How can I check how many Play Hosts have variable value X in Ansible?

I have the following Ansible problem.

Each host a variable, let's call it is_master, which can either be "0" or "1".

I now want to check if the number of hosts having is_master == "1" is grather than 1.

So I want to do something like

- set_fact: master_counter = "0"
- set_fact: {{ master_counter + 1 }}
  when: {{ hostvars['{{ item }}']['is_master'] }} == "1"
  loop: "{{ ansible_play_hosts_all }}"
  delegate_to: localhost
  run_once: true
- debug: msg="There is more than one master!"
  when: master_counter > 1

Solution

For example, given the inventory for testing

test:
  hosts:
    host_A:
      is_master: '1'
    host_B:
      is_master: '1'
    host_C:
      is_master: '0'

count the masters

  master_counter: "{{ ansible_play_hosts_all|
                      map('extract', hostvars, 'is_master')|
                      select('eq', '1')|length }}"

and display the result

    - debug:
        msg: "master_counter={{ master_counter }}. There is more than one master!"
      when: master_counter|int > 1
      run_once: true

gives

  msg: master_counter=2. There is more than one master!

^{Example of a complete playbook for testing

- hosts: all

vars:

master_counter: "{{ ansible_play_hosts_all|
map('extract', hostvars, 'is_master')|
select('eq', '1')|length }}"

tasks:

- debug:
var: is_master

- debug:
msg: "master_counter={{ master_counter }}. There is more than one master!"
when: master_counter|int > 1
run_once: true

It's more practical to use boolean instead of stings

test:
hosts:
host_A:
is_master: true
host_B:
is_master: true
host_C:
is_master: false

The counting is a bit simpler
master_counter: "{{ ansible_play_hosts_all|
map('extract', hostvars, 'is_master')|
select()|length }}"

The above options count hosts limited by the play ansible_play_hosts_all. You can count all hosts in the inventory if you want to

master_counter: "{{ hostvars|
json_query('*.is_master')|
select()|length }}"}