Ansible - how to launch playbook only on the first host of each inventory group

Question

I want to launch my playbook only on the first host of each inventory group.

[ego]
server1
server2

[asp]
server1
server2

[bre]
server1
server2

I saw there is an option to specify group_name but this is just for 1 group_name

hosts: group_name[0]

This option will only run it on 1 host in general and doesn't take into account the groups.

The run_once:True

Is there a way to run the playbook or tasks over the first host of all groups without defining which groups in the playbook.

So when executing the playbook i want to be able to select different groups as a limit and only run the play against the first host of each group.

Answer 1

You can use add_host module to build a new group containing only the first host of every group in the inventory.

- hosts: all
  gather_facts: no
  tasks:
  # groups.keys() gets all groups names. I will exclude 'all' and 'ungrouped'
  # solutions to do that in a better way are welcomed
  - set_fact:
      groups_names: "{{ groups.keys() | difference(['all'])| difference(['ungrouped']) }}"
    run_once: true
 
  # Add first host from every group to new_group
  - add_host:
      name: "{{ groups[item][0] }}"
      groups: new_group
    loop: "{{ groups_names }}"
    changed_when: false

  - debug:
      msg: "{{ inventory_hostname }}"
    when: inventory_hostname in groups['new_group']

My inventory:

[test]
test-001
test-002

[staging]
staging-001
staging-002
staging-003

[prod]
prod-001
prod-002

The output:

PLAY [all] *************************************************

TASK [set_fact] ********************************************
ok: [test-001]

TASK [add_host] ********************************************
ok: [test-001] => (item=test)
ok: [test-001] => (item=staging)
ok: [test-001] => (item=prod)

TASK [debug] ***********************************************
skipping: [test-002]
ok: [test-001] => {
    "msg": "test-001"
}
skipping: [staging-002]
ok: [staging-001] => {
    "msg": "staging-001"
}
skipping: [staging-003]
skipping: [prod-002]
ok: [prod-001] => {
    "msg": "prod-001"
}

PLAY RECAP ***************************************************
prod-001    : ok=1    changed=0    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0   
prod-002    : ok=0    changed=0    unreachable=0    failed=0    skipped=1    rescued=0    ignored=0   
staging-001 : ok=1    changed=0    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0   
staging-002 : ok=0    changed=0    unreachable=0    failed=0    skipped=1    rescued=0    ignored=0   
staging-003 : ok=0    changed=0    unreachable=0    failed=0    skipped=1    rescued=0    ignored=0   
test-001    : ok=3    changed=0    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0   
test-002    : ok=0    changed=0    unreachable=0    failed=0    skipped=1    rescued=0    ignored=0