Crawling programs for wrapper-based applications

Many large web sites provide pages containing highly valuable data. In order to extract data from these pages several methods and techniques have been developed to generate web wrappers, that is, programs that convert into a structured format the data embedded into HTML pages. These techniques easy the burden of writing applications that make reuse of data from the web. However the generation of wrappers is just one of the ingredients needed to the development of such applications. A necessary yet underestimated task is that of developing programs for driving a crawler towards the pages that contain the target data. We present a method and an associated tool to support this activity. Our method relies on a data model whose constructs allows a designer to define an intensional description of the organization of data in a web site. Based on the model, we introduce the concepts of (i) intensional navigation, which represents an abstract description of the navigation to be performed to reach pages of interest, and (ii) extensional navigation, which represents the actual set of navigation paths (i.e. sequences of links to be followed) that lead the target pages. The method is supported by a tool that infers an intensional navigation, i.e. the crawling program, from one sample extensional navigation. The tool, which has been developed as a Firefox plug-in, supports the designer in the task of defining and verifying the sample navigation and the inferred crawling program.