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MongoDB is an open-source document database that provides high performance, high availability, and automatic scaling. MongoDB obviates the need for an Object Relational Mapping (ORM) to facilitate development. 
A record in MongoDB is a document, which is a data structure composed of field and value pairs. MongoDB documents are similar to JSON objects. The values of fields may include other documents, arrays, and arrays of documents.

The Xillio Unified Datamodel uses MongoDB as its database. 

To read more about how to use MongoDB, read the Getting started with MongoDB article on mongodb.org.

It is important to realize that MongoDB stores a working set (which is the data you are most working with). This working set is loaded into the RAM of themachine you are working on. This is no problem as long as your RAM is big enough to store the entire data in. When it is not big enough, MongoDB works really slow because of the overhead.

Due to this slow processing of MongoDB the following error could occur: "MongoCursorNotFoundException in huge Mongo.find loop". The default TimeOut is set to 10 minutes. It is possible to change this so you can give MongoDB some extra processing time. The following statement can be used to change the TimeOut. "--setParameter cursorTimeoutMillis=1800000".

For more information you can also go to: https://jira.mongodb.org/browse/SERVER-8188