Is it possible to specify hosts for an Ansible role based on a map value?

Question

I have hosts, where I can setup application on a single node architecture or distributed.

So I have an inventory.

[STG]
node1

[LIVE]
app_node
db_node
gateway_node

So a variable with default value be single but can be changed on CLI to distributed.

I have a role definition

- hosts:
  gather_facts: no
  roles:
      - {role: setup, tags: ['setup', 'orchestra']}

So I want host line to be dynamic based on map value

- hosts: 'if single then host == STG else LIVE'

Answer 1

There are more options:

1. Put the logic into the expression of hosts:

shell> cat pb.yml
- hosts: "{{ (map_value == 'single')|ternary('STG', 'LIVE') }}"
  tasks:
    - debug:
        var: ansible_play_hosts
      run_once: true

gives what you want

shell> ansible-playbook pb.yml -e map_value=single

PLAY [single] ********************************************************************************

TASK [debug] *********************************************************************************
ok: [node1] => 
  ansible_play_hosts:
  - node1

PLAY RECAP ***********************************************************************************
node1: ok=1    changed=0    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0   

shell> ansible-playbook pb.yml -e map_value=distributed

PLAY [distributed] ***************************************************************************

TASK [debug] *********************************************************************************
ok: [app_node] => 
  ansible_play_hosts:
  - app_node
  - db_node
  - gateway_node

PLAY RECAP ***********************************************************************************
app_node: ok=1    changed=0    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0

---

2. Create children (group aliases)

shell> cat hosts 
[STG]
node1

[single:children]
STG

[LIVE]
app_node
db_node
gateway_node

[distributed:children]
LIVE

Then, the playbook gives the same results

shell> cat pb.yml
- hosts: "{{ map_value }}"
  tasks:
    - debug:
        var: ansible_play_hosts
      run_once: true

If you can't change the inventory file put the aliases into a separate file. For example,

shell> tree inventory/
inventory/
├── 01-hosts
└── 02-aliases

shell> cat inventory/01-hosts 
[STG]
node1

[LIVE]
app_node
db_node
gateway_node

shell> cat inventory/02-aliases 
[single:children]
STG

[distributed:children]
LIVE

Then, the playbook gives the same results

shell> ansible-playbook -i inventory pb.yml -e map_value=single
...
shell> ansible-playbook -i inventory pb.yml -e map_value=distributed
...

---

3. Use the inventory plugin constructed. See

shell> ansible-doc -t inventory constructed

For example, the inventory

shell> tree inventory
inventory
├── 01-hosts
└── 02-constructed.yml

shell> cat inventory/01-hosts 
[STG]
node1

[STG:vars]
map_group_value=single

[LIVE]
app_node
db_node
gateway_node

[LIVE:vars]
map_group_value=distributed

shell> cat inventory/02-constructed.yml 
plugin: constructed
use_extra_vars: true
compose:
  map_group: map_value
groups:
  map_group: map_group == map_group_value

Then, the playbook

- hosts: map_group
  tasks:
    - debug:
        var: ansible_play_hosts
      run_once: true

gives the same results

shell> ansible-playbook -i inventory pb.yml -e map_value=single
...
shell> ansible-playbook -i inventory pb.yml -e map_value=distributed
...

---

If you insist on testing the use case with a role create one

shell> cat roles/setup/tasks/main.yml 
- debug:
    var: ansible_play_hosts
  run_once: true

and use it in the playbook with any tags you like

shell> cat pb.yml
- hosts: map_group
  roles:
    - role: setup