How to use facts from one host to perform actions on another host?

Question

In Ansible I'm trying to generate a variable from a task on one host and then use the variable to do something to another host. Specifically, I'm trying to create a Kubernetes cluster - On the control plane node I can save the join command into a variable, but this appears to be scoped to the source host only - I can't work out how to access that variable from another host.

- name: read join command from the controlPlane
  shell: kubeadm token create --print-join-command
  when: inventory_hostname in groups['controlPlane']
  register: joinCmd

- name: join worker nodes to cluster
  command: "{{ joinCmd.stdout }}"
  when: inventory_hostname in groups['workerNodes']

The above fails on the second task (The task includes an option with an undefined variable. the error was: 'dict object' has no attribute 'stdout'). The standard approach seems to be save the line to a file locally, copy the file to the worker nodes and execute it locally which seems cumbersome (and leaves the join token lying around).

How can I achieve this with variables only?

Answer 1

I found a solution - the trick here is delegate_to and delegate_facts:

- hosts: all
  remote_user: ansible
  become: false

  tasks:
  - name: read join line from controlPlane
    shell: kubeadm token create --print-join-command
    when: inventory_hostname in groups['workerNodes']
    register: joinCmd
    delegate_to: "{{ item }}"
    delegate_facts: true
    with_items: "{{ groups['controlPlane'] }}"

  - name: join all worker nodes to cluster
    command: "{{ joinCmd.results[0].stdout }}"
    become: yes
    when: inventory_hostname in groups['workerNodes']

The first task kind of reads backwards to my mind - when says 'run this task on all the workerNode hosts...' but delegate_to says '..but actually run it on a different host'. It works for me though (I've only got 1 host in controlPlane group, not sure of outcome with more hosts).

Answer 2

Another option here would be to run tasks on the master node(s) to create the join command and just run the role for nodes joining based on the same conditions:

- hosts: cluster_masters,cluster_workers
  pre_tasks:
    - name: Create the token to join
      ansible.builtin.shell: |
        kubeadm token create --print-join-command
      register: join_command
      when: inventory_hostname in groups.cluster_masters
    - name: Set token as fact
      ansible.builtin.set_fact:
        join_command: "{{ join_command.stdout }}"
      run_once: true
      when: inventory_hostname in groups.cluster_masters
  roles:
    - role: applications/kubernetes/node
      when: inventory_hostname in groups.cluster_workers and join_command is defined

The join_command is now availabe in the role