Ansible playbook-wide variable

I have a playbook with multiple hosts section. I would like to define a variable in this playbook.yml file that applies only within the file, for example:

vars:
  my_global_var: 'hello'

- hosts: db
  tasks:
   -shell: echo {{my_global_var}} 

- hosts: web
  tasks:
   -shell: echo {{my_global_var}} 

The example above does not work. I have to either duplicate the variable for each host section (bad) or define it at higher level, for example in my group_vars/all (not what I want, but works). I am also aware that variables files can be included, but this affects readibility. Any suggestion to get it in the right scope (e.g the playbook file itself)?

The set_fact module will accomplish this if group_vars don't suit your needs.

https://docs.ansible.com/ansible/latest/collections/ansible/builtin/set_fact_module.html

This module allows setting new variables. Variables are set on a host-by-host >basis just like facts discovered by the setup module. These variables will >survive between plays during an Ansible run, but will not be saved across >executions even if you use a fact cache.

- hosts: db:web
  tasks:
  - set_fact: my_global_var='hello'

- hosts: db
  tasks:
  -shell: echo {{my_global_var}} 

- hosts: web
  tasks:
  -shell: echo {{my_global_var}}