Titan is a research open source tool
that is using various NoSQL platforms for storage(i.e. Casandra, HBase, Hadoop)and "Gremlin" graph traversal language.
ThinkAurelius consulting that are developers of Titan
One To Watch: Aurelius's distributed graph database, Titan
With all those good efforts, there are not standardized "service interfaces"
for distributed data, that would preserve semantics and enable flexibility.
Idea of Semantic Web Services, preferably much simpler and with a better name, not realized yet...
Neo4j graph database appears to have a good business momentum,
with recently released version 2.0
On the other side, with ever improving computing hardware, in particular large memory and SSDs, make even "classic" SQL databases run faster when data are in-memory... (Oracle 12, SQL Server 2014)
With all those good efforts, there are not standardized "service interfaces"
for distributed data, that would preserve semantics and enable flexibility.
Idea of Semantic Web Services, preferably much simpler and with a better name, not realized yet...