JanusGraph is a scalable graph database optimized for storing and querying graphs containing hundreds of billions of vertices and edges distributed across a multi-machine cluster. JanusGraph is a transactional database that can support thousands of concurrent users executing complex graph traversals in real time.

In addition, JanusGraph provides the following features:

Elastic and linear scalability for a growing data and user base.
Data distribution and replication for performance and fault tolerance.
Multi-datacenter high availability and hot backups.
Support for ACID and eventual consistency.
Support for various storage backends:
Apache Cassandra
Apache HBase
Oracle BerkeleyDB
Support for global graph data analytics, reporting, and ETL through integration with big data platforms:
Apache Spark
Apache Giraph
Apache Hadoop
Support for geo, numeric range, and full-text search via:
ElasticSearch
Solr
Lucene
Native integration with the TinkerPop graph stack:
Gremlin graph query language
Gremlin graph server
Gremlin applications
Open source under the Apache 2 license.

https://github.com/JanusGraph/janusgraph/
http://janusgraph.org/