Problem

So if you have hiera.yaml with a hierarchy like this

# hiera.yaml
---
:backends:
 - yaml
:hierarchy:
 - "hosts/%{clientcert}"
 - "default"

following content in the yaml files

# default.yaml
---
testmodule::hash_expr:
  key1: val1
  key2: val2
testmodule::array_expr:
  - item1
  - item2

# hosts/yourhostname.yaml
---
testmodule::hash_expr:
  key3: val3
  key4: val4
testmodule::array_expr:
  - item3
  - item4

and an included puppet class like this

# testmodule/init.pp
class testmodule (
  $hash_expr,
  $array_expr,
){
  notify{$hash_expr:}
  notify{$array_expr:}
}

you will get the values from hosts/yourhostname.yaml only, still if you expect and want to have a behavior like with hash merge or array merge lookups.

bash $ puppet agent -t
...
Notice: {"key3"=>"val3", "key4"=>"val4"}
Notice: item4
Notice: item3

Solution

You can change your class like this

# testmodule/init.pp
class testmodule (
  $hash_expr,
  $array_expr,
){

  #hiera lookup
  $hiera_hash_expr = hiera_hash("${module_name}::hash_expr",undef)
  $fin_hash_expr = $hiera_hash_expr ? {
    undef   => $hash_expr,
    default => $hiera_hash_expr,
  }

  $hiera_array_expr = hiera_array("${module_name}::array_expr",undef)
  $fin_array_expr = $hiera_array_expr ? {
    undef   => $array_expr,
    default => $hiera_array_expr,
  }


  notify{$fin_hash_expr:}
  notify{$fin_array_expr:}
}

In case of sucessful hiera lookup we use the results from it, otherwise the parameters submitted to the class. So, the class can be used on the regular way without hiera, but if it’s used with hiera it will use array and hash lookups.

bash $ puppet agent -t
...
Notice: {"key1"=>"val1", "key2"=>"val2", "key3"=>"val3", "key4"=>"val4"}
Notice: item1
Notice: item2
Notice: item4
Notice: item3

Puppet hiera hash merge and automatic parameter lookup

Problem

Solution