Execute on host conditionally, Ansible [closed] - ansible

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I'm attempting to create a conditional in my playbook that switches based on an ENV defined in a Dockerfile. The playbook is copied in by the Dockerfile, and Ansible runs it against localhost to do some configuration. However, these playbooks are also used to configure an ordinary server.
Here's my question: Is is possible to use a conditional when defining hosts?
- hosts: actual_server
roles:
- some_roles
when:
- DOCKER_CONTAINER is not defined
- hosts: docker_container
roles:
- almost_same_roles
when:
- DOCKER_CONTAINER is true
I've hunted through the Ansible docs, and found several examples where multiple hosts are targeted in a single playbook, but I an unsure if my syntax for the conditional is wrong, or if what I want to accomplish simply cannot be done. Am I limited to defining conditionals only on a role-by-role basis?
I'd be glad to edit this question, to make it more clear, since it has been closed for being "unclear"... but I'm not sure how much MORE clear I could be, than stating exactly what I'm asking in the above. Glad for more specific feedback if anyone is willing to give it.

Answer 1

It honestly feels like this behavior should be pushed lower into the tasks as opposed to the role level. Roles describe how to accomplish a specific state through a list of tasks, and the tasks themselves perform the steps in order to accomplish that specific state.
With that said, it makes sense to have the logic in the task to determine whether or not it will execute. This means that your role some_roles would look like this (warning: untested).
# In tasks/playbook.yml
- hosts: actual_server
roles:
- some_roles
vars:
environment_type: Server
- hosts: docker_container
roles:
- some_roles
vars:
environment_type: Docker
# In vars/main.yml file
environment_type: Server
# In tasks/main.yml
- name: Download a tarball
get_url:
url: "https://example.com/some_package.tar.gz"
dest: /tmp
mode: 0750
when: environment_type == 'Docker'

Answer 2

If you want to run one ansible script for the different environment.
Here is your answer, you can go with inventory file for each environment generated by docker(if needed).
ansible-playbook playbook.yml -i development.ini
ansible-playbook playbook.yml -i testing.ini
ansible-playbook playbook.yml -i staging.ini
development.ini
[actual_server]
192.168.0.100 # some IP address
[docker_container]
127.0.0.1
testing.ini
[actual_server]
192.168.0.101 # some IP address
[docker_container]
127.0.0.1
https://docs.ansible.com/ansible/latest/user_guide/intro_inventory.html
https://symfonycasts.com/screencast/ansible/hosts-inventory

Answer 3

Not the ideal answer, but you can use a when condition on each role:
- hosts: actual_server
roles:
- role: role1
when: DOCKER_CONTAINER is not defined
- role: role2
when: DOCKER_CONTAINER is not defined
...
- hosts: docker_container
roles:
- role: role3
when: DOCKER_CONTAINER is true
- role: role4
when: DOCKER_CONTAINER is true
...