ansible/docs/docsite/rst/user_guide/playbooks_intro.rst

Intro to Playbooks
==================

.. _about_playbooks:

About Playbooks
```````````````

Playbooks are a completely different way to use ansible than in ad-hoc task execution mode, and are
particularly powerful.

Simply put, playbooks 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.

There are also some full sets of playbooks illustrating a lot of these techniques in the
`ansible-examples repository <https://github.com/ansible/ansible-examples>`_.  We'd recommend
looking at these in another tab as you go along.

There are also many jumping off points after you learn playbooks, so hop back to the documentation
index after you're done with this section.

.. _playbook_language_example:

Playbook Language Example
`````````````````````````

Playbooks are expressed in YAML format (see :ref:`yaml_syntax`) and have a minimum of syntax, which intentionally
tries to not be a programming language or script, but rather a model of a configuration or a process.

Each playbook is composed of one or more 'plays' in a list.

The goal of a play is to map a group of hosts to some well defined roles, represented by
things ansible calls tasks.  At a basic level, a task is nothing more than a call
to an ansible module (see :doc:`modules`).

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.

"plays" are more or less a sports analogy.  You can have quite a lot of plays that affect your systems
to do different things.  It's not as if you were just defining one particular state or model, and you
can run different plays at different times.

For starters, here's a playbook that contains just one play::

    ---
    - hosts: webservers
      vars:
        http_port: 80
        max_clients: 200
      remote_user: root
      tasks:
      - name: ensure apache is at the latest version
        yum:
          name: httpd
          state: latest
      - name: write the apache config file
        template:
          src: /srv/httpd.j2
          dest: /etc/httpd.conf
        notify:
        - restart apache
      - name: ensure apache is running
        service:
          name: httpd
          state: started
      handlers:
        - name: restart apache
          service:
            name: httpd
            state: restarted

Playbooks can contain multiple plays. You may have a playbook that targets first
the web servers, and then the database servers. For example::

    ---
    - hosts: webservers
      remote_user: root

      tasks:
      - name: ensure apache is at the latest version
        yum:
          name: httpd
          state: latest
      - name: write the apache config file
        template:
          src: /srv/httpd.j2
          dest: /etc/httpd.conf

    - hosts: databases
      remote_user: root

      tasks:
      - name: ensure postgresql is at the latest version
        yum:
          name: postgresql
          state: latest
      - name: ensure that postgresql is started
        service:
          name: postgresql
          state: started

You can use this method to switch between the host group you're targeting,
the username logging into the remote servers, whether to sudo or not, and so
forth. Plays, like tasks, run in the order specified in the playbook: top to
bottom.

Below, we'll break down what the various features of the playbook language are.

.. _playbook_basics:

Basics
``````

.. _playbook_hosts_and_users:

Hosts and Users
+++++++++++++++

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 :doc:`intro_patterns`
documentation.  The ``remote_user`` is just the name of the user account::

    ---
    - hosts: webservers
      remote_user: root

.. note::

    The ``remote_user`` parameter was formerly called just ``user``. It was renamed in Ansible 1.4 to make it more distinguishable from the **user** module (used to create users on remote systems).

Remote users can also be defined per task::

    ---
    - hosts: webservers
      remote_user: root
      tasks:
        - name: test connection
          ping:
          remote_user: yourname

Support for running things as another user is also available (see :doc:`become`)::

    ---
    - hosts: webservers
      remote_user: yourname
      become: yes

You can also use keyword ``become`` on a particular task instead of the whole play::

    ---
    - hosts: webservers
      remote_user: yourname
      tasks:
        - service: 
            name: nginx
            state: started
          become: yes
          become_method: sudo


You can also login as you, and then become a user different than root::

    ---
    - hosts: webservers
      remote_user: yourname
      become: yes
      become_user: postgres

You can also use other privilege escalation methods, like su::

    ---
    - hosts: webservers
      remote_user: yourname
      become: yes
      become_method: su

If you need to specify a password to sudo, run ``ansible-playbook`` with ``--ask-become-pass`` or
when using the old sudo syntax ``--ask-sudo-pass`` (``-K``).  If you run a become playbook and the
playbook seems to hang, it's probably stuck at the privilege escalation prompt.
Just `Control-C` to kill it and run it again adding the appropriate password.

.. important::

   When using ``become_user`` to a user other than root, the module
   arguments are briefly written into a random tempfile in ``/tmp``.
   These are deleted immediately after the command is executed.  This
   only occurs when changing privileges from a user like 'bob' to 'timmy',
   not when going from 'bob' to 'root', or logging in directly as 'bob' or
   'root'.  If it concerns you that this data is briefly readable
   (not writable), avoid transferring unencrypted passwords with
   `become_user` set.  In other cases, ``/tmp`` is not used and this does
   not come into play. Ansible also takes care to not log password
   parameters.


.. _order:

.. versionadded:: 2.4

You can also control the order in which hosts are run. The default is to follow the order supplied by the inventory::

    - hosts: all
      order: sorted
      gather_facts: False
      tasks:
        - debug:
            var: inventory_hostname

Possible values for order are:

inventory:
    The default. The order is 'as provided' by the inventory
reverse_inventory:
    As the name implies, this reverses the order 'as provided' by the inventory
sorted:
    Hosts are alphabetically sorted by name
reverse_sorted:
    Hosts are sorted by name in reverse alphabetical order
shuffle:
    Hosts are randomly ordered each run


.. _tasks_list:

Tasks list
++++++++++

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.  It is important to understand that, within a play,
all hosts are going to get the same task directives.  It is the purpose of a play to map
a selection of hosts to tasks.

When running the playbook, which runs top to bottom, 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 should be idempotent, that is, running a module multiple times
in a sequence should have the same effect as running it just once. One
way to achieve idempotency is to have a module check whether its desired
final state has already been achieved, and if that state has been achieved,
to exit without performing any actions. If all the modules a playbook uses
are idempotent, then the playbook itself is likely to be idempotent, so
re-running the playbook should be safe.

The **command** and **shell** modules will typically rerun the same command again,
which is totally ok if the command is something like
``chmod`` or ``setsebool``, etc.  Though there is a ``creates`` flag available which can
be used to make these modules also idempotent.

Every task should have a ``name``, which is included in the output from
running the playbook.   This is human readable output, and so it is 
useful to provide good descriptions of each task step.  If the name
is not provided though, the string fed to 'action' will be used for
output.

Tasks can be declared using the legacy ``action: module options`` format, but
it is recommended that you use the more conventional ``module: options`` format.
This recommended format is used throughout the documentation, but you may
encounter the older format in some playbooks.

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
       service:
         name: httpd
         state: started

The **command** and **shell** modules are the only modules that just take a list
of arguments and don't use the ``key=value`` form.  This makes
them work as simply as you would expect::

   tasks:
     - name: enable selinux
       command: /sbin/setenforce 1

The **command** and **shell** module care about return codes, so if you have a command
whose successful exit code is not zero, you may wish to do this::

   tasks:
     - name: run this command and ignore the result
       shell: /usr/bin/somecommand || /bin/true

Or this::

   tasks:
     - name: run this command and ignore the result
       shell: /usr/bin/somecommand
       ignore_errors: True


If the action line is getting too long for comfort you can break it on
a space and indent any continuation lines::

    tasks:
      - name: Copy ansible inventory file to client
        copy: src=/etc/ansible/hosts dest=/etc/ansible/hosts
                owner=root group=root mode=0644

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 }}
       template:
         src: somefile.j2
         dest: /etc/httpd/conf.d/{{ vhost }}

Those same variables are usable in templates, which we'll get to later.

Now in a very basic playbook all the tasks will be listed directly in that play, though it will usually
make more sense to break up tasks as described in :doc:`playbooks_reuse`.

.. _action_shorthand:

Action Shorthand
````````````````

.. versionadded:: 0.8

Ansible prefers listing modules like this::

    template:
        src: templates/foo.j2
        dest: /etc/foo.conf

Early versions of Ansible used the following format, which still works::

    action: template src=templates/foo.j2 dest=/etc/foo.conf


.. _handlers:

Handlers: Running Operations On Change
``````````````````````````````````````

As we've mentioned, modules should 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 block of tasks in a play, and will only be
triggered once even if notified by multiple different tasks.

For instance, multiple resources may indicate
that apache needs to be restarted because they have changed a config file,
but apache will only be bounced once to avoid unnecessary restarts.

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
     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 a globally unique name, and are notified 
by notifiers.  If nothing notifies a handler, it will not
run.  Regardless of how many tasks 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
          service:
            name: memcached
            state: restarted
        - name: restart apache
          service:
            name: apache
            state: restarted

As of Ansible 2.2, handlers can also "listen" to generic topics, and tasks can notify those topics as follows::

    handlers:
        - name: restart memcached
          service:
            name: memcached
            state: restarted
          listen: "restart web services"
        - name: restart apache
          service:
            name: apache
            state:restarted
          listen: "restart web services"

    tasks:
        - name: restart everything
          command: echo "this task will restart the web services"
          notify: "restart web services"

This use makes it much easier to trigger multiple handlers. It also decouples handlers from their names,
making it easier to share handlers among playbooks and roles (especially when using 3rd party roles from
a shared source like Galaxy).

.. note::
   * Notify handlers are always run in the same order they are defined, `not` in the order listed in the notify-statement. This is also the case for handlers using `listen`.
   * Handler names and `listen` topics live in a global namespace.
   * If two handler tasks have the same name, only one will run.
     `* <https://github.com/ansible/ansible/issues/4943>`_
   * You cannot notify a handler that is defined inside of an include. As of Ansible 2.1, this does work, however the include must be `static`.

Roles are described later on, but it's worthwhile to point out that:

* handlers notified within ``pre_tasks``, ``tasks``, and ``post_tasks`` sections are automatically flushed in the end of section where they were notified;
* handlers notified within ``roles`` section are automatically flushed in the end of ``tasks`` section, but before any ``tasks`` handlers.

If you ever want to flush all the handler commands immediately you can do this::

    tasks:
       - shell: some tasks go here
       - meta: flush_handlers
       - shell: some other tasks

In the above example any queued up handlers would be processed early when the ``meta``
statement was reached.  This is a bit of a niche case but can come in handy from
time to time.

.. _executing_a_playbook:

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

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

.. _playbook_ansible-pull:

Ansible-Pull
````````````

Should you want to invert the architecture of Ansible, so that nodes check in to a central location, instead
of pushing configuration out to them, you can.

The ``ansible-pull`` is a small script that will checkout a repo of configuration instructions from git, and then
run ``ansible-playbook`` against that content.

Assuming you load balance your checkout location, ``ansible-pull`` scales essentially infinitely.

Run ``ansible-pull --help`` for details.

There's also a `clever playbook <https://github.com/ansible/ansible-examples/blob/master/language_features/ansible_pull.yml>`_ available to configure ``ansible-pull`` via a crontab from push mode.

.. _tips_and_tricks:

Tips and Tricks
```````````````

To check the syntax of a playbook, use ``ansible-playbook`` with the ``--syntax-check`` flag. This will run the
playbook file through the parser to ensure its included files, roles, etc. have no syntax problems.

Look at the bottom of the playbook execution for a summary of the nodes that were targeted
and how they performed.   General failures and fatal "unreachable" communication attempts are
kept separate in the counts.

If you ever want to see detailed output from successful modules as well as unsuccessful ones,
use the ``--verbose`` flag.  This is available in Ansible 0.5 and later.


To see what hosts would be affected by a playbook before you run it, you
can do this::

    ansible-playbook playbook.yml --list-hosts

.. seealso::

   :ref:`yaml_syntax`
       Learn about YAML syntax
   :ref:`playbooks_best_practices`
       Various tips about managing playbooks in the real world
   :ref:`all_modules`
       Learn about available modules
   :ref:`developing_modules`
       Learn how to extend Ansible by writing your own modules
   :ref:`intro_patterns`
       Learn about how to select hosts
   `Github examples directory <https://github.com/ansible/ansible-examples>`_
       Complete end-to-end playbook examples
   `Mailing List <https://groups.google.com/group/ansible-project>`_
       Questions? Help? Ideas?  Stop by the list on Google Groups