Apache Lucene is one of the most matured implementations of the inverted index. Lucene is an open source full-text search library. It's very high performing, entirely written in Java. Any application that requires text search can use Lucene. It allows adding full-text search capabilities to any application. Elasticsearch uses Apache Lucene to manage and create its inverted index. To learn more about Apache Lucene, please visit http://lucene.apache.org/core/.
We will talk about how distributed search works in Elasticsearch in the next section.
The term index is used both by Apache Lucene (inverted index) and Elasticsearch index. For the remainder of the book, unless specified the term index refers to an Elasticsearch index.
