Internals:
Lucene or Apache Lucene is an open-source Java library used as a search engine. Elasticsearch is built on top of Lucene. Elasticsearch converts Lucene into a distributed system/search engine for scaling horizontally.
- Elasticsearch is built over Lucene and provides a JSON based REST API to refer to Lucene features.
- Elasticsearch provides a distributed system on top of Lucene. A distributed system is not something Lucene is aware of or built for. Elasticsearch provides this abstraction of distributed structure.
- Elasticsearch provides other supporting features like thread-pool, queues, node/cluster monitoring API, data monitoring API, Cluster management, etc.