Ansible won't see a handler when using the group_by

I used to have simple playbook (something like this) which I run on all over my machines (RH & Debian based) to update them, and for each machine which was updated run a script (notify handler).

Recently I tried to test a new module called group_by, so instead using the when condition to run yum update when ansible_distribution == "CentOS", I will first gather the facts and group the host based on there ansible_pkg_mgr as key and then I was looking to run yum update on all the hosts which the key is PackageManager_yum , see the play book example:

---
- hosts: all
  gather_facts: false
  remote_user: root
  tasks:

    - name: Gathering facts
      setup:

    - name: Create a group of all hosts by operating system
      group_by: key=PackageManager_{{ansible_pkg_mgr}}

- hosts: PackageManager_apt
  gather_facts: false
  tasks:
    - name: Update DEB Family
      apt: 
        upgrade=dist
        autoremove=yes
        install_recommends=no
        update_cache=yes
      when: ansible_os_family == "Debian"
      register: update_status
      notify: updateX
      tags: 
        - deb
        - apt_update
        - update

- hosts: PackageManager_yum
  gather_facts: false
  tasks:
    - name: Update RPM Family
      yum: name=* state=latest
      when: ansible_os_family == "RedHat"
      register: update_status
      notify: updateX
      tags: 
        - rpm
        - yum
        - yum_update

  handlers:
    - name: updateX
      command: /usr/local/bin/update

And this is the error message I get,

PLAY [all] ********************************************************************

TASK [Gathering facts] *********************************************************
Wednesday 21 December 2016  11:26:17 +0200 (0:00:00.031)       0:00:00.031 **** 
....

TASK [Create a group of all hosts by operating system] *************************
Wednesday 21 December 2016  11:26:26 +0200 (0:00:01.443)       0:00:09.242 **** 

TASK [Update DEB Family] *******************************************************
Wednesday 21 December 2016  11:26:26 +0200 (0:00:00.211)       0:00:09.454 **** 
ERROR! The requested handler 'updateX' was not found in either the main handlers list nor in the listening handlers list

thanks in advance.

You defined handlers only in one of your plays. It's quite clear if you look at the indentation.

The play which you execute for PackageManager_apt does not have the handlers at all (it has no access to the updateX handler defined in a separate play), so Ansible complains.

If you don't want to duplicate the code, you can save the handler to a separate file (let's name it handlers.yml) and include in both plays with:

  handlers:
    - name: Include common handlers
      include: handlers.yml

---

Note: there's a remark in Handlers: Running Operations On Change section regarding including handlers:

You cannot notify a handler that is defined inside of an include. As of Ansible 2.1, this does work, however the include must be static.

---

Finally, you should rather consider converting your playbook to a role.

A common method to achieve what you want is to include the tasks (in tasks/main.yml) using file names with the architecture in their names:

- include: "{{ architecture_specific_tasks_file }}"
  with_first_found:
    - "tasks-for-{{ ansible_distribution }}.yml"
    - "tasks-for-{{ ansible_os_family }}.yml"
  loop_control:
    loop_var: architecture_specific_tasks_file

Handlers are then defined in handlers/main.yml.