Big Sack
Big Sack: A lightweight Java Key/Value store with undo and disk cache.
...It has the convenience of Java SDK TreeMap and TreeSet classes and is used the same easy way, but it includes rollback through undo logging to checkpoint data so it does not wind up in an unknown state regardless of failures. Data storage in the exabyte range is possible using filesystem and/or memory-mapped IO. Three levels of configurable write-through caching at different granularities ensure performance.