FlockDB
Twitter uses FlockDB to store social graphs, such as who follows whom, as
well as for some secondary indexes. It is free and open source, using the
Apache 2.0 license. It is simpler than other graph databases as it seems to try
to solve fewer problems and is designed for one primary use. FlockDB is
designed for online, low-latency, high-throughput environments such as
websites like Twitter; even then, it’s only for storing specific types of data.
References
http://www.oracle.com/database/berkeley-db/db.html—The main Berkeley
DB website
http://cassandra.apache.org—The main website for Cassandra
http://www.scylladb.com—The main website for Scylla
http://www.memcached.org—The main website for Memcached
http://memcachedb.org—The main website for MemcacheDB
https://coreos.com/etcd/—The main website for etcd
http://redis.io—The main website for Redis
http://basho.com/products/—The main website for Riak
http://couchdb.apache.org—The main website for CouchDB
http://mongodb.org—The main website for MongoDB
http://basex.org—The main website for BaseX
http://research.google.com/archive/bigtable.html—A paper describing
BigTable
http://research.google.com/archive/mapreduce.html—A paper
describing MapReduce
http://hadoop.apache.org—The main website for Hadoop
http://hbase.apache.org—The main website for HBase, the Hadoop
database
http://research.google.com/archive/gfs.html—The paper describing
Google File System
http://neo4j.com—The main website for Neo4j
http://orientdb.org—The main website for OrientDB