Getting started with Ruby and Logstash

I've been able to get a lot done with your help here and with documentation on Logstash.

Now I want to start learning how to program in Logstash with Ruby. How does one get started with understanding of the Logstash runtime environment: how to interact with the pipe line processing, how to fetch event data, how to recognize matches, how to add to the events, how to modify events, etc?

Is there a Logstash programming primer or a set of examples to analyze to start this journey in Logstash?

Have you read It should at least answer some of your questions.