Compass is a powerful search engine mapping built on Lucene. It is designed for fast search and support mapping from data object to the search engine. Compass is not a traditional database, it is more fit to a search engine.
Shay Banon developed Compass at 2006 when he found there was not a good recipe management software and decided to develop one by himself, named iCook. He then extended to searching on a lot of domain models and created Compass.
Compass has a lot of sub-indexes and each sub-index is mapping to a Lucene Index, which is an inverted index. Compass used index partitioning to manage complexity and increase performance.