Xml Matters #7

Comparing W3C XML Schemas and Document Type Definitions (DTD's)


David Mertz, Ph.D.
Idempotentate, Gnosis Software, Inc.
January 2001

The "buzz" about XML Schemas is that they are the next big technology to think about within the XML universe. Specifically, a widespread sentiment exists that Schemas will soon replace DTDs as the means of specifying XML document types. In the author's opinion, much of the praise for XML Schemas is overstated, but XML Schemas are nonetheless an invaluable tool in an XML developer's arsenal. This article tries to sort out just what is going on in the XML Schema world.

Introduction

Much of the point of using XML as a data representation format is the possibility of specifying structural requirements for documents: rules for exactly what types of content and subelements may occur within elements (and in what order, cardinality, etc). In traditional SGML circles, document rules have been represented as DTD's--and indeed the formal specification of the W3C XML 1.0 Recommendation explicitly provides for DTD's. However, there are some things that DTD's cannot accomplish that are fairly common constraints; the main limitation of DTD's is the poverty of their expression of data types (you can specify that an element must contain PCDATA, but not that it must contain, e.g., a nonNegativeInteger). As a side matter, DTD's do not make the speficication of subelement cardinality easy (you can compactly specify "one or more" of a subelement, but specifying "between seven and twelve" is, while possible, excessively verbose, or even outright contorted).

In answer to some of the limitations of DTD's, some XML users have called for alternative ways of specifying document rules. It has always been possible to programmatically examine conditions in XML documents, but being able to impose the more rigid standard that a document not meeting a set of formal rules is invalid per se is often preferable. W3C XML Schemas are one major answer to these calls (but not the only Schema option out there). Steven Holzner, in Inside XML has a characterization of XML Schemas that is worth repeating:

Over time, many people have complained to the W3C about the complexity of DTDs and have asked for something simpler. W3C listened, assigned a committee to work on the problem, and came up with a solution that is much more complex than DTDs ever were (p.199)

Holzner continues--and most all XML programmers will agree (myself included)--that despite their complexity, W3C XML Schemas provide a lot of important capabilities, and are worth using for many classes of validation rules.

At least two wrinkles remain for any "Schemas everywhere" goal. That is, two fundamental and conceptual issues; at a more pragmatic level, tools for working with XML Schemas are less mature than those for working with DTD's (especially in regard to validation, which is the core issue). The first issue is that the W3C XML Schema Candidate Recommendation which just ended its review period on December 15, 2000 does not include any provision for entitites; by extension this includes parametric entities. The second issue is that despite their enhanced expressiveness, there are still many document rules that cannot be expressed in XML Schemas (some proposals have been made to utilize XSLT to enhance validation expressiveness, but other means are possible and utilized also). In other words, Schemas cannot quite do everything DTD's have long been able to, on the one hand, while on the other hand, Schemas also cannot express a whole set of further rules one might wish to impose on documents.

The whole state of XML document validation rules remains messy. Unfortunately, I am not able to prognosticate how everything will finally shake out. But in the meanwhile, let us look at some specifics of what DTD's and XML Schemas are capable of expressing.

Rich Typing

The place where W3C XML Schemas really shine is in expressing type constraints on attribute values and element contents. This is where DTD's are weakest. Beyond providing an extremely rich set of built-in simpleType's, XML Schemas allow you to derive new simpleType's using a regular-expression-like syntax. The built-ins include those you would expect if you have worked with programming languages: string, int, float, unsignedLong, byte, etc; but they also include some types most programming languages lack natively timeInstant (i.e. date/time), recurringDate (day-of-year), uriReference, language, nonNegativeInteger. For example, in a DTD one might have a declaration like:

DTD "item" Element Definition

<!ELEMENT item (prodName+,USPrice,shipDate?)
<!ATTLIST item partNum CDATA>
<!ELEMENT prodName (#PCDATA)>
<!ELEMENT USPrice (#PCDATA)>
<!ELEMENT shipDate (#PCDATA)>

In W3C XML Schema, one can be more specific (modified slightly from the W3C Schema primer):

XML Schema "item" Element Definition

<xsd:element name="item">
   <xsd:complexType>
      <xsd:sequence>
         <xsd:element name="prodName" type="xsd:string" maxOccurs="5"/>
         <xsd:element name="USPrice"  type="xsd:decimal"/>
         <xsd:element name="shipDate" type="xsd:date" minOccurs="0"/>
      </xsd:sequence>
      <xsd:attribute name="partNum" type="SKU"/>
   </xsd:complexType>
</xsd:element>

<!-- Stock Keeping Unit, a code for identifying products -->
<xsd:simpleType name="SKU">
   <xsd:restriction base="xsd:string">
      <xsd:pattern value="\d{3}-[A-Z]{2}"/>
   </xsd:restriction>
</xsd:simpleType>

Two striking, if superficial, features stand out in these element definitions. One is that the Schema is itself a well-formed XML instance with its tags using the "xsd" namespace (actually, so is the DTD, but it has only processing instructions, no content as such); the second (and consequence of the first) is that the Schema is far more verbose than the DTD.

Beyond the syntactic niceties, we can see that the Schema example does several things that are imposible with DTD's. The type of "prodName" is basically the same between the definitions. But "USPrice" and "shipDate" are specified in the Schema as types decimal and date. Considered as a text file, an XML instance with these elements contains some ASCII (or Unicode) characters inside the elements. But a validator that is Schema-aware can demand much more specific formatting of the characters inside decimal and date elements (and likewise other types). Much more interesting is the attribute "partNum" which is of a derived specialized type. The type SKU is not a built-in type, but rather a sequence of characters following the pattern given in the "SKU" declaration (specifically, it must have three digits, a dash, and two capital letters, in that order). SKU could also be used for an element type, it is just coincidence that it defines an attribute in this case.

In the DTD version of our element definition, all these interesting (and potentially rather complicated, if specialized) types must simply get called PCDATA, with no further say as to what that character data looks like (CDATA in the case of attributes).

In richly typing element/attribute values, Schemas shade subtly from describing the syntax of an XML instance to describing its semantics. Parsing purists might take issue with my characterization: built-in Schema types are defined syntactically, and patterns built on those built-in are thusly also formally syntactic. But in practical terms, when we declare that a given element must be a date, what we really want is...well...for the element to contain a date. Expressing semantic information is not a bad thing, of course; but one might argue that that is better confined to an application level as such, rather than a format declaration. After all, there are semantic features--even simple ones--that elude Schemas but might be just as important in an application as what Schemas express. For example, sure a "stock keeping unit" must look like "999-AA"; but maybe also widgets are shipped out only in baker's dozens: divisibility on an integer by 13 is not expressible in XML Schemas (and therefore widgetquantity still cannot be given the needed constraints at that level). The point here is that even with the extra capabilities of Schemas (over DTD's), one still might need to do "post-validation" at an application level to determine if an XML document is "functionally valid."

Occurence Constaints

As well as powerful type declaration, XML Schemas improve upon the DTD's ability to declare the cardinality of subelement patterns. However, DTD's always had a clumsier way of expressing every occurence constraint (cardinality) that XML Schemas can.

In DTD's, cardinality is quanitified by one of the symbols "?", "*" and "+" which specify, respectively, "zero or one," "zero or more," "one or more." That is, except for the question mark being able to say "it is there or it isn't," nothing in the DTD syntax seems to limit the number of occurences of a given pattern (whether a single subtag, or a nested sequence of them). So expressing the 1-5 occurences of prodName in the above example Schema seems to be a problem. Likewise, without having the XML Schema attribute "minOccurs", we do not seem to be able to express the requirement that something occurs some specific number of times (other than "at least once"). But actually, DTD's minimum quantifiers are good enough, if inelegant at times. The following constraints are equivalent:

XML Schema syntax for "seven to twelve" donuts

<xsd:element name="donutorder">
   <xsd:complexType>
      <xsd:sequence>
         <xsd:element name="donut" type="xsd:string"
                      minOccurs="7" maxOccurs="12" />
      </xsd:sequence>
   </xsd:complexType>
</xsd:element>


<!ELEMENT donut (#PCDATA)>
<!ELEMENT donutorder
          (donut,donut,donut,donut,donut,donut,donut,
           donut?,donut?,donut?,donut?,donut?)

Of course, if you get orders by the gross, DTD's start to look really inelegant!

Enumeration

Both DTD's and W3C XML Schemas allow use of enumerated types in attributes. But Schemas are a great improvement in also allowing enumerated types in element contents. The lack of those, in my opinion, is a genuine shortcoming of DTD's. Furthermore, Schemas' approach to enumeration is general and elegant. A specialized simpleType can contain an enumeration "facet." And such a simpleType is automatically suitable for either an attribute or element value type.

Let us illustrate each syntax:

XML Schema syntax for enumerated attribute

<xsd:simpleType name="shoe_color">
   <xsd:restriction base="xsd:string">
      <xsd:enumeration value="red"/>
      <xsd:enumeration value="green"/>
      <xsd:enumeration value="blue"/>
      <xsd:enumeration value="yellow"/>
   </xsd:restriction>
</xsd:simpleType>
<xsd:element name="person" type="person_type">
   <xsd:attribute name="shoes" type="shoe_color"/>
</xsd:element>


<!ATTLIST person shoes (red | green | blue | yellow)>

The DTD attribute declaration appears just as good (maybe better in its conciseness). But if your model puts shoe_color in an element content instead, the DTD falls flat:

XML Schema syntax for enumerated element

<xsd:element name="shoes" type="shoe_color">


Whither

W3C XML Schemas let XML programmers express a new set of declarative constraints on documents that DTD's are insufficient for. For many programmers, the use of XML instance syntax in Schemas also brings a greater measure of consistency to different parts of XML work (other disagree, of course). Schemas are certainly destined to grow in significance and scope as they become more familiar, and more tools are enhanced to work with XML Schemas.

One way to get a jump start on Schema work is to automate conversion of existing DTD's to XML Schema format. Obviously, automated conversions cannot add the new expressive capabilities of XML Schemas themselves; but automation can create good templates from which to specify the specific typing constraints one wishes to impose. The Resources section provides two links to automated DTD-to-Schema conversion tools.

Resources

The W3C Candidate Recommendation 24 October 2000 is the basic standard for W3C XML Schemas:

http://www.w3.org/TR/xmlschema-0/

The Extensible Markup Language (XML) 1.0 (Second Edition) W3C Recommendation 6 October 2000 can be found at:

http://www.w3.org/TR/REC-xml

To keep matters sufficiently complicated, the W3C's XML Schemas are not the only Schema options out there. RELAX (Regular Expression Language for XML) is now ISO/IEC DIS (Draft International Standard) 22250-1. This standard is most widely used in Japan, but is not language or culture specific. A good starting place is:

http://www.xml.gr.jp/relax/

The XML Schema Specification in Context is a nice compact summary of the comparative capabilities of W3C XML Schemas (compared to a number of other descriptive formats). Find it at:

http://www.ascc.net/~ricko/XMLSchemaInContext.html

Yuichi Koike's conversion tool from DTDs to XML Schemas can be found at the following link. It requires Perl:

http://www.w3.org/2000/04/schema_hack/

The author has created his own Python-based tool for converting DTDs to W3C XML Schemas. Once stable, this tool should be usable as a module within larger Python programs, as well as standalone. At the time of writing, consider this alpha level code, however:

http://gnosis.cx/download/dtd2schema.py

A nice thick, informative--but perhaps somewhat rambling-- introduction to most all matters XML is Inside XML, Steven Holzner, New Riders, 2001 (ISBN 0-7357-1020-1). This column excerpts a particular pithy and humorous sentence.

About The Author

Picture of Author David Mertz, in his gnomist aspirations, wishes he had coined the observation that the great thing about standards is that there are so many to choose from. But then, he is also fuzzy on OS design. David may be reached at mertz@gnosis.cx; his life pored over athttp://gnosis.cx/publish/. Suggestions and recommendations on this, past, or future, columns are welcomed.