ansible - how to iterate children groups in ansible?

Question

I do have an inventory file as below

[ParentGroup]
ChildrenGroup1
ChildrenGroup2

[ChildrenGroup1]
host1
host2
host3

[ChildrenGroup2]
host4
host5
host6

Now i want to iterate Children wise..

i.e. Perform my task in parallel on host1,host2, host3 i.e only on hosts exists inChildrenGroup1 and once this is success, i Need to go with ChildrenGroup2 i.e on host4, host5, host6

Points to be taken care ?

1. if there is any failure on any one of the childrengroup hosts then we need to wait/pause before proceeding with next children group
2. I shall have many children groups on my inventory
3. I need to action my task only on one chidlrengroup at a time.
4. I shall make sure all the childrengroups are addressed in one-shot too.

Can you suggest on how to take this forward ?

Answer 1

The critical limitation here is the fact that a playbook can't start another playbook. The only option is import_playbook. Imported files must be available when a playbook starts. As a result, the solution is a two-step process. Create the playbooks in the first step and then run them. For example, given the inventory

shell> cat hosts
[ParentGroup:children]
ChildrenGroup1
ChildrenGroup2

[ChildrenGroup1]
host1
host2
host3

[ChildrenGroup2]
host4
host5
host6

you want to run the playbook pb.yml as described in the question. Take the playbook and create the template by putting {{ item }} to hosts:

shell> cat pb.yml.j2
- hosts: "{{ item }}"
  gather_facts: false
  tasks:
    - debug:
        msg: "{{ inventory_hostname }}: Playbook started."

---

1. Create playbooks

The playbook below creates the list of the groups my_groups in the first task. Then the template task iterates this list and creates playbooks for the groups. The next template task imports these playbooks into the playbook pb-groups.yml

shell> cat pb-init.yml
- hosts: localhost
  vars:
    groups_other: [ParentGroup, all, ungrouped]
  tasks:
    - set_fact:
        my_groups: "{{ groups.keys()|difference(groups_other) }}"
    - template:
        src: pb.yml.j2
        dest: "pb-{{ item }}.yml"
      loop: "{{ my_groups }}"
    - template:
        src: pb-groups.yml.j2
        dest: pb-groups.yml

shell> cat pb-groups.yml.j2
- hosts: localhost
  gather_facts: false

{% for group in my_groups %}
- import_playbook: pb-{{ group }}.yml
{% endfor %}

See created files

shell> cat pb-ChildrenGroup1.yml 
- hosts: "ChildrenGroup1"
  gather_facts: false
  tasks:
    - debug:
        msg: "localhost: Playbook started."

shell> cat pb-ChildrenGroup2.yml 
- hosts: "ChildrenGroup2"
  gather_facts: false
  tasks:
    - debug:
        msg: "localhost: Playbook started."

shell> cat pb-groups.yml
- hosts: localhost
  gather_facts: false

- import_playbook: pb-ChildrenGroup1.yml
- import_playbook: pb-ChildrenGroup2.yml

---

2. Run created playbooks

shell> ansible-playbook pb-groups.yml

PLAY [localhost] ****

PLAY [ChildrenGroup1] ****

TASK [debug] ****
ok: [host1] => 
  msg: 'localhost: Playbook started.'
ok: [host2] => 
  msg: 'localhost: Playbook started.'
ok: [host3] => 
  msg: 'localhost: Playbook started.'

PLAY [ChildrenGroup2] ****

TASK [debug] ****
ok: [host4] => 
  msg: 'localhost: Playbook started.'
ok: [host5] => 
  msg: 'localhost: Playbook started.'
ok: [host6] => 
  msg: 'localhost: Playbook started.'

PLAY RECAP ****

  ...

---

Many children groups on my inventory

Change the inventory. For example

shell> cat hosts
[ParentGroup:children]
ChildrenGroup1
ChildrenGroup2
ChildrenGroup3

[ChildrenGroup1]
host1
host2

[ChildrenGroup2]
host4
host5

[ChildrenGroup3]
host3
host6

The commands below work as expected

shell> ansible-playbook pb-init.yml
  ...
shell> ansible-playbook pb-groups.yml

PLAY [localhost] ****

PLAY [ChildrenGroup1] ****

TASK [debug] ****
ok: [host1] => 
  msg: 'localhost: Playbook started.'
ok: [host2] => 
  msg: 'localhost: Playbook started.'

PLAY [ChildrenGroup2] ****

TASK [debug] ****
ok: [host4] => 
  msg: 'localhost: Playbook started.'
ok: [host5] => 
  msg: 'localhost: Playbook started.'

PLAY [ChildrenGroup3] ****

TASK [debug] ****
ok: [host3] => 
  msg: 'localhost: Playbook started.'
ok: [host6] => 
  msg: 'localhost: Playbook started.'

PLAY RECAP ****

  ...