Control ansible play execution inventory hosts - ansible

Question

Need assistance how to make one particular task in the playbook run on all hosts.
I have a below task inside a role
---
- name: Task 1
shell: echo "Must be run on one node"
- name: Task 2
shell: echo "Must be run on one node"
- name: Task 3
shell: echo "This must run on all nodes inside inventory"
I am running this task by passing --limit but "Task 3" must run on all hosts in the inventory. I tried using below block but not its not executing
- name: Verify limit is set
debug:
msg: "Must use --limit"
when: ansible_limit is not defined
run_once: true
- name: Print all nodes
debug:
msg: "Running on node {{ item }}"
with_items: "{{ ansible_play_hosts_all }}"

Answer 1

There are many options. For example, create a dictionary that will control the flow
shell> cat group_vars/all
task_hosts:
1: [host_1] # task 1 runs on host_1 only
2: [host_2] # task 2 runs on host_2 only
3: "{{ groups.all }}" # task 3 runs on all hosts
Given the inventory
shell> cat hosts
host_1
host_2
host_3
The playbook
shell> cat pb.yml
- hosts: all
gather_facts: false
tasks:
- name: Task 1
debug:
msg: echo "Must be run on one node"
when: inventory_hostname in task_hosts.1
- name: Task 2
debug:
msg: echo "Must be run on one node"
when: inventory_hostname in task_hosts.2
- name: Task 3
debug:
msg: echo "This must run on all nodes inside inventory"
when: inventory_hostname in task_hosts.3
gives
shell> ansible-playbook pb.yml
PLAY [all] ***********************************************************************************
TASK [Task 1] ********************************************************************************
skipping: [host_2]
ok: [host_1] =>
msg: echo "Must be run on one node"
skipping: [host_3]
TASK [Task 2] ********************************************************************************
skipping: [host_1]
skipping: [host_3]
ok: [host_2] =>
msg: echo "Must be run on one node"
TASK [Task 3] ********************************************************************************
ok: [host_1] =>
msg: echo "This must run on all nodes inside inventory"
ok: [host_2] =>
msg: echo "This must run on all nodes inside inventory"
ok: [host_3] =>
msg: echo "This must run on all nodes inside inventory"
PLAY RECAP ***********************************************************************************
host_1: ok=2 changed=0 unreachable=0 failed=0 skipped=1 rescued=0 ignored=0
host_2: ok=2 changed=0 unreachable=0 failed=0 skipped=1 rescued=0 ignored=0
host_3: ok=1 changed=0 unreachable=0 failed=0 skipped=2 rescued=0 ignored=0
Notes:
Add one more level if there are more plays in the playbook
shell> cat group_vars/all
play_task_hosts:
1: # play 1
1: [host_1] # task 1 runs on host_1 only
2: [host_2] # task 2 runs on host_2 only
3: "{{ groups.all }}" # task 3 runs on all hosts
You might want to automate the updates if there are many tasks in many plays in many playbooks
shell> cat update_pb.yml
- hosts: localhost
vars:
my_dir: "{{ playbook_dir }}"
my_pb: pb.yml
my_flow: play_task_hosts
pb: "{{ lookup('file', my_pb)|from_yaml }}"
_tasks: "{{ pb|json_query('[].tasks') }}"
_update_tasks: |
{% for play in _tasks %}
-
{% set play_index = loop.index %}
{% for task in play %}
{% set condition = ["inventory_hostname in ",
my_flow, ".",
play_index, ".",
loop.index]|join() %}
- {{ task|combine({"when": [condition]}, list_merge='append_rp') }}
{% endfor %}
{% endfor %}
update_tasks: "{{ _update_tasks|from_yaml }}"
_update_pb: |
{% for play in pb %}
- {{ play|combine({"tasks": update_tasks[loop.index0]}) }}
{% endfor %}
update_pb: "{{ _update_pb|from_yaml }}"
tasks:
- block:
- debug:
var: _tasks
- debug:
var: update_tasks
- debug:
var: update_pb
when: debug|d(false)|bool
- copy:
dest: "{{ my_dir }}/{{ my_pb }}.update"
content: |
{{ update_pb|to_nice_yaml }}

Answer 2

Generally, if you have a different task list for different hosts, it begs for having different roles. Basically, the best practice for Ansible is to have plays to control where code is been executed.
If role start to decide which host should have which tasks, it twists Ansible upside down, and make role wig the inventory and plays. Don't do it. Role should do what was told and do not make cross-host decisions.
If it's a single task and you don't want to have two roles, you may pass a variable from inventory/play to the role. It can be either flag (like doo_foo), configured in inventory, which is checked as when: doo_foo, or as a delegation:
- name: Do foo
run_once: true
do: foo
delegate_to: '{{ foo_delegation_host }}'
Note: there is run_once flag, which forces Ansible to do task one, and delegate_to is controlling where it runs. The value foo_delegation_host should come from the play (as role parameter or a play variable).