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 keywords like ``serial``.
The default behavior described above is the :ref:`linear strategy<linear_strategy>`. Ansible offers other strategies, including the :ref:`debug strategy<debug_strategy>` (see also :ref:`playbook_debugger`) and the :ref:`free strategy<free_strategy>`, which allows each host to run until the end of the play as fast as it can::
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::
All strategies are implemented as :ref:`strategy plugins<strategy_plugins>`. Please review the documentation for each strategy plugin for details on how it works.
In addition to strategies, several :ref:`keywords<playbook_keywords>` also affect play execution. You can set a number, a percentage, or a list of numbers of hosts you want to manage at a time with ``serial``. Ansible completes the play on the specified number or percentage of hosts before starting the next batch of hosts. You can restrict the number of workers allotted to a block or task with ``throttle``. You can control how Ansible selects the next host in a group to execute against with ``order``. You can run a task on a single host with ``run_once``. These keywords are not strategies. They are directives or options applied to a play, block, or task.
By default, Ansible runs in parallel against all the hosts in the :ref:`pattern <intro_patterns>` you set in the ``hosts:`` field of each play. If you want to manage only a few machines at a time, for example during a rolling update, you can define how many hosts Ansible should manage at a single time using the ``serial`` keyword::
---
- name: test play
hosts: webservers
serial: 2
gather_facts: False
tasks:
- name: first task
command: hostname
- name: second task
command: hostname
In the above example, if we had 4 hosts in the group 'webservers', Ansible would execute the play completely (both tasks) on 2 of the hosts before moving on to the next 2 hosts::
PLAY [webservers] ****************************************
PLAY RECAP ***********************************************
web1 : ok=2 changed=2 unreachable=0 failed=0
web2 : ok=2 changed=2 unreachable=0 failed=0
web3 : ok=2 changed=2 unreachable=0 failed=0
web4 : ok=2 changed=2 unreachable=0 failed=0
You can also specify a percentage with the ``serial`` keyword. Ansible applies the percentage to the total number of hosts in a play to determine the number of hosts per pass::
---
- name: test play
hosts: webservers
serial: "30%"
If the number of hosts does not divide equally into the number of passes, the final pass contains the remainder. In this example, if you had 20 hosts in the webservers group, the first batch would contain 6 hosts, the second batch would contain 6 hosts, the third batch would contain 6 hosts, and the last batch would contain 2 hosts.
You can also specify batch sizes as a list. For example::
---
- name: test play
hosts: webservers
serial:
- 1
- 5
- 10
In the above example, the first batch would contain a single host, the next would contain 5 hosts, and (if there are any hosts left), every following batch would contain either 10 hosts or all the remaining hosts, if fewer than 10 hosts remained.
You can list multiple batch sizes as percentages::
---
- name: test play
hosts: webservers
serial:
- "10%"
- "20%"
- "100%"
You can also mix and match the values::
---
- name: test play
hosts: webservers
serial:
- 1
- 5
- "20%"
..note::
No matter how small the percentage, the number of hosts per pass will always be 1 or greater.
Restricting execution with ``throttle``
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The ``throttle`` keyword limits the number of workers for a particular task. It can be set at the block and task level. Use ``throttle`` to restrict tasks that may be CPU-intensive or interact with a rate-limiting API::
If you have already restricted the number of forks or the number of machines to execute against in parallel, you can reduce the number of workers with ``throttle``, but you cannot increase it. In other words, to have an effect, your ``throttle`` setting must be lower than your ``forks`` or ``serial`` setting if you are using them together.
Other keywords that affect play execution include ``ignore_errors``, ``ignore_unreachable``, and ``any_errors_fatal``. These options are documented in :ref:`playbooks_error_handling`.
If you want a task to run only on the first host in your batch of hosts, set ``run_once`` to true on that task::
---
# ...
tasks:
# ...
- command: /opt/application/upgrade_db.py
run_once: true
# ...
Ansible executes this task on the first host in the current batch and applies all results and facts to all the hosts in the same batch. This approach is similar to applying a conditional to a task such as::
- command: /opt/application/upgrade_db.py
when: inventory_hostname == webservers[0]
However, with ``run_once``, the results are applied to all the hosts. To run the task on a specific host, instead of the first host in the batch, delegate the task::
- command: /opt/application/upgrade_db.py
run_once: true
delegate_to: web01.example.org
As always with :ref:`delegation <playbooks_delegation>`, the action will be executed on the delegated host, but the information is still that of the original host in the task.
..note::
When used together with ``serial``, tasks marked as ``run_once`` will be run on one host in *each* serial batch. If the task must run only once regardless of ``serial`` mode, use
