OrientDB is a NoSQL DBMS which can store 150,000 documents per second on common hardware. Even with a document-based database, the relationships are managed as in graph databases, with direct connections among records. You can traverse entire or parts of trees and graphs of records in a few milliseconds. It supports schema-less, schema-full, and schema-mixed modes, has a strong security profiling system based on users and roles, and supports SQL between the query languages. Thanks to the SQL layer, it's straightforward to use for people skilled in the relational world.
Orient Key/Value Server is based on the Document Database technology and is accessible as an embedded repository via Java APIs or via HTTP using a RESTful API. It uses a new algorithm called RB+Tree, derived from the Red-Black Tree to maintain tree balance, and from the B+Tree storing the links to records in pages to optimize memory consumption and loading time. Orient Key/Value Server scales out very well in a cluster with thousands of running machines: Orient will divide the load among all the nodes. Clustering, by default, works in auto-discovery mode: when a node starts, it attaches itself to the cluster if one is available. When a node goes down, the cluster automatically rebalances itself.