Run once

The run_once parameter can be set on any task. It will ensure that a task is run only once, even when multiple hosts are in scope.

For example, if you are running a playbook on multiple nodes, but you need to run one specific task just once on a host, you can use run_once. This can be used to select a single host to run a task on, for example to run a task on one node of a cluster.

- name: Install a cluster
  hosts: cluster_a
  become: yes
  
  tasks:
    - name: Check something on one node
      ansible.builtin.assert:
        that:
          - ansible_distribution == "Ubuntu"
      run_once: true
    
    - name: Do something to all nodes
      ansible.builtin.debug:
        msg: "I'm doing something to all nodes."

Use-cases for run_once:

• Initialize a cluster.
• Create a directory on the delegated host localhost.
• Interact with an API after installation of the API software on all nodes.

Assignment

Write a playbook that:

1. Creates a directory on the localhost once.
2. Fetch “install.log” (or so) from the managed nodes to the controller node.

Solution

A possible solution here.