files.

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.

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

The above would be the equivalent of:

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

In a future release, the yum and apt modules will use with_items to execute fewer package manager transactions.

Selecting Files And Templates Based On Variables¶

Sometimes a configuration file you want to copy, or a template you will use may depend on a variable. The following construct (new in 0.4) selects the first available file appropriate for the variables of a given host, which is often much cleaner than putting a lot of if conditionals in a template.

The following example shows how to template out a configuration file that was very different between, say, CentOS and Debian:

- name: template a file
  action: template src=$item dest=/etc/myapp/foo.conf
  first_available_file:
    - /srv/templates/myapp/${ansible_distribution}.conf
    - /srv/templates/myapp/default.conf

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

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

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:

ansible-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

Pull-Mode Playbooks¶

The use of playbooks in local mode (above) is made extremely powerful with the addition of ansible-pull in the 0.4 release. A script for setting up ansible-pull is provided in the examples/playbooks directory of the source checkout.

The basic idea is to use Ansible to set up a remote copy of ansible on each managed node, each set to run via cron and update playbook source via git. This interverts the default push architecture of ansible into a pull architecture, which has near-limitless scaling potential. The setup playbook can be tuned to change the cron frequency, logging locations, and parameters to ansible-pull.

This is useful both for extreme scale-out as well as periodic remediation. Usage of the ‘fetch’ module to retrieve logs from ansible-pull runs would be an excellent way to gather and analyze remote logs from ansible-pull.