Automatic lookup of class parameters

Puppet looks up the values for class parameters in Hiera, using the fully qualified name of the parameter (myclass::parameter_one) as a lookup key.

Most classes need configuration, and you can specify them as parameters to a class as this looks up the needed data if not directly given when the class is included in a catalog. Puppet sets values for class parameters in this order:

  1. If you're doing a resource-like declaration, Puppet uses parameters that are explicitly set (if explicitly setting undef, a looked up value or default is used).

  2. Puppet uses Hiera, using <CLASS NAME>::<PARAMETER NAME> as the lookup key. For example, it looks up ntp::servers for the ntp class's $servers parameter.

  3. If a parameter still has no value, Puppet uses the default value from the parameter's default value expression in the class's definition.

  4. If any parameters have no value and no default, Puppet fails compilation with an error.

For example, you can set servers for the NTP class like this: 

# /etc/puppetlabs/code/production/data/nodes/web01.example.com.yaml
---
ntp::servers:
  - time.example.com
  - 0.pool.ntp.org
The best way to manage this is to use the roles and profiles method, which allows you to store a smaller amount of more meaningful data in Hiera.

Automatic lookup of class parameters uses the "first" merge method by default. You cannot change the default. If you want to get deep merges on keys, use the lookup_options feature.

This feature is often referred to as Automatic Parameter Lookup (APL).