See also
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, or they can automate steps of a manual ordered process. 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.
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.
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. This is just like the first parameter to /usr/bin/ansible.
Each play gets to designate it’s own choice of patterns.
Playbook steps on the remote system can be executed as any user. The default is root, but you can specify others. Sudo support is pending.:
user: mdehaan
The vars’ section contains a list of variables and values that can be used in the plays. These can be used in templates or tasks and are dereferenced using `jinja2 syntax 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.
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.
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.
The action line is the name of an ansible module followed by parameters in key=value form:
- 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. Simple:
- 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:
- 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.
As we’ve mentioned, nearly all modules are written to be ‘idempotent’ and can signal 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
Next up, we’ll show what a handler looks like.
Note
Notify handlers are always run in the order written.
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.
Now that you have the basics down, let’s learn some more advanced things you can do with playbooks.
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
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.
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.
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
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.
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
Note
Don’t forget to check out the Github examples directory for examples of playbooks in action, so you can see how all of these features can be put together.