Ansible play runs on one group of hosts, but depends on facts from others

Question

Here's the basic use case:

I have an NGINX reverse proxy I want to set up and so I specific a play that only runs on the "nginx" group.

However, in order to know the ips to reverse proxy to I need to gather facts from the "upstreams" group. This doesn't happen since the play does not run setup on the "upstreams".

This answer contains a solution I've used before, but I'd like to be able to have it all self contained in a single hosts play that I can run independently of others.

Answer 1

Use Delegated Facts, pre_tasks, and delegate the facts to the hosts they belong to.

- hosts: nginx
  become: yes
  tags:
    - nginx
  vars:
    listen_address: "x.x.x.x"
  pre_tasks:
    - name: 'gather upstream facts.'
      setup:
      delegate_to: "{{item}}"
      delegate_facts: True
      with_items: "{{groups['upstreams']}}"
  roles:
    - role: nginx
      upstreams: "{{ groups['upstreams'] | map('extract', hostvars, ['ansible_all_ipv4_addresses']) | ipaddr('x.x.x.x') | first | list }}"