With the release of Ansible 2.0, a new way to control play execution behavior was introduced: strategy. A strategy defines how Ansible coordinates each task across the set of hosts. Each strategy is a plugin, and two come with Ansible – linear and free. The linear strategy, which is the default strategy, is how Ansible has always behaved. As a play is executed, all the hosts for a given play execute the first task.

Once all are complete, Ansible moves to the next task. The serial directive can create batches of hosts to operate in this way, but the base strategy remains the same. All the targets for a given batch must complete a task before the next task is executed. The free strategy breaks from this traditional behavior. When using the free strategy, as soon as a host completes a task, Ansible will execute the next task for that host, without waiting for any other hosts to complete.

This will happen for every host in the set, for every task in the play. The hosts will complete the tasks as fast as each are able to, minimizing the execution time of each specific host. While most playbooks will use the default linear strategy, there are situations where the free strategy would be advantageous; for example, upgrading a service across a large set of hosts. If the play has numerous tasks to perform the upgrade, which starts with shutting down the service, then it would be more important for each host to suffer as little downtime as possible.

Allowing each host to independently move through the play as fast as it is able to will ensure that each host is down only for as long as necessary. Without using free, the entire set will be down for as long as the slowest host in the set takes to complete the tasks.

Execution strategies are implemented as a plugin and, as such, custom strategies can be developed to extend Ansible behavior. Development of such plugins is beyond the scope of this book.