Why Ansible variable override strategy doesn't seem to behave as documented?

Question

Suppose I have a parameter (ansible variable) which I want to keep the same for several inventories except for one.

By using the playbook level group_vars and inventory level group_vars, according to the docs, I could define a group for all inventories and still be able to override variables in the inventory by targeting the children because the children have precedence over the parents.

Example

inventories/testervm/hosts

[central:children]
testervm

[testervm]
127.0.0.1 ansible_connection=local

group_vars/central.yaml

parameter: "I want to be overridden"

inventories/testervm/group_vars/central.yaml

parameter: "This override shouldn't work."

inventories/testervm/group_vars/testervm.yaml

parameter: "This override should work"

playbook.yaml

- hosts: all
  roles:
    - the_role

roles/the_role/tasks/main.yaml

- name: printing the parameter
  debug:
    msg: "{{ parameter }}"

When I run

$ ansible --version
ansible 2.9.7
[...]

$ ansible-playbook -i inventories/testervm playbook.yaml 

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

TASK [Gathering Facts] *********************************************************************************************
ok: [127.0.0.1]

TASK [the_role : printing the parameter] ***************************************************************************
ok: [127.0.0.1] => {
    "msg": "I want to be overridden"
}

PLAY RECAP *********************************************************************************************************
127.0.0.1                  : ok=2    changed=0    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0

Needless to say I would have expected to see "msg": "This override should work".

What am I getting wrong ?

[EDIT] fixed wrong paths in file paths

[EDIT] PS: Thanks to Vladimir I ended up doing this trick, maybe somebody can find it useful. In the playbook I added this task:

    - name: include variable overrides
      include_vars:
        file: "{{ item }}"
      when: item is file
      with_items: "{{ [inventory_dir ~ '/group_vars/_overrides/'] | product(groups.keys()) | map('join') | product(['.yaml']) | map('join') | list }}"
      tags: always

Example: inventories/testervm/group_vars/_overrides/testervm.yaml

This makes possible to have this precedence order in the group vars:

• inventory group_vars/_overrides (highest)
• playbook group_vars
• inventory group_vars (lowest)

Answer 1

Your assumption is wrong. playbook group_vars/* (7.) is a higher precedence than inventory group_vars/* (6.). See Understanding variable precedence.

---

The result of the test is correct

shell> cat playbook.yaml 
- hosts: all
  roles:
    - the_role

shell> cat roles/the_role/tasks/main.yaml 
- name: printing the parameter
  debug:
    msg: "{{ parameter }}"

shell> cat inventories/testervm/hosts 
[central:children]
testervm

[testervm]
127.0.0.1 ansible_connection=local

shell> cat inventories/testervm/group_vars/central.yaml
parameter: "Inventory group_vars/central.yaml"

shell> cat inventories/testervm/group_vars/testervm.yaml
parameter: "Inventory group_vars/testervm.yaml"

shell> cat group_vars/central.yaml
parameter: "Playbook group_vars/central.yaml"

gives (abridged)

shell> ansible-playbook -i inventories/testervm playbook.yaml

    "msg": "Playbook group_vars/central.yaml"

---

Q: "Override a variable only in one specific inventory ... 12 options (above 10.) how to override host_vars and group_vars none of which applies to groups."

A: All of the 12 options apply, of course. This is the consequence of higher precedence. For example, the playbook below shows how to override parameter by include_vars (precedence 18.) if the host is a member of the group testervm

- hosts: all
  roles:
    - the_role
  tasks:
    - include_vars: files/testervm.yaml
      when: inventory_hostname in groups.testervm
    - debug:
        msg: "{{ parameter }}"

shell> cat files/testervm.yaml
parameter: "Include_vars files/testervm.yaml"

gives (abridged)

shell> ansible-playbook -i inventories/testervm playbook.yaml

    "msg": "Playbook group_vars/central.yaml"
    "msg": "Include_vars files/testervm.yaml"