Hiera provides an interesting function called hiera_include, which allows users to include all the classes defined for a given key.

This, in practice, exploits the Hiera flexibility to provide classes to nodes as an External Node Classifier does.

It's enough to place the following line in our site.pp file at /etc/puppet/manifests/:

hiera_include('classes')

Define in our data sources a classes key with an array of the classes to be included.

In a YAML-based backend, it would look like the following:

---
classes:
  - apache
  - mysql
  - php

This is exactly the same as having some declarations in our site.pp as follows:

include apache
include mysql
include php

The classes key (can have any name, but classes is a standard de facto) contains an array that is merged along the hierarchy. So we can define in a common.yaml file the classes that we want to include in all our nodes, and include specific classes for specific servers, adding them to the different layers of our hierarchy...