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.
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.
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:
---
- hosts: webservers
vars:
http_port: 80
max_clients: 200
user: root
tasks:
- include: base.yml somevar=3 othervar=4
- 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:
- include: handlers.yml
The hosts line is a list of one or more groups or host patterns, seperated by colons, asdescribed in the ‘patterns’ documentation. This is just like the first parameter to /usr/bin/ansible.
A list of variables and values that can be used in the plays. These can be used in templates or ‘action’ lines 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 play’s 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.
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. Usually these are expressed in key=value form, except for the command module, which looks just like a Linux/Unix command line. See the module documentation for more info.
Variables, as mentioned above, can be used in action lines. So if, hypothetically, you wanted to make a directory on each system named after the hostname ... yeah, that’s I know silly ... you could do it like so:
- name: make a directory
- action: mkdir /tmp/{{ facter_hostname }}
Nearly all modules are written to be ‘idempotent’ and can signal when they have affected a change on the remote system. If a notify statement is used, the named handler will be run against each system where a change was effected, but NOT on systems where no change occurred. This happens after all of the tasks are run. For example, if notifying Apache and potentially replacing lots of configuration files, you could have Apache restart just once, at the end of a run. If you need Apache restarted in the middle of a run, you could just make a task for it, no harm done. Notifiers are optional.
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.
Not all tasks have to be listed directly in the main file. An include file can contain a list of tasks (in YAML) as well, optionally passing extra variables into the file. Variables passed in can be deferenced like this (assume a variable named ‘user’):
{{ user }}
For instance, if deploying multiple wordpress instances, I could contain all of my tasks in a 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.
The format of an included list of tasks or handlers looks just like a flat list of tasks. Here is an example of what base.yml might look like:
---
- name: no selinux
action: command /usr/sbin/setenforce 0
- name: no iptables
action: service name=iptables state=stopped
- name: this is just to show variables work here, favcolor={{ favcolor }}
action: command /bin/true
As you can see above, variables in include files work just like they do in the main file. Including a variable in the name of a task is a contrived example, you could also pass them to the action command line or use them inside a template file.
Note that include statements are only usable from the top level playbook file. At this time, includes can not include other includes.
Include files are best used to reuse logic between playbooks. You could imagine a playbook describing your entire infrastructure like this:
---
- hosts: atlanta-webservers
vars:
datacenter: atlanta
tasks:
- include: base.yml
- include: webservers.yml database=db.atlanta.com
handlers:
- include: generic-handlers.yml
- hosts: atlanta-dbservers
vars:
datacenter: atlanta
tasks:
- include: base.yml
- include: dbservers.yml
handlers:
- include: generic-handlers.yml
There is one (or more) play defined for each group of systems, and each play maps each group includes one or more ‘class definitions’ telling the systems what they are supposed to do or be.
Using a common handlers file could allow one task in ‘webservers’ to define ‘restart apache’, and it could be reused between multiple plays.
Variables like ‘database’ above can be used in templates referenced from the configuration file to generate machine specific variables.
(Information on this feature is pending)