If you ever start working with large, semi-heterogenous groups of VMs and want to manage them with Ansible, you quickly start running into problems trying to make it work with complex groups.

As an example, let's assume we have webservers and jobservers spread out across 2 cloud providers, for 3 different applications, in 3 environments (production, staging, and test). How would you set variables for all webservers for application A? Or all production servers across all providers and applications? Or all webservers, but not jobservers?

The typical solution is to have a lot of duplication in your inventory files. For example:

all:
  children:
    env_prod:
      children:
        provider_gcp:
          children:
            type_webserver:
              children:
                app_a:
                  children:
                    app_a_webservers:
                      hosts:
                        web[01:03].app-a.prod.gcp.mycorp.net:
                app_b:
                  children:
                    app_b_webservers:
                      hosts:
                        web[01:03].app-b.prod.gcp.mycorp.net:
            type_jobserver:
              children:
                app_a:
                  children:
                    app_a_jobservers:
                      hosts:
                        job[01:03].app-a.prod.gcp.mycorp.net:
                app_b:
                  children:
                    app_b_jobservers:
                      hosts:
                        job[01:03].app-b.prod.gcp.mycorp.net:

Imagine many, many more lines like this, for other environments and providers. The only thing we're not repeating in this example is the top-level group (which in this case is env_prod, but it's up to you how to structure your hierarchy) and the lowest level group.

The alternative: Using constructed

# inventory/main.yml
all:
  children:
    app_a_webservers:
      hosts:
        web[01:03].app-a.prod.gcp.mycorp.net:
        web[01:05].app-a.prod.aws.mycorp.net:
        web[01:02].app-a.stag.gcp.mycorp.net:
        web[01:02].app-a.test.gcp.mycorp.net:

The constructed plugin lets you define jinja statements for groups. Basically, if the Jinja statement evaluates to true, the server will be added to this group. Any built-in Ansible variables (not group_vars, host_vars) are available here, so you can check things like the hostname, which user you're using to connect with, which non-constructed groups the server is a part of, and more. inventory_hostname is a useful one, and because it's a string you can do a lot of useful Python operations on it, such as .split and .startswith. You can also use Ansible filters.

# inventory/zz-constructed.yml
plugin: constructed
strict: true

groups:
  provider_gcp: inventory_hostname.endswith('.gcp.mycorp.net')
  provider_aws: inventory_hostname.split('.')[-3] == 'aws'
  env_prod: inventory_hostname.split('.')[-4] == 'prod'
  env_staging: '.stag.' in inventory_hostname
  env_test: inventory_hostname.endswith(('.stag.gcp.mycorp.net', '.stag.aws.mycorp.net'))
  type_webserver: inventory_hostname.startswith('web')
  type_jobserver: inventory_hostname | regex_search('^job\d+\.')
  app_a: inventory_hostname.split('.')[1] == 'app-a'
  app_b: inventory_hostname | regex_search('[a-z0-9]+\.')