A Web Odyssey: from Codd to XML

The Web presents the database area with vast opportunities and commensurate challenges. Databases and the Web are organically connected at many lev els. Web sites are increasingly pow ered b y databases.Collections of linked Web pages distributed across the Internet are themselves tempting targets for a database. The emergence of XML as the lingua franc a of the Web brings some m uchneeded order and will greatly facilitate the use of database techniques to manage Web information. This paper will discuss some of the developments related to the Web from the viewpoint of database theory. As we shall see, the Web scenario requires revisiting some of the basic assumptions of the area. T o be sure, database theory remains as valid as ev er in the classical setting, and the database industry will continue to represent a multi-billion dollar target of applicability for the foreseeable future. But the Web represents an opportunity of an entirely di erent scale. We are th us at an important juncture. Database theory could retain its classical focus and turn in w ards.Or, it could attempt to take heads-on the challenge of the Web and contribute to an important part of its formal foundations. T o do so, it will have to leave its familiar shores and reinvent itself. There are good signs that the journey has already begun. What makes theWeb scenario di erent from classical databases? In short, everything. A classical database is a coheren tly designed system. The system imposes rigid structure, and provides queries, updates, as well as transactions, concurrency, integrity, and recovery, in a con trolled environment. The Web escapes any suc h con trol.It is a freeevolving, ever-changing collection of data sources of various