Variables of another host not accessible via the `hostvars` variable?

I have a playbook running plays against 2 different hosts. Here are the plays:

- name: Backup database
  hosts: host1
  roles:
    - backup_database
  vars:
    db_backup_path: "{{ hostvars.localhost.temp_dir.path + '/' + backup_filename }}"

- name: Check the backup exists
  hosts: localhost
  roles:
    - aws
  tasks:
    - debug:
        var: hostvars['host1']['db_backup_path']

Running this threw an error:
> The task includes an option with an undefined variable. The error was: 'ansible.vars.hostvars.HostVarsVars object' has no attribute 'db_backup_path'.

I obviously assigned that variable in the previous play, so why would the second play not be able to access it?

This is described in the variables documentation page:

> You can decide where to set a variable based on the scope you want
> that value to have. Ansible has three main scopes:
>
> * Global: this is set by config, environment variables and the command line
> * Play: each play and contained structures, vars entries (vars; vars_files; vars_prompt), role defaults and vars.
> * Host: variables directly associated to a host, like inventory, include_vars, facts or registered task outputs

<sup>Source: https://docs.ansible.com/ansible/latest/playbook_guide/playbooks_variables.html#scoping-variables&lt;/sup&gt;

So, in your case, by using a vars at the play level, your variable db_backup_path is only visible for that play.

What you can do, if you want to variable to be accessible by both your role and your second play is to use a set_fact task in a pre_tasks.

So, in your case, something like:

- hosts: host1
  gather_fact: false

  pre_tasks:
    - set_fact:
        db_backup_path: >-
          {{ hostvars.localhost.temp_dir.path }}/{{ backup_filename }}

  roles:
    - backup_database
    

- hosts: localhost
  gather_fact: false

  roles:
    - aws

  tasks:
    - debug:
        var: hostvars.host1.db_backup_path

Mind that there are also voices in the Ansible team itself that would prefer the usage of include_role and import_role over the role keyword, which is a exception in Ansible.

So, your playbook could also be written:

- hosts: host1
  gather_fact: false    

  tasks:
    - set_fact:
        db_backup_path: >-
          {{ hostvars.localhost.temp_dir.path }}/{{ backup_filename }}

    - import_role: 
        name: backup_database
    

- hosts: localhost
  gather_fact: false

  tasks:
    - import_role: 
        name: aws

    - debug:
        var: hostvars.host1.db_backup_path

Which is, in my opinion, easier to read as the tasks are going to happen in the same chronological order as you read them.