How to run tasks on some nodes only

How to run tasks on some nodes only

Only on first node in group

Check the group array

For example

- name: Ping first node listed in group "tower"
  ping:
  when: inventory_hostname == groups['tower'][0]

Only if node is in group

Is the host in group node_b?

---
- hosts:
  - node_a
  - node_b
  gather_facts: no

  tasks:
  - ping:
    when: inventory_hostname in groups['node_b']