Ansible target specific host (not delegate_to) - ansible

Question

There are 3 hosts in my play.
[machines]
MachineA
MachineB
MachineC
MongoDB runs on these servers. And one of these servers can be a MasterDB of Mongo.
So, each of these machines can be a 'Master'. This is determined by setting the fact if the machine is master, in this example only MachineA is targeted:
- name: check if master
shell: 'shell command to check if master'
set_fact: MasterHost="machineA"
when: 'shell command to check if master'.stdout == "true"
This is also done for MachineB and MachineC.
Mission to achieve: To run commands only on on the Master machine, which has the fact "MasterHost".
I tried the delegate_to module, but delegate_to also uses the two other machines:
- name: some task
copy: src=/tmp/test.txt dest=/tmp/test.txt
delegate_to: "{{ MasterHost }}"
I want to target the master it in my playbook and run only commands on the master, not in the shell via the --limit option.

Answer 1

Assuming the command run to check whether the host is the master or not is not costly, you can go without setting a specific fact:
- name: check if master
shell: 'shell command to check if master'
register: master_check
- name: some task
copy: src=/tmp/test.txt dest=/tmp/test.txt
when: master_check.stdout == "true"
Run the play on all hosts and only the one that is the master will run some task.

Answer 2

Eventually, this was my answer. Sorry for the first post, still learning how to make a good post. Hihi
- name: Check which host is master
shell: mongo --quiet --eval 'db.isMaster().ismaster'
register: mongoMaster
- name: Set fact for mongoMasterr
set_fact: MongoMasterHost="{{ item }}"
with_items: "{{ groups['HOSTS'] }}"
when: mongoMaster.stdout == "true"
- name: Copy local backup.tgz to master /var/lib/mongodb/backup
copy: src=/tmp/backup.tgz dest=/var/lib/backup/backup.tgz
when: mongoMaster.stdout == "true"
Thanks for helping and pointing me toward the right direction.