Understanding the Ansible Playbook Error: run-once

When developing Ansible playbooks, it's crucial to ensure efficient execution and adhere to best practices. This includes using strategies that align with the desired behavior of your tasks and plays. In the world of Ansible, the run_once directive is a powerful tool. It is used to ensure that a particular task runs only once in a playbook. However, its use can become problematic if not handled correctly, especially when used in conjunction with the strategy: free configuration.

What is run_once?

In Ansible, the run_once directive allows you to specify that a task should execute only once, regardless of how many hosts are involved in the playbook. This is particularly useful when you have a task that should be performed only on the control node or a single target host, even in a scenario where multiple hosts are being managed.

Here's a quick example:

``yaml

  • name: Ensure setup is done once

hosts: all

gather_facts: no

tasks:

- name: Perform setup

ansible.builtin.debug:

msg: "This setup should run once"

run_once: true

`

In this example, the debug task will execute only once, even if it's part of a playbook applied to multiple hosts.

Ansible Lint Output

`bash

WARNING Listing 2 violation(s) that are fatal

yaml[truthy]: Truthy value should be one of [false, true]

run_once.yml:4

run-once[task]: Using run_once may behave differently if strategy is set to free.

run_once.yml:6 Task/Handler: Perform setup

Read documentation for instructions on how to ignore specific rule violations.

Rule Violation Summary

count tag profile rule associated tags

1 yaml[truthy] basic formatting, yaml

1 run-once[task] idiom

Failed: 2 failure(s), 0 warning(s) on 1 files. Last profile that met the validation criteria was 'min'.

`

The Issue with run_once and strategy: free

One common pitfall occurs when you use the run_once directive in combination with the strategy: free setting. The strategy: free tells Ansible to parallelize tasks as much as possible, which can lead to unexpected results when used with run_once. This combination may not guarantee that the task runs only once due to the parallel nature of strategy: free.

To prevent this misalignment of strategies, Ansible Lint has a rule called run-once. This rule checks if the run_once directive is used when the playbook's strategy is set to free. If it encounters this scenario, it issues a warning to prompt the user to reconsider their strategy choices.

Proper Usage and Resolutions

To ensure that your playbook works as expected, consider the following best practices:

1. Avoid strategy: free with run_once: If your playbook includes tasks with the run_once directive, it's a good practice to avoid using strategy: free for that specific playbook. A playbook-level strategy: free`