Elasticsearch, also based on the Apache Lucene engine, is an open source distributed search engine that focuses on modern concepts like REST APIs and JSON documents. Its approach to scaling makes it easy to take Elasticsearch clusters from gigabytes to petabytes of data with low operational overhead.
As part of the ELK stack (Elasticsearch, Logstash, and Kibana, all developed by Elasticsearch’s creators, Elastic), Elasticsearch has found its killer app as an open source Splunk replacement for log analysis. Companies like Netflix, Facebook, Microsoft, and LinkedIn run large Elasticsearch clusters for their logging infrastructure. Furthermore, the ELK stack is finding its way into other domains, such as fraud detection and domain-specific business analytics, spreading the use of Elasticsearch throughout the enterprise.
-- Ian Pointer