XML is increasingly used by business partners in application-to-application (intraenterprise) and business-to-business (interenterprise) application integration projects to establish agreements on the structure of content (see Note 1). Part of the reason for this trend is XML's unique value as a portable data standard. XML-based agreements define business data such as purchase orders and securities transactions. The most common way to express such agreements in XML is by creating a DTD, Document Type Definition, which is part of the XML-based specifications (see Research Note T-07-9653, XML Specifications Proliferate — Hierarchically).

DTDs — The Upside. Driven by XML's adoption (see Research Note QA-08-0150, XML Will Ultimately Pay for Itself in Application Integration), DTDs are already widely used to express agreements formally on the format of XML-defined content (see Note 2). For example, the FIX technical committee, hosted by major securities traders (e.g., Fidelity, Goldman Sachs) has redefined its FIX protocol using XML (i.e., FIXML).

Enterprises and industry groups use DTDs to express agreements on data structures for two key reasons: they are relatively easy to read, define and understand; and they are based on the increasingly accepted XML specifications. DTDs cleanly separate the agreement on structure from instances that conform to that structure. In the typical scenario, a DTD is created, posted on a Web site for general availability and referenced from within XML documents. XML parsers can obtain the DTD reference from within XML documents and access it to validate content. This technique enables any application — anywhere — to receive and process XML content and parse it against a common, shared DTD.

DTDs — The Downside. DTDs suffer from severe limitations in semantic expressive ability. DTDs have no standard way of expressing even primitive data types (e.g., text, numeric) or value constraints (e.g., "income less than $50,000"). These limitations are typically addressed via nonstandard XML extensions, external documentation or software libraries to enforce constraints. Thus, because XML parsers cannot do full data validation, and because of the negative impact of validation on performance, enterprises rarely use the XML parser validating feature in application integration projects. This also renders most DTDs incomplete and, in some cases, incompatible with likely future XML specifications (e.g., XML Schema). Despite their severe technical limitations, DTDs are the de facto standard method for using XML to express agreements on the structure of content.

XML Schema — A Better DTD. To address DTDs' limitations, the W3C XML Schema (see Note 3) working group has been considering a variety of proposals (e.g., XML-Data, DCD) that extend DTDs' expressive ability. W3C's release of a working draft of XML Schema is a significant step toward "closing the loophole" in DTDs' limited form of expression, and potentially enabling validation to occur at the parser level rather than in custom application logic. Moreover, there is now a movement within the W3C to merge the RDF Schema and XML Schema specifications — further consolidation. XML Schema, a superset of DTD, will be a W3C recommendation in 1H00 (0.8 probability).

Until the XML Schema is approved, enterprises should use DTDs. However, there is no standard way in DTDs to express data types and value constraints. To work around this, enterprises should use XML comments (see Note 4), external documentation or software library routines to enforce constraints. Enterprises should avoid using proprietary XML extensions to express such constraints since these will likely be incompatible with specifications such as XML Schema. After XML Schema becomes a W3C recommendation, enterprises should consider using that instead of DTDs, particularly to take advantage of XML Schema's increased expressive abilities (e.g., for standard XML data types and value constraints).

Bottom Line: Despite their severely limited expressive abilities, DTDs are the de facto standard for enterprises to use to establish agreements on the structure of XML-based content. To address limitations, DTDs should be used in conjunction with XML comments, external documentation or application logic. Once XML Schema is an approved specification, it should be used instead of DTDs.

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