This chapter is from the book
This chapter is from the book
This chapter is from the book
The Importance of Document Validation
Before digging deeper into DTDs, I’d like to address a topic that has been already been touched on a few times: document validation. I’ve already mentioned that the primary purpose of schemas is to provide a means of validating XML documents. What I didn’t point out is that an XML document doesn’t necessarily have to be valid in order to be useful. So, there is a little more to the document validation story than I’ve let on.
In addition to document validity, XML establishes the concept of a well-formed document, which is a document that meets all of the general language rules of XML. A well-formed document follows the five XML commandments you learned about in the previous hour, but it doesn’t necessarily qualify as being a valid document. The reason is because a well-formed document doesn’t have to be associated with a schema. Looking at XML documents in this light, it becomes apparent that there are two different degrees of "correctness" with respect to XML documents. The first degree is met by well-formed documents, which must meet the strict language requirements of XML. Taking things a step further leads us to valid documents, which are well-formed documents that also adhere to a schema of some sort (DTD or XSD, for example).
The terms valid and well-formed are extremely important to XML and are used throughout the remainder of the book. Whether or not a document needs to be valid is up to the particular application in which it is being used. However, all XML documents must be well-formed or you will undoubtedly encounter errors when the document is processed. To help keep valid and well-formed documents clear, remember that well-formed documents must adhere to the general XML rules, whereas valid documents must also adhere to a schema. You learn how to use DTDs and XSDs to validate XML documents in Hour 8.