Feature Grammars

We propose a grammatical view of the problem of integrating different data items under a database perspective. We introduce a variant of context-free grammars, called feature grammars, whose parsers may rewrite their input stream. This allows us to provide a simple mechanism for describing and maintaining indexes to Internet multimedia documents. Integration of parser instances as mediators into a database system provides a remarkably transparent framework for indexing external sources and also facilitates the use of plug-in modules provided by third parties. Rewriting the input stream allows to (1) interpret input data and replace them by their interpretations, and (2) integrate data from different sources by linking them into the input stream in the spirit of a structuring schema. The techniques described are used in the Dutch Acoi project.