ansible/rst/playbooks.rst

Playbooks: Ansible for Deployment, Configuration Management, and Orchestration
==============================================================================

.. seealso::

   :doc:`YAMLScripts`
       Learn about YAML syntax
   :doc:`modules`
       Learn about available modules and writing your own
   :doc:`patterns`
       Learn about how to select hosts


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.


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::

    ---
    - 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

Hosts line
``````````

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.

Vars section
````````````

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.

Tasks list
``````````

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.

Task name and action
`````````````````````

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 }}

Notify statements
`````````````````

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
````````

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.

Includes
````````

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.

Using Includes To Assign Classes of Systems
```````````````````````````````````````````

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.

Asynchronous Actions and Polling
````````````````````````````````

(Information on this feature is pending)

Executing A Playbook
````````````````````

To run a playbook::

    ansible-playbook playbook.yml