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The next generation database--XDB: combining XML''s document flexibility with SQL''s data integrity (XML & Databases).

Article Excerpt
Imagine the Internet as an ocean and yourself on the beach gazing out on the horizon. If you look far enough, you'll see the crest of a big wave forming. You can see that it's beginning to take shape and that its impact will be major. It will affect all that lies in its path. What is this next big wave? You've probably already guessed: XDB.

Application processing will change dramatically with the ability to exchange information transparently. With the advent of Web services, applications are now free to communicate in a common format--that of an XML document--anywhere on the Web. Where the Web was once built on static content linked together via hypertext, XML takes it to the next level. Instead of users surfing the Internet via HTML pages linked with hyperlinks, we can now build Web-based applications that can be linked via XML documents. Imagine a user clicking on a link to a Web site. This in turn fires off an exchange of an XML document to another application.

Here's the key: the XML document. This will be the primary means of information exchange and message passing. With the need to process XML documents comes the need to be able to store, retrieve, and report on them. Hence the need for a management system to handle the flood of XML documents that an application will process. This is where an XML database, XDB, comes in.

What Is an XDB?

So what is an XML database, or an XDB? In this article I define what it is, when and why you will need to use one, and what impact it will have on the business world. By the time you finish reading, you just may realize the importance of an XDB and will want to grab your surfboard to ride the next big wave.

Before I define what an XDB is, let's briefly review the evolution of databases. We can consider the first generation of databases as file-based systems. VSAM, ISAM, Btrieve, and dbase--examples of file-based databases--store data in a file and use various types of index methods to retrieve and update it. The second generation of databases is relational database management systems, or RDBMSs. Oracle, SQL Server, and Sybase are examples. They use a common language, SQL, to access the data stored in normalized tables composed of rows and columns. Now, with the emergence of XML, the need to quickly store, retrieve, and update XML documents in real time is imperative. The next generation of databases, XDBs, will need to excel at providing this capability.

We can define an XDB as a database management system that stores, updates, and retrieves XML documents. It uses as its fundamental unit of storage an XML document. Just as the first generation uses a file (or table) as its base unit, and the second generation, RDBMSs, uses a row in a table as its base unit of storage, in an XDB all operations function at the XML document level.

In addition to using the XML document as its base unit, an XDB provides a set of tools to manage the XML documents. These tools conform to a logical data model that abstracts the logical structures from the physical structures. Four major data model specifications are currently proposed by the W3C:

1. Document Object Model (DOM) 1.0 Level 3

2. XML Information Set model

3. XPath 1.0 data model

4. XQuery 1.0 and XPath 2.0 data model

Types of XDBs

There are no requirements for how an XDB is expected to physically store XML documents. Some XDBs are built on an object database, others might use compressed files with an indexing scheme, and still others might be built on top of a relational database. At this time XDBs can be classified into two basic types (with a third type on the horizon): native and XML enabled.

Natiue XML database

A native XML database (NXDB) is simply one that was designed from the ground up to store XML documents. It might make use of a preexisting technology such as object-oriented data storage techniques, but its mission is to store, retrieve, and update XML documents....

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