Elasticsearch internals: an overview by Njal Karevoll.
From the post:
This article gives an overview of the Elasticsearch internals. I will present a 10,000 foot view of the different modules that Elasticsearch is composed of and how we can extend or replace built-in functionality using plugins.
Using Freemind, Njal has created maps of the namespaces and modules of ElasticSearch for your exploration.
The full module view reminds me of SGML productions, except less complicated.