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

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 throttle keyword also affects execution and can be set at the block and task level. This keyword limits the number of workers up to the maximum set with the forks setting or serial. Use throttle to restrict tasks that may be CPU-intensive or interact with a rate-limiting API:

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

The order keyword controls the order in which hosts are run. Possible values for order are:

inventory:
(default) The order provided in the inventory
reverse_inventory:
The reverse of the order provided by the inventory
sorted:
Sorted alphabetically sorted by name
reverse_sorted:
Sorted by name in reverse alphabetical order
shuffle:
Randomly ordered on each run

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

Intro to Playbooks
An introduction to playbooks
Roles
Playbook organization by roles
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