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#D2 Persistence A Document-Based Object Storage System for Java

##Overview

The D2 object storage system is a new persistence tool for the Java platform. D2 differs from most other Java persistence mechanisms in that it does not use object-relational mapping. Instead of relying on a traditional relational database, D2 can use any key-value storage system. Modern key-value storage systems, often referred to as a “NoSQL” databases, are often high performance and very scalable. Locating a stored objects is done using an pluggable indexing system, which does not need to be directly related to the storage system. In contrast with object oriented databases , D2 does not store each instance of each object as a separate node in the stored object graph. Instead, objects wholly-owned by the parent object are assumed to be part of, and therefore stored with, the parent object. D2 automatically maintains the many-to-many relationships between root-level object instances. In some ways, then, D2 utilizes “object-document” mapping, where a tree of objects is serialized into a single document, and multiple documents are linked together to form the full object graph.

##Contents

• The Genesis of D2
• Goals
• Concepts
• Sample Code
• Limitations
• Fixes Needed
• Roadmap