How and When to Create Your Own XML Document Type Definitions ( DTD )

XML Document Type Definitions (DTD’s) already exist for almost every imaginable sort of information, including newspaper articles, transcriptions of medieval plays, press releases, and bird-watching trip reports.

You should look closely at these existing DTDs. Even if you don’t find one that is suitable for your own use, you can choose to use part of an existing XML Document Type Definitions (DTD’s) and build on it, or to learn form its mistakes and move on.

There are a number of compelling reasons for using an industry standard DTD:

· Other people have already solved difficult problems, for example How to represent cross-references to a bibliography or how to mark up a purchase order

· Other people in your industry may already be familiar with the standard DTDs, so hiring and training become easier

· You can exchange information with other people; that is, you can make your documents available, and you can also use the documents of others

· A program written to work with one DTD might or might not work with documents using a different DTD. General XML tools work with any DTD, but applications using XNL may have specific requirements. When you write your own DTD, you may make it more difficult to use other people’s software. Look for available software that depends on a relevant DTD or a DTD fragment such as OASIS tables or MathML

· If you need style sheets for printing your XML documents or software to convert the documents to another format such as HTML or RTF, you may well find available free documents that you could use immediately if you used the appropriate DTD. For example, a lot of software uses the DocBook DTD, a popular document-centric DTD

· Off-the-shelf XML tools such as editors, formatters and even databases may support industry-standard DTDs

If you decide not to create your own DTD, you should still follow the steps outlined in the “Process People” to ensure that everyone involved agrees to use the DTD you select.

If you do create a DTD of your own, consider making it public so that others can build on your work and perhaps even contribute software or ideas. You can get a lot of publicity and visibility for your organization within your industry in this way and make valuable contacts, all for relatively small cost. The experiences of others can also help you avoid costly mistakes.

The Building Blocks

A DTD consists of two parts. The first part contains a set of declarations of elements, entitles m and so forth, and it is called the document type declaration. The second part of a DTD includes documentation and conventions that specify how your documents and processed. Documentation is an integral part of the markup process.

The following are the most important declarations that you will use in writing a document type declaration:

Element Declaration
General Entity Declaration
Comments
Conditional Sections
Parameter Entitles

Processes and People

Designing a DTD isn’t just a matter of syntax. It’s about getting a group of people to agree to use a particular markup for their documents so that they can be more productive. Make clear that people that once you finish the design of your document architecture, changes may be expensive.

One way to do this is with a formal change and request procedure, which can also become a record of why each change was made and for whom. Correcting design defects in a DTD is cheap before the DTD is finished. If you find errors after you create documents that use it, you may need to go back and correct the documents.

If you share the DTD and documents with other departments or organizations, you will need a formal change request producer, and every one involved will have to update their copies that differ from other industry-standard XML Document Type Definitions (DTD’s)

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