4. Lucene
• Scalable, High-Performance Indexing
• Powerful, Accurate and Efficient Search Algorithms
• Cross-Platform Solution
• The Apache Software Foundation
http://lucene.apache.org/core
5. Solr
Solr is the popular, blazing-fast, open source enterprise search
platform built on Apache Lucene.
http://lucene.apache.org/solr/
6. Elasticsearch
Elasticsearch is a distributed, JSON-based search and analytics engine
designed for horizontal scalability, maximum reliability, and easy
management.
https://www.elastic.co