apache.yaml

collectd::core_plugins:
  'apache':
    'apache80':
      'URL': 'http://localhost/server-status?auto'

common.yaml

collectd::core_plugins:
  'syslog': ~
  'conntrack': ~
  'contextswitch': ~
  'entropy': ~
  'load': ~
  'memory': ~
  'disk':
    'Disk': '/^[vhs]d[a-f][0-9]?$/'
    'IgnoreSelected': 'false'
  'df':
    'MountPoint': '/'
  'interface':
    'Interface': 'lo'
    'IgnoreSelected': 'true'

hiera.yaml

---
:backends:
  - yaml

:hierarchy:
  - "%{::hostname}"
  - "%{::environment}"
  - apache
  - common

:yaml:
  :datadir: /etc/puppet/nodes

:merge_behavior: deeper

init.pp

class collectd (
  $core_plugins = undef,
  $perl_plugins = undef
) {

    # Hiera deep merge magic
    if $core_plugins != undef {
        $collectd_core_plugins = hiera_hash('collectd::core_plugins')

        validate_hash($collectd_core_plugins)
    }

    if $perl_plugins != undef {
        $collectd_perl_plugins = hiera_hash('collectd::perl_plugins')

        validate_hash($collectd_perl_plugins)
    }

    # ... there's a template call that uses collectd_core_plugins and collectd_perl_plugins later here
}

webapp.yaml

# nothing here