Elasticsearch

Elasticsearch is a distributed search and analytics engine built on Apache Lucene. Reach for it when your system needs search that a relational database can't handle efficiently: full-text search with relevance ranking, autocomplete, faceted filtering, and log analytics at scale.

Underneath the clustering and the JSON API, Elasticsearch is a search engine built on an inverted index. That phrase is worth holding on to, because the inverted index decides both what the engine answers in a millisecond and what it quietly refuses to promise you.