Can I use ansible-playbook "--extra-vars" to execute roles conditionally - ansible

Question

I have an Ansible playbook I am working on and I am trying to make the execution a little more dynamic. I have two roles that don't always need to be run in conjunction, sometimes only one of them needs to be run. I have been digging into the Ansible docs, and I am wondering if I can pass --extra-vars parameters to only run a specific role.
Currently, my playbook looks like this:
---
- hosts: default
become: true
roles:
- role: upgrades
when: {{ upgrades_role }}
- role: custom-packages
when: {{ custom_packages_role }}
So, the goal is to be able to run:
ansible-playbook playbook.yml -e "upgrades_role=upgrades"
And this would only run the upgrades role and skip the custom_packages role.
Similarly, if I want to run both roles on the same hosts/system:
ansible-playbook playbook.yml -e "upgrades_role=upgrades custom_packages_role=custom-packages"
This would run both roles.
Based on my understating of Ansible syntax and the --extra-vars, -e parameter, this seems like it should work. I just want to be sure I am doing this the proper way and avoiding anti-patterns.
Ansible Version: 2.14

Answer 1

Let me provide you with a framework to automate the use case that you described:
Pass --extra-vars parameters to only run a specific role
Create the project
shell> ls -1
ansible.cfg
hosts
playbook.yml.j2
roles
setup.yml
shell> cat ansible.cfg
[defaults]
gathering = explicit
collections_path = $HOME/.local/lib/python3.9/site-packages/
inventory = $PWD/hosts
roles_path = $PWD/roles
retry_files_enabled = false
stdout_callback = yaml
shell> cat hosts
localhost
The playbook setup.yml
Gets the list of the roles. Fit the variable my_roles_dir to your needs.
Creates file my_roles_order.yml with the list my_roles_order. The purpose of this file is to create the order of the roles.
Creates file my_roles_enable.yml with the dictionary my_roles_enable. The purpose of this file is to create defaults.
Creates file playbook.yml from the template. Fit the template to your needs.
shell> cat setup.yml
- hosts: localhost
vars:
my_roles_dir: "{{ lookup('config', 'DEFAULT_ROLES_PATH') }}"
tasks:
- set_fact:
my_roles: "{{ my_roles|default([]) +
lookup('pipe', 'ls -1 ' ~ item).splitlines() }}"
loop: "{{ my_roles_dir }}"
- copy:
dest: "{{ playbook_dir }}/my_roles_order.yml"
content: |
my_roles_order:
{{ my_roles|to_nice_yaml|indent(2, true) }}
force: "{{ my_roles_order_force|default(false) }}"
- include_vars: my_roles_order.yml
- copy:
dest: "{{ playbook_dir }}/my_roles_enable.yml"
content: |
my_roles_enable:
{% for role in my_roles %}
{{ role }}: false
{% endfor %}
force: "{{ my_roles_enable_force|default(false) }}"
- include_vars: my_roles_enable.yml
- template:
src: playbook.yml.j2
dest: "{{ playbook_dir }}/playbook.yml"
shell> cat playbook.yml.j2
- hosts: localhost
become: true
vars_files:
- my_roles_enable.yml
roles:
{% for role in my_roles_order %}
- role: {{ role }}
when: {{ role }}_role|default(my_roles_enable.{{ role }})|bool
{% endfor %}
Test the trivial roles
shell> tree roles/
roles/
├── current_packages
│   └── tasks
│   └── main.yml
├── custom_packages
│   └── tasks
│   └── main.yml
├── stable_packages
│   └── tasks
│   └── main.yml
└── upgrades
└── tasks
└── main.yml
8 directories, 4 files
shell> cat roles/*/tasks/main.yml
- debug:
msg: Role current_packages running ...
- debug:
msg: Role custom_packages running ...
- debug:
msg: Role stable_packages running ...
- debug:
msg: Role upgrades running ...
Run the playbook setup.yml
shell> ansible-playbook setup.yml
PLAY [localhost] ****************************************************************************************
TASK [set_fact] *****************************************************************************************
ok: [localhost] => (item=/scratch/tmp7/test-204/roles)
TASK [copy] *********************************************************************************************
changed: [localhost]
TASK [include_vars] *************************************************************************************
ok: [localhost]
TASK [copy] *********************************************************************************************
changed: [localhost]
TASK [include_vars] *************************************************************************************
ok: [localhost]
TASK [template] *****************************************************************************************
changed: [localhost]
PLAY RECAP **********************************************************************************************
localhost: ok=6 changed=3 unreachable=0 failed=0 skipped=0 rescued=0 ignored=0
This creates playbook.yml
shell> cat playbook.yml
- hosts: localhost
become: true
vars_files:
- my_roles_enable.yml
roles:
- role: current_packages
when: current_packages_role|default(my_roles_enable.current_packages)|bool
- role: custom_packages
when: custom_packages_role|default(my_roles_enable.custom_packages)|bool
- role: stable_packages
when: stable_packages_role|default(my_roles_enable.stable_packages)|bool
- role: upgrades
when: upgrades_role|default(my_roles_enable.upgrades)|bool
and the files
shell> cat my_roles_order.yml
my_roles_order:
- current_packages
- custom_packages
- stable_packages
- upgrades
shell> cat my_roles_enable.yml
my_roles_enable:
current_packages: false
custom_packages: false
stable_packages: false
upgrades: false
By default, the playbook runs nothing. Here you can "pass --extra-vars parameters to only run a specific role". For example, enable the role upgrades
shell> ansible-playbook playbook.yml -e upgrades_role=true
PLAY [localhost] *****************************************************************************
TASK [current_packages : debug] **************************************************************
skipping: [localhost]
TASK [custom_packages : debug] ***************************************************************
skipping: [localhost]
TASK [stable_packages : debug] ***************************************************************
skipping: [localhost]
TASK [upgrades : debug] **********************************************************************
ok: [localhost] =>
msg: Role upgrades running ...
PLAY RECAP ***********************************************************************************
localhost: ok=1 changed=0 unreachable=0 failed=0 skipped=3 rescued=0 ignored=0
If you want to change the order and/or the enablement defaults of the roles edit the files my_roles_order.yml and my_roles_enable.yml. For example, put the role upgrades in the first place and enable it by default
shell> cat my_roles_order.yml
my_roles_order:
- upgrades
- current_packages
- custom_packages
- stable_packages
shell> cat my_roles_enable.yml
my_roles_enable:
current_packages: false
custom_packages: false
stable_packages: false
upgrades: true
Update the playbook
shell> ansible-playbook setup.yml
Test it
shell> ansible-playbook playbook.yml
PLAY [localhost] *****************************************************************************
TASK [upgrades : debug] **********************************************************************
ok: [localhost] =>
msg: Role upgrades running ...
TASK [current_packages : debug] **************************************************************
skipping: [localhost]
TASK [custom_packages : debug] ***************************************************************
skipping: [localhost]
TASK [stable_packages : debug] ***************************************************************
skipping: [localhost]
PLAY RECAP ***********************************************************************************
localhost: ok=1 changed=0 unreachable=0 failed=0 skipped=3 rescued=0 ignored=0
It's practical to create a file if you want to enable/disable multiple roles on the command line. For example,
shell> cat myroles.yml
upgrades_role: false
stable_packages_role: true
custom_packages_role: true
Use it in the command line
shell> ansible-playbook playbook.yml -e #myroles.yml
PLAY [localhost] *****************************************************************************
TASK [upgrades : debug] **********************************************************************
skipping: [localhost]
TASK [current_packages : debug] **************************************************************
skipping: [localhost]
TASK [custom_packages : debug] ***************************************************************
ok: [localhost] =>
msg: Role custom_packages running ...
TASK [stable_packages : debug] ***************************************************************
ok: [localhost] =>
msg: Role stable_packages running ...
PLAY RECAP ***********************************************************************************
localhost: ok=2 changed=0 unreachable=0 failed=0 skipped=2 rescued=0 ignored=0
Don't display skipped hosts
shell> ANSIBLE_DISPLAY_SKIPPED_HOSTS=false ansible-playbook playbook.yml -e #myroles.yml
PLAY [localhost] *****************************************************************************
TASK [custom_packages : debug] ***************************************************************
ok: [localhost] =>
msg: Role custom_packages running ...
TASK [stable_packages : debug] ***************************************************************
ok: [localhost] =>
msg: Role stable_packages running ...
PLAY RECAP ***********************************************************************************
localhost: ok=2 changed=0 unreachable=0 failed=0 skipped=2 rescued=0 ignored=0

Answer 2

The most common way will be to use tags; updating the example in the question:
---
- hosts: default
become: true
roles:
- role: upgrades
tags: upgrades
- role: custom-packages
tags: packages
...
So, the goal is to be able to run:
# Execute all the roles
ansible-playbook playbook.yml
# Execute only the upgrade
ansible-playbook playbook.yml -t upgrades
# Execute only the setup of custom packages
ansible-playbook playbook.yml -t packages
Here is the documentation for tags.
Using variables and when is possible, but you'll need to ensure to cast it to bool.