Each role can have it's own unique set of variables, and there are numerous ways to define the variables that will be used by a role. This is in the order of precedence, top down, where the option higher in the list takes precedence.

• Define variables using the --extra-vars command line option
• Define variables using the vars plugin
• Define variables in vars/main.yml
• Define variables in defaults/main.yml

For example, let's say you have two roles, "foo" and "bar". Here is how to define variables using the vars plugin.

---
- hosts: all
  roles:
    - role: foo
      vars:
        var: Hello

    - role: bar
      vars:
        var: World

Or like this.

---
- hosts: all
  roles:
    - { role: foo, vars: { var: "Hello" } }
    - { role: bar, vars: { var: "World" } }

Let's say /etc/ansible/roles/foo|bar/tasks/main.yml contains:

- name: variable test
  debug:
    msg: "{{ var }}"

Invoking master.yml should return the following.

TASK [/etc/ansible/roles/foo: variable test] 
ok: [server1.example.com] => {
    "msg": "Hello"
}

TASK [/etc/ansible/roles/bar: variable test] 
ok: [server1.example.com] => {
    "msg": "World"
}

Or, variables can be put in the defaults/main.yml file or the vars/main.yml file. Let's say /etc/ansible/roles/foo/vars/main.yml contains:

var: "Hello"

Let's say /etc/ansible/roles/bar/vars/main.yml contains:

var: "World"

Invoking master.yml should return the following.

TASK [/etc/ansible/roles/foo: variable test] 
ok: [server1.example.com] => {
    "msg": "Hello"
}

TASK [/etc/ansible/roles/bar: variable test] 
ok: [server1.example.com] => {
    "msg": "World"
}