Modules layout evolution

Over the years, different modules layouts have been explored, following the evolution of Puppet's features and the refinement of usage patterns.

There has never been a unique way of doing a module, but patterns and best practices have emerged and we are going to review the most relevant ones.

Class parameters—from zero to data bindings

The introduction of parameterized classes, with Puppet 2.6, has been a crucial step in standardizing the interfaces of classes. On earlier versions there was no unique way to pass data to a class. Variables defined anywhere could be dynamically used inside Puppet code or in templates to manage the module's behavior; there was no standard API to access or set them. We used to define parameter less classes as follows:

class apache {
  # Variables used in DSL or in templates were dynamically scoped 
  # and referenced without using their fully qualified name.
  # IE: $port, not $apache::port or $::apache_port
}

To declare them always and only...