SGML Architectures: A More Abstract Formalism for Interchanging Structured Information

By itself, SGML offers a means whereby the syntax and semantic associated with a given information construct (an element) can be expressed: an element definition in a DTD. Whenever such an information construct must be used in a document, the corresponding DTD must allow this construct to appear there. Problems can arise, however, when it becomes necessary for whole sections of various DTDs to resemble each other. While the method of using master DTD fragments that are inserted into actual DTDs is usually workable, this method is often needlessly and cripplingly rigid. The use of architectures, rather than sets of DTD fragments, for formalizing the structures needed to permit interchange of information common to several document types, permits the architects of DTDs to retain full and optimum control of SGML's validation apparatus, while still guaranteeing information interchangeability. DTDs can be permitted to change in any way that does not violate the constraints imposed by the architecture. Each architecture can be developed in such a way that it imposes no constraints on document structure that are not actually necessary for interchange of the information with which the architecture is concerned. The DTDs themselves can then be used to impose as much further constraint on actual document instances as desired, so none of the constraining power of SGML is lost to any particular document type simply because of a need to allow the interchange of information elements whose general outline must appear in more than one type of document.