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# Controlling playbook execution: strategies and more

By default, Ansible runs each task on all hosts affected by a play before starting the next task on any host, using 5 forks. If you want to change this default behavior, you can use a different strategy plugin, change the number of forks, or apply one of several play-level keywords like `serial`.

- Selecting a strategy

- Setting the number of forks

- Using keywords to control execution

## Selecting a strategy

The default behavior described above is the linear strategy. Ansible offers other strategies, including the debug strategy (see also Playbook Debugger) and the free strategy, which allows each host to run until the end of the play as fast as it can:

    - hosts: all
      strategy: free
      tasks:
      ...

You can select a different strategy for each play as shown above, or set your preferred strategy globally in `ansible.cfg`, under the `defaults` stanza:

    [defaults]
    strategy = free

All strategies are implemented as strategy plugins. Please review the documentation for each strategy plugin for details on how it works.

## Setting the number of forks

If you have the processing power available and want to use more forks, you can set the number in `ansible.cfg`:

    [defaults]
    forks = 30

or pass it on the command line: ansible-playbook -f 30 my_playbook.yml.

## Using keywords to control execution

Several play-level keyword also affect play execution. The most common one is `serial`, which sets a number, a percentage, or a list of numbers of hosts you want to manage at a time. Setting `serial` with any strategy directs Ansible to ‘batch’ the hosts, completing the play on the specified number or percentage of hosts before starting the next ‘batch’. This is especially useful for rolling updates.

The second keyword to affect execution is `throttle`, which can also be used at the block and task level. This keyword limits the number of workers up to the maximum set via the forks setting or `serial`. This can be useful in restricting tasks that may be CPU-intensive or interact with a rate-limiting API:

    tasks:
    - command: /path/to/cpu_intensive_command
      throttle: 1

Other keywords that affect play execution include `ignore_errors`, `ignore_unreachable`, and `any_errors_fatal`. Please note that these keywords are not strategies. They are play-level directives or options.

See also

About Playbooks

  An introduction to playbooks

Roles

  Playbook organization by roles

User Mailing List

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irc.freenode.net

  #ansible IRC chat channel