1.. _playbooks_strategies:
2
3Controlling playbook execution: strategies and more
4===================================================
5
6By 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``.
7
8.. contents::
9   :local:
10
11Selecting a strategy
12--------------------
13The 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::
14
15    - hosts: all
16      strategy: free
17      tasks:
18      ...
19
20You can select a different strategy for each play as shown above, or set your preferred strategy globally in ``ansible.cfg``, under the ``defaults`` stanza::
21
22    [defaults]
23    strategy = free
24
25All strategies are implemented as :ref:`strategy plugins<strategy_plugins>`. Please review the documentation for each strategy plugin for details on how it works.
26
27Setting the number of forks
28---------------------------
29If you have the processing power available and want to use more forks, you can set the number in ``ansible.cfg``::
30
31    [defaults]
32    forks = 30
33
34or pass it on the command line: `ansible-playbook -f 30 my_playbook.yml`.
35
36Using keywords to control execution
37-----------------------------------
38
39In 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.
40
41.. _rolling_update_batch_size:
42
43Setting the batch size with ``serial``
44^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
45
46By 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::
47
48    ---
49    - name: test play
50      hosts: webservers
51      serial: 3
52      gather_facts: False
53
54      tasks:
55        - name: first task
56          command: hostname
57        - name: second task
58          command: hostname
59
60In the above example, if we had 6 hosts in the group 'webservers', Ansible would execute the play completely (both tasks) on 3 of the hosts before moving on to the next 3 hosts::
61
62
63    PLAY [webservers] ****************************************
64
65    TASK [first task] ****************************************
66    changed: [web3]
67    changed: [web2]
68    changed: [web1]
69
70    TASK [second task] ***************************************
71    changed: [web1]
72    changed: [web2]
73    changed: [web3]
74
75    PLAY [webservers] ****************************************
76
77    TASK [first task] ****************************************
78    changed: [web4]
79    changed: [web5]
80    changed: [web6]
81
82    TASK [second task] ***************************************
83    changed: [web4]
84    changed: [web5]
85    changed: [web2]
86
87    PLAY RECAP ***********************************************
88    web1      : ok=2    changed=2    unreachable=0    failed=0
89    web2      : ok=2    changed=2    unreachable=0    failed=0
90    web3      : ok=2    changed=2    unreachable=0    failed=0
91    web4      : ok=2    changed=2    unreachable=0    failed=0
92    web5      : ok=2    changed=2    unreachable=0    failed=0
93    web6      : ok=2    changed=2    unreachable=0    failed=0
94
95
96You 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::
97
98    ---
99    - name: test play
100      hosts: webservers
101      serial: "30%"
102
103If 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.
104
105You can also specify batch sizes as a list. For example::
106
107    ---
108    - name: test play
109      hosts: webservers
110      serial:
111        - 1
112        - 5
113        - 10
114
115In 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.
116
117You can list multiple batch sizes as percentages::
118
119    ---
120    - name: test play
121      hosts: webservers
122      serial:
123        - "10%"
124        - "20%"
125        - "100%"
126
127You can also mix and match the values::
128
129    ---
130    - name: test play
131      hosts: webservers
132      serial:
133        - 1
134        - 5
135        - "20%"
136
137.. note::
138     No matter how small the percentage, the number of hosts per pass will always be 1 or greater.
139
140Restricting execution with ``throttle``
141^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
142
143The ``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::
144
145    tasks:
146    - command: /path/to/cpu_intensive_command
147      throttle: 1
148
149If 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.
150
151Ordering execution based on inventory
152^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
153
154The ``order`` keyword controls the order in which hosts are run. Possible values for order are:
155
156inventory:
157    (default) The order provided in the inventory
158reverse_inventory:
159    The reverse of the order provided by the inventory
160sorted:
161    Sorted alphabetically sorted by name
162reverse_sorted:
163    Sorted by name in reverse alphabetical order
164shuffle:
165    Randomly ordered on each run
166
167Other keywords that affect play execution include ``ignore_errors``, ``ignore_unreachable``, and ``any_errors_fatal``. These options are documented in :ref:`playbooks_error_handling`.
168
169.. _run_once:
170
171Running on a single machine with ``run_once``
172^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
173
174If you want a task to run only on the first host in your batch of hosts, set ``run_once`` to true on that task::
175
176    ---
177    # ...
178
179      tasks:
180
181        # ...
182
183        - command: /opt/application/upgrade_db.py
184          run_once: true
185
186        # ...
187
188Ansible 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::
189
190        - command: /opt/application/upgrade_db.py
191          when: inventory_hostname == webservers[0]
192
193However, 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::
194
195        - command: /opt/application/upgrade_db.py
196          run_once: true
197          delegate_to: web01.example.org
198
199As 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.
200
201.. note::
202     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
203     :code:`when: inventory_hostname == ansible_play_hosts_all[0]` construct.
204
205.. note::
206    Any conditional (in other words, `when:`) will use the variables of the 'first host' to decide if the task runs or not, no other hosts will be tested.
207
208.. note::
209    If you want to avoid the default behavior of setting the fact for all hosts, set ``delegate_facts: True`` for the specific task or block.
210
211.. seealso::
212
213   :ref:`about_playbooks`
214       An introduction to playbooks
215   :ref:`playbooks_delegation`
216       Running tasks on or assigning facts to specific machines
217   :ref:`playbooks_reuse_roles`
218       Playbook organization by roles
219   `User Mailing List <https://groups.google.com/group/ansible-devel>`_
220       Have a question?  Stop by the google group!
221   `irc.libera.chat <https://libera.chat/>`_
222       #ansible IRC chat channel
223