XML in a Nutshell, 3rd Edition

Maintaining Compatibility

Maintaining backward compatibility with existing documents and processing software is a primary concern for XML applications that are widely used by diverse audiences. Standards organizations face formidable difficulties when updating a popular application (such as HTML). While few applications will become as widespread as HTML, some thought should be given in advance to how new versions of a schema or DTD will interact with existing documents.

One possible, although problematic, approach to maintaining backward compatibility is to create a new, distinct namespace that will be used to mark new element declarations or perhaps to change the namespace of the entire document to reflect a substantially changed version. This has substantial costs, however, and generally makes sense only when the new functionality is itself a separate vocabulary. Working with documents that have parts written in different namespace-indicated versions is a tough problem for developers.

A better strategy is only to extend existing applications without removing prior functionality. In this case, it is a good idea to ensure that each instance document for an application has some readily identifiable marker that associates it with a particular version of a DTD or schema. The good news is that the highly transformable nature of XML makes it very easy to migrate old documents to new document formats.

Removing functionality is possible, but frequently difficult, once a format is widely used. ...