Mediators and Wrappers

Abstract

Search engines and directories, as described in Chapter 1, provide Internet users with rapid retrieval of information, but do not provide a database-like query language to retrieve information based, for example, on the underlying structure of the HTML documents. The lack of such query languages is due largely to the semistructured nature of Web data [1], which is unsuitable for retrieval and storage in a relational database form. Systems based on mediators and data warehouses have been introduced to overcome the inability to query Web data using a full-fledged database query language. In contrast to the approaches described in Chapter 2, which employ search engines as backends, mediators and data warehouses are based on a database management system (DBMS). Thus, the query languages in Chapter 2 would be implemented using search engines and directories as backends. Such a query system would generate, from the user’s query, a query or a set of queries that can be executed on the search engines. The responses returned from the search engines would then be compiled and supplied to the user. In the mediator or data warehouse approach, on the other hand, the user interacts with the DBMS, which in turn interacts with the Web.