CloudKit provides schema-free, auto-versioned, RESTful JSON storage with optional OpenID and OAuth support, including OAuth Discovery.
Quick Start: http://code.google.com/p/persevere-framework/w/list
The Apache Jackrabbit™ content repository is a fully conforming implementation of the Content Repository for Java Technology API (JCR, specified in JSR 170 and 283). A content repository is a hierarchical content store with support for structured and unstructured content, full text search, versioning, transactions, observation, and more.
ConclusionDocument databases store and retrieve documents and basic atomic stored unit is a document. As always your requirement leads into the decision. You need to think about your data-access patterns / use-cases to create a smart document-model. When your domain model can be split and partitioned across some documents, a document-database will be a suitable one for you. For example for a blog-software, a CMS or a wiki-software a document-db works extremely well. But at the same time a non-relational database is not better than a relational one in some cases where your database have a lot of relations and normalization.
Just check the following link from stackoverflow also to cover the pros/cons of Relational Vs Document based databases.
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