The database market is facing a radical shift in emphasis with the introduction of XML and the requirement to maintain persistent XML content. XML is becoming all-pervasive, and although much of the early work with XML focused on the transactional nature and possibilities of XML (the idea that XML would become the EDI-killer), there is now recognition that the possibilities go much further than originally imagined.
The amount of XML content that needs to be persisted is growing and systems need to be put in place that can handle the documents at a higher level of granularity than is provided by relational systems. Typically the storage of persistent XML documents has fallen within the domain of content management systems, but the market is extending to take into account a different level of granularity that can be obtained from a single XML document.
Databases are collections of data, and the distinction between the individual data elements and the collection and storage facilities for multiple data elements is an important one. Data elements are created in one of two forms, and their use or purpose also takes one of two forms.
Data elements can be created as a single entity that exists by and of itself; they can also be created by decomposing documents that contain individual data elements. Similarly their purpose can be as a single entity or as a collection of inter-related elements that form an informational document.
Data elements can be re-ordered and re-purposed to provide different information. The data elements that exist within, for example, an XML document can have a life and a purpose outside of that specific document. This purpose can only be realised if the physical implementation that is used to store the documents allows it to take place, or does not constrain that extended use.