https://github.com/ansible/ansible/issues/16933

hosts

0
1
2
3
4
5
6
7
8
9

play.yml

- hosts: all
  gather_facts: false
  #serial: 1
  # serial 1 is needed to enforce is deterministic order in the grouped by group, to allow to expect the first node to be the master
  # see https://github.com/ansible/ansible/issues/16933
  tasks:
  - group_by:
      key: foobar

- hosts: all
  gather_facts: false
  tasks:
  - debug:
      var: groups.foobar
    run_once: yes

result-with-serial-1.log

 serge  ~/tmp/issues16933  ansible-playbook -i hosts  play.yml 

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

TASK [group_by] ****************************************************************
ok: [0]

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

TASK [group_by] ****************************************************************
ok: [1]

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

TASK [group_by] ****************************************************************
ok: [2]

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

TASK [group_by] ****************************************************************
ok: [3]

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

TASK [group_by] ****************************************************************
ok: [4]

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

TASK [group_by] ****************************************************************
ok: [5]

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

TASK [group_by] ****************************************************************
ok: [6]

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

TASK [group_by] ****************************************************************
ok: [7]

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

TASK [group_by] ****************************************************************
ok: [8]

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

TASK [group_by] ****************************************************************
ok: [9]

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

TASK [debug] *******************************************************************
ok: [0] => {
    "groups.foobar": [
        "0", 
        "1", 
        "2", 
        "3", 
        "4", 
        "5", 
        "6", 
        "7", 
        "8", 
        "9"
    ]
}

PLAY RECAP *********************************************************************
0                          : ok=2    changed=0    unreachable=0    failed=0   
1                          : ok=1    changed=0    unreachable=0    failed=0   
2                          : ok=1    changed=0    unreachable=0    failed=0   
3                          : ok=1    changed=0    unreachable=0    failed=0   
4                          : ok=1    changed=0    unreachable=0    failed=0   
5                          : ok=1    changed=0    unreachable=0    failed=0   
6                          : ok=1    changed=0    unreachable=0    failed=0   
7                          : ok=1    changed=0    unreachable=0    failed=0   
8                          : ok=1    changed=0    unreachable=0    failed=0   
9                          : ok=1    changed=0    unreachable=0    failed=0   

result-without-serial-1.log

 serge  ~/tmp/issues16933  ansible-playbook -i hosts  play.yml 

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

TASK [group_by] ****************************************************************
ok: [0]
ok: [3]
ok: [1]
ok: [2]
ok: [4]
ok: [5]
ok: [6]
ok: [7]
ok: [8]
ok: [9]

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

TASK [debug] *******************************************************************
ok: [0] => {
    "groups.foobar": [
        "0", 
        "3", 
        "1", 
        "2", 
        "4", 
        "5", 
        "6", 
        "7", 
        "8", 
        "9"
    ]
}

PLAY RECAP *********************************************************************
0                          : ok=2    changed=0    unreachable=0    failed=0   
1                          : ok=1    changed=0    unreachable=0    failed=0   
2                          : ok=1    changed=0    unreachable=0    failed=0   
3                          : ok=1    changed=0    unreachable=0    failed=0   
4                          : ok=1    changed=0    unreachable=0    failed=0   
5                          : ok=1    changed=0    unreachable=0    failed=0   
6                          : ok=1    changed=0    unreachable=0    failed=0   
7                          : ok=1    changed=0    unreachable=0    failed=0   
8                          : ok=1    changed=0    unreachable=0    failed=0   
9                          : ok=1    changed=0    unreachable=0    failed=0   

test.sh

ansible-playbook -i hosts  play.yml