TomP2P is a P2P-based high performance key-value pair storage library. Each peer has a table (either disk-based or memory-based) to store its values. A single value can be queried or updated with a secondary key. The underlying communication framework uses Java NIO to handle many concurrent connections.
Sirix is a versioned tree-based storage system. It provides Snapshot Isolation through MVCC and is best suited for flash-based storage. The tree encoding is suited for heavy update operations. It provides diffing algorithms either to import diffs between currently shredded (imported) XML documents to update a tree structure or to get the differences between versions of a tree structure. A GUI provides several visualizations for comparing imported tree structures.