ElasticSearch - Open Source, Distributed, RESTful Search Engine
ElasticSearch is an open source, distributed, RESTful search engine built on top of Lucene. Its features include:
- Distributed and Highly Available Search Engine.
- Each index is fully sharded with a configurable number of shards.
- Each shard can have zero or more replicas.
- Read / Search operations performed on either replica shard.
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Source: High Scalability