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.
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.
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 Inventory & Patterns 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.
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 like this:
$varname
In templates, the full power of the Jinja2 templating language is also available, which looks like this:
{{ varname }}
The Jinja2 documentation provides information about how to construct loops and conditionals for those who which to use more advanced templating. This is optional and the $varname format still works in template files.
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.
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 and shell 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.
The command and shell modules 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 and shell modules are the one modules that just takes a list of arguments, and don’t use the key=value form. This makes them 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: create a virtual host file for $vhost
action: template src=somefile.j2 dest=/etc/httpd/conf.d/$vhost
Those same variables are usable in templates, which we’ll get to later.
As we’ve mentioned, nearly all modules are written to be ‘idempotent’ and can relay when they have made 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.
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
You 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
Variables passed in can be used in the included files. You can reference them like this:
$user
In addition to the explicitly passed in parameters, all variables from the vars section are also available for use here as well.
Note
Include statements are only usable from the top level playbook file. This means includes can not include other includes. This may be implemented in a later release.
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 handlers.yml that looks 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.
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