In Java, data structures such as Maps and Sets are very useful, but when dealing with large amounts of data there are practical limits on memory size. Oftentimes you want to persist this data to disk or database. Key/value stores are ubiquitous and there are many implemtations, but I haven't found any that implement the Map/Set interfaces. I come from a Perl background and the lack of a simple tie-like interface for hashes was driving me crazy. To address this, Pickle provides this capability with implementations of Map and Set, using persistent key/value stores as underlying engines. Any Serializable objects can be used as keys and values; Kryo is used for serialization.

Current implementations:

• Accumulo (http://accumulo.apache.org/)
• LevelDB (http://code.google.com/p/leveldb/)
• Cdb (http://cr.yp.to/cdb.html)
• SQLite (http://www.sqlite.org/)
• MapDB (http://www.mapdb.org/)

Planned implementations:

• Berkeley DB (http://www.oracle.com/technetwork/products/berkeleydb/)
• JDBM (http://jdbm.sourceforge.net/)

Other potential implementations:

• Voldemort (http://www.project-voldemort.com/)
• MongoDB (http://www.mongodb.org/)
• ... (other suggestions welcome)

Planned features:

• BTree implementation for navigable maps/sets
• Generic Pickle set leveraging Pickle map
• PickleException
• Compression