ansible/docs/docsite/rst/user_guide/playbooks_delegation.rst

.. _playbooks_delegation:

Controlling where tasks run: delegation and local actions
=========================================================

By default Ansible gathers facts and executes all tasks on the machines that match the ``hosts`` line of your playbook. This page shows you how to delegate tasks to a different machine or group, delegate facts to specific machines or groups, or run an entire playbook locally. Using these approaches, you can manage inter-related environments precisely and efficiently. For example, when updating your webservers, you might need to remove them from a load-balanced pool temporarily. You cannot perform this task on the webservers themselves. By delegating the task to localhost, you keep all the tasks within the same play.

.. contents::
   :local:

Tasks that cannot be delegated
------------------------------

Some tasks always execute on the controller. These tasks, including ``include``, ``add_host``, and ``debug``, cannot be delegated.

.. _delegation:

Delegating tasks
----------------

If you want to perform a task on one host with reference to other hosts, use the 'delegate_to' keyword on a task. This is ideal for managing nodes in a load balanced pool or for controlling outage windows. You can use delegation with the :ref:`serial <rolling_update_batch_size>` keyword to control the number of hosts executing at one time::

    ---
    - hosts: webservers
      serial: 5

      tasks:
        - name: take out of load balancer pool
          command: /usr/bin/take_out_of_pool {{ inventory_hostname }}
          delegate_to: 127.0.0.1

        - name: actual steps would go here
          yum:
            name: acme-web-stack
            state: latest

        - name: add back to load balancer pool
          command: /usr/bin/add_back_to_pool {{ inventory_hostname }}
          delegate_to: 127.0.0.1

The first and third tasks in this play run on 127.0.0.1, which is the machine running Ansible. There is also a shorthand syntax that you can use on a per-task basis: 'local_action'. Here is the same playbook as above, but using the shorthand syntax for delegating to 127.0.0.1::

    ---
    # ...

      tasks:
        - name: take out of load balancer pool
          local_action: command /usr/bin/take_out_of_pool {{ inventory_hostname }}

    # ...

        - name: add back to load balancer pool
          local_action: command /usr/bin/add_back_to_pool {{ inventory_hostname }}

You can use a local action to call 'rsync' to recursively copy files to the managed servers::

    ---
    # ...

      tasks:
        - name: recursively copy files from management server to target
          local_action: command rsync -a /path/to/files {{ inventory_hostname }}:/path/to/target/

Note that you must have passphrase-less SSH keys or an ssh-agent configured for this to work, otherwise rsync
will need to ask for a passphrase.

To specify more arguments, use the following syntax::

    ---
    # ...

      tasks:
        - name: Send summary mail
          local_action:
            module: mail
            subject: "Summary Mail"
            to: "{{ mail_recipient }}"
            body: "{{ mail_body }}"
          run_once: True

The `ansible_host` variable reflects the host a task is delegated to.

.. _delegate_facts:

Delegating facts
----------------

Delegating Ansible tasks is like delegating tasks in the real world - your groceries belong to you, even if someone else delivers them to your home. Similarly, any facts gathered by a delegated task are assigned by default to the `inventory_hostname` (the current host), not to the host which produced the facts (the delegated to host). To assign gathered facts to the delegated host instead of the current host, set `delegate_facts` to `True`::

    ---
    - hosts: app_servers

      tasks:
        - name: gather facts from db servers
          setup:
          delegate_to: "{{item}}"
          delegate_facts: True
          loop: "{{groups['dbservers']}}"

This task gathers facts for the machines in the dbservers group and assigns the facts to those machines, even though the play targets the app_servers group. This way you can lookup `hostvars['dbhost1']['ansible_default_ipv4']['address']` even though dbservers were not part of the play, or left out by using `--limit`.

.. _local_playbooks:

Local playbooks
---------------

It may be useful to use a playbook locally on a remote host, rather than by connecting over SSH.  This can be useful for assuring the configuration of a system by putting a playbook in a crontab.  This may also be used
to run a playbook inside an OS installer, such as an Anaconda kickstart.

To run an entire playbook locally, just set the "hosts:" line to "hosts: 127.0.0.1" and then run the playbook like so::

    ansible-playbook playbook.yml --connection=local

Alternatively, a local connection can be used in a single playbook play, even if other plays in the playbook
use the default remote connection type::

    ---
    - hosts: 127.0.0.1
      connection: local

.. note::
    If you set the connection to local and there is no ansible_python_interpreter set, modules will run under /usr/bin/python and not
    under {{ ansible_playbook_python }}. Be sure to set ansible_python_interpreter: "{{ ansible_playbook_python }}" in
    host_vars/localhost.yml, for example. You can avoid this issue by using ``local_action`` or ``delegate_to: localhost`` instead.

.. seealso::

   :ref:`playbooks_intro`
       An introduction to playbooks
   :ref:`playbooks_strategies`
       More ways to control how and where Ansible executes
   `Ansible Examples on GitHub <https://github.com/ansible/ansible-examples>`_
       Many examples of full-stack deployments
   `User Mailing List <https://groups.google.com/group/ansible-devel>`_
       Have a question?  Stop by the google group!
   `irc.freenode.net <http://irc.freenode.net>`_
       #ansible IRC chat channel