Chapter 8 - Storing: XML and Databases

Abstract

This chapter explores the various facilities through which Extensible Markup Language (XML) data can be stored persistently and illustrates the resulting implications on querying such persistent XML. It explains pros and cons of using database technology vs. ordinary file systems for storing and querying XML documents, and reviews shredding as a mechanism for storing XML documents into ordinary relational (or, indeed, other sorts of) databases. Several types of database management systems are in wide use by enterprises of all sorts, but only three are most common ones are used to store and manage XML data: relational, object-oriented, and pure XML. All of these types of database provide the ability not only to store and retrieve XML documents but also to search that data through the use of query languages of some sort. Relational database management systems (RDBMSs) have been on the scene since the early 1980s and have arguably become the most widely used form of DBMS. Relational database systems such as Oracle's Oracle database, IBM's DB2, and Microsoft's SQL Server are tremendously scalable, often able to handle thousands of concurrent users accessing many terabytes of data. Unlike the RDBMS products, OODBMS products suffered from not having a formal data model on which their design was based. The Chapter also examines the SQL standard's new built-in XML type, its relationship to shredding, and the implications on the APIs that application programs use to access Structured Query Language (SQL) database management systems. Finally, the chapter highlights the nature of streaming XML, its uses, and the difficulties rose when querying such non-persistent XML data. When the XML data is inherently unsuitable for storing, the queries against streaming XML is desired; therefore, most of the applications are better served by storing XML in some persistent medium and then by querying that persistent XML data.