- High Scalability

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.
Multi Tenant with Multi Types.
- Support for more than one index.
- Support for more than one type per index.
- Index level configuration (number of shards, index storage, ...).
Various set of APIs.
- HTTP RESTful API.
- Native Java API.
- All APIs perform automatic node operation rerouting.
Document oriented.
- No need for upfront schema definition.
- Schema can be defined per type for customization of the indexing process.
Reliable, Asynchronous Write Behind for long term persistency.
(Near) Real Time Search.
Built on top of Lucene.
- Each shard is a fully functional Lucene index.
- All the power of Lucene easily exposed through simple configuration / plugins.
Per operation consistency.
- Single document level operations are atomic, consistent, isolated and durable.
Open Source under Apache 2 License.

Check out the complete overview, download it, read the docs, and watch/fork it. Follow @elasticsearch for news and updates.