Ansible can we use if condition at hosts - ansible

Question

How to run the playbook on a specific set of hosts with a conditional variable
vars_file.yml
deployment: blue
hosts_file.yml
[east1]
127.0.0.1
127.0.0.2
[west2]
127.0.0.3
127.0.0.4
playbook.yml
---
hosts: all
vars_files:
- 'vars_file.yml'
tasks:
- copy: src=config dest=/tmp/
hosts: {{ east1[0] if deployment == "blue" else west2[0]}}
vars_files:
- 'vars_file.yml'
tasks:
- shell: "./startup_script restart"
Note: I cant pass variables through the command line and I cant segregate task to a new playbook.

Answer 1

You can access variables defined on another host by targeting the hostvars dictionary key of that host.
In order to do that though, you need to register the variable on the host, with set_fact, importing it won't be enough.
Here is an example, given the inventory:
all:
children:
east1:
hosts:
east_node_1:
ansible_host: node1
east_node_2:
ansible_host: node4
west2:
hosts:
west_node_1:
ansible_host: node2
west_node_2:
ansible_host: node3
And the playbook:
- hosts: localhost
gather_facts: no
vars_files:
- vars_file.yml
tasks:
- set_fact:
deployment: "{{ deployment }}"
- hosts: >-
{{ groups['east1'][0]
if hostvars['localhost'].deployment == 'blue'
else groups['west2'][0]
}}
gather_facts: no
tasks:
- debug:
This would yield the recaps:
PLAY [localhost] *******************************************************************************************************************
TASK [set_fact] ********************************************************************************************************************
ok: [localhost]
PLAY [east_node_1] *****************************************************************************************************************
TASK [debug] ***********************************************************************************************************************
ok: [east_node_1] => {
"msg": "Hello world!"
}
PLAY RECAP *************************************************************************************************************************
east_node_1 : ok=2 changed=0 unreachable=0 failed=0 skipped=0 rescued=0 ignored=0
localhost : ok=2 changed=0 unreachable=0 failed=0 skipped=0 rescued=0 ignored=0
when vars_file.yml contains
deployment: blue
And
PLAY [localhost] *******************************************************************************************************************
TASK [set_fact] ********************************************************************************************************************
ok: [localhost]
PLAY [west_node_1] *****************************************************************************************************************
TASK [debug] ***********************************************************************************************************************
ok: [west_node_1] => {
"msg": "Hello world!"
}
PLAY RECAP *************************************************************************************************************************
west_node_1 : ok=2 changed=0 unreachable=0 failed=0 skipped=0 rescued=0 ignored=0
localhost : ok=2 changed=0 unreachable=0 failed=0 skipped=0 rescued=0 ignored=0
when vars_file.yml contains
deployment: red
Another equivalent construction, using patterns to target the group, would just see the host target changed to:
- hosts: >-
{{ 'east1[0]'
if hostvars['localhost'].deployment == 'blue'
else 'west2[0]'
}}