Storage Techniques and Mapping Schemas for XML

As an increasing amount of XML data is being exchanged between Web applications, reliable XML storage systems that make this data persistent and process it efficiently are becoming necessary. The flexibility of representation offered by XML raises challenging issues when storing XML data in relational or object systems. Major database vendors: IBM, Microsoft and Oracle provide a declarative mapping schema in which database administrators (DBAs) can express how to map XML data into their system. One of the main benefits of using a mapping schema is to make mappings transparent to DBAs and thus, help them modify and combine mappings when tuning an XML storage system. Unfortunately, existing mapping schemas for XML storage are proprietary and can be used for only one storage backend. We developed a new mapping schema that identifies orthogonal aspects of mapping XML into relations and provides a powerful tool in which most existing research and commercial solutions for storing XML can be expressed. If mapping information is made accessible, optimizing and reusing applications on top of XML stores will be easier. In particular, an application such as XML data exchange would greatly benefit from the knowledge of how XML data is stored in the underlying systems. We designed a mapping interface that provides a simple way of querying mappings expressed in our schema. This paper presents an overview of storage techniques and mapping schemas for XML and discusses related open issues.