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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?

thanks,

Jim

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  • i may be mistaken, but you may need a separate play. I seem to remember that host vars are re-evaluated when a new play is started – Bruce Becker May 25 at 5:29
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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).

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