Ansible: Conditional role dependencies

I’ve recently been working on an Ansible role that applies to both Ubuntu and OSX hosts. It has some dependencies which are only needed on OSX. There doesn’t seem to be a central document on all the options available for solving this problem, so here are my notes.


The role which must apply to both Ubuntu and OSX hosts builds a Rust compiler capable of cross-compiling to Android, so I call it crosscompiler. To run the crosscompiler role on a Mac, you need the xcode role installed, but applying the xcode role to an Ubuntu host will fail.

A simplified version of this setup looks like:

├── galaxy_roles.yaml
├── hosts
├── roles
│   ├── crosscompiler
│   │   ├── defaults
│   │   │   └── main.yaml
│   │   ├── meta
│   │   │   └── main.yaml
│   │   └── tasks
│   │       └── main.yaml
│   └── xcode
│       ├── defaults
│       │   └── main.yaml
│       ├── meta
│       │   └── main.yaml
│       └── tasks
│           └── main.yaml
└── site.yaml

Here are a bunch of different ways to avoid applying a Mac-specific task to an Ubuntu host, or vice versa. Note that any of the following steps in isolation will solve the problem – it should not be necessary to use more than one of them.

Check OS on each task of the role

Add the line when: ansible_os_family == 'Darwin' at the end of each task in roles/xcode/tasks/main.yaml.

This needlessly bloats the code and makes it more difficult to read.

Refactor depended role to ignore non-target platforms

Move the entire contents of roles/xcode/main.yaml into roles/xcode/osx.yaml, then create a new main.yaml containing:

- include: osx.yaml
  when: ansible_os_family == 'Darwin'

This avoids the bloat induced by running the conditional on each task, while accomplishing the same goal. Now the xcode role looks like:

├── defaults
│   └── main.yaml
├── meta
│   └── main.yaml
└── tasks
    ├── main.yaml
    └── osx.yaml

This is the best solution for a role which might later expand to support additional platforms.

Make the dependency conditional in meta/main.yaml of depending role

Edit ansible-configs/roles/crosscompiler/main.yaml so that the dependency on xcode reads:

  - { role: 'xcode', when: ansible_os_family == 'Darwin' }

This is the best solution when the inner role will only ever target one platform, as is the case with xcode.

Install role conditionally from site.yaml

Edit ansible-configs/site.yaml to read:

- name: Provision cross-compile hosts
  hosts: xcompilehosts
    - { role: xcode, when: ansible_os_family == 'Darwin' }
    - crosscompiler

This is problematic because if I was to distribute the crosscompiler role on the Ansible Galaxy, its dependency logic would not be distributed to other users correctly.


You can conditionally include dependencies in your roles. It’s helpful to end users when galaxy roles only try to apply platform-specific tasks to their target platforms, since you can’t be sure how others will use your code.