Apache Lucene-Solr

Solr is highly reliable, scalable and fault tolerant, providing distributed indexing, replication and load-balanced querying, automated failover and recovery, centralized configuration and more. Solr powers the search and navigation features of many of the world’s largest internet sites.

Web: http://lucene.apache.org/solr/

Code: https://github.com/apache/lucene-solr