Delegation, Rolling Updates, and Local Actions
==============================================

Ansible is great at doing things on one host on behalf of another, and in particular this is very applicable
when setting up continuous deployment infrastructure or zero downtime rolling updates.

Rolling Update Batch Size
`````````````````````````

.. versionadded:: 0.7

By default, Ansible will try to manage all of the machines referenced in a play in parallel.  For a rolling updates
use case, you can define how many hosts Ansible should manage at a single time by using the ''serial'' keyword::


    - name: test play
      hosts: webservers
      serial: 3

In the above example, if we had 100 hosts, 3 hosts in the group 'webservers'
would complete the play completely before moving on to the next 3 hosts.

Maximum Failure Percentage
``````````````````````````

.. versionadded:: 1.3

By default, Ansible will continue executing actions as long as there are hosts in the group that have not yet failed.
In some situations, such as with the rolling updates described above, it may be desireable to abort the play when a 
certain threshold of failures have been reached. To acheive this, as of version 1.3 you can set a maximum failure 
percentage on a play as follows::

    - hosts: webservers
      max_fail_percentage: 30
      serial: 10

In the above example, if more than 3 of the 10 servers in the group were to fail, the rest of the play would be aborted.

.. note::

     The percentage set must be exceeded, not equaled. For example, if serial were set to 4 and you wanted the task to abort 
     when 2 of the systems failed, the percentage should be set at 49 rather than 50.

Delegation
``````````

.. versionadded:: 0.7

This isn't actually rolling update specific but comes up frequently in those cases.

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 placing nodes in a load balanced pool, or removing them.  It is also very useful for controlling
outage windows.  Using this with the 'serial' keyword to control the number of hosts executing at one time is also
a good idea::

    ---
    - 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


These commands will 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 }}

A common pattern is to use a local action to call 'rsync' to recursively copy files to the managed servers.
Here is an example::

    ---
    # ...
      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.