XML in a Nutshell, 3rd Edition

Choosing a Validation Method

The first major implementation decision when designing a new XML application is what type of validation (if any) will be performed on instance documents. In many cases, prototyping a set of instance documents is the best way to determine what level of validation must be performed.

If your application is simply saving some internal program state between invocations (such as window positions or menu configurations within a GUI application), the structure is fixed by the program logic itself. Even though these configuration documents will always be written and read by the same program, writing a schema and validating documents on input can detect file corruption, not to mention bugs in the software itself. All too often we’ve watched our computers crash because various software (most often Microsoft Word) went down in flames when it encountered content in a document it had assumed could not possibly be present (most recently while working on Chapter 27 of this book). Validation may be a key defense against such attacks, intentional or otherwise.

Validation is even more important when XML documents are exchanged between different related systems that are not maintained by the same development organization. In this case, a DTD or schema can serve as a definitive blueprint to ensure that all systems are sending and receiving information in the expected formats. If something does go wrong and one process begins rejecting the other processes’ inputs, validation ...