There are two supported ways to debug Ruby code with puppetserver—ruby-debug and Pry. Both of these will implement a Read Eval Print Loop (REPL). A REPL allows you to type commands and have them evaluated in real time. When working with puppetserver, this gives you the power to inspect the puppetserver process while it is compiling a catalog or processing a report. Both ruby-debug and Pry are installed as Ruby gems. To use them with puppetserver, you have to install them by using the puppetserver gem command. I prefer to use Pry. So, I will show you how to use Pry in this example. We'll inspect calls to the hiera function by using Pry. To start, install Pry for use within puppetserver, as follows:

[root@puppet ~]# puppetserver gem install pry
Fetching: coderay-1.1.0.gem (100%)
Successfully installed coderay-1.1.0
Fetching: slop-3.6.0.gem (100%)
Successfully installed slop-3.6.0
Fetching: method_source-0.8.2.gem (100%)
Successfully installed method_source-0.8.2
Fetching: spoon...