Editing XML schema (section)

=== W3C XML Schema ===
{{main|XML Schema (W3C)}}

==== Advantages over DTDs ====

Features available in XSD that are missing from DTDs include:

* Names of elements and attributes are namespace-aware
* Constraints ("simple types") can be defined for the textual content of elements and attributes, for example to specify that they are numeric or contain dates. A wide repertoire of simple types are provided as standard, and additional user-defined types can be derived from these, for example by specifying ranges of values, regular expressions, or by enumerating the permitted values.
* Facilities for defining uniqueness constraints and referential integrity are more powerful: unlike the ID and IDREF constraints in DTDs, they can be scoped to any part of a document, can be of any data type, can apply to element as well as attribute content, and can be multi-part (for example the combination of first name and last name must be unique).
* Many requirements that are traditionally handled using parameter entities in DTDs have explicit support in XSD: examples include substitution groups, which allow a single name (such as "block" or "inline") to refer to a whole class of elements; complex types, which allow the same content model to be shared (or adapted by restriction or extension) by multiple elements; and model groups and attribute groups, which allow common parts of component models to be defined in one place and reused.
* XSD 1.1 adds the ability to define arbitrary assertions (using XPath expressions) as constraints on element content.

XSD schemas are conventionally written as XML documents, so familiar editing and transformation tools can be used.

As well as validation, XSD allows XML instances to be annotated with type information (the [[PSVI|Post-Schema-Validation Infoset (PSVI)]]) which is designed to make manipulation of the XML instance easier in application programs. This may be by mapping the XSD-defined types to types in a programming language such as Java ("data binding") or by enriching the type system of XML processing languages such as XSLT and XQuery (known as "schema-awareness").

==== Commonality with RELAX NG ====
RELAX NG and W3C XML Schema allow for similar mechanisms of specificity. Both allow for a degree of modularity in their languages, including, for example, splitting the schema into multiple files. And both of them are, or can be, defined in{{Clarify|date=July 2015}} an XML language.

==== Advantages over RELAX NG ====
{{See also|XML Schema (W3C)#Criticism}}
RELAX NG does not have any analog to [[PSVI]]. Unlike W3C XML Schema, RELAX NG was designed so that validation and augmentation (adding type information and default values) are separate.

W3C XML Schema has a formal mechanism for attaching a schema to an XML document, while RELAX NG intentionally avoids such mechanisms for security and interoperability reasons.

RELAX NG has no ability to apply default attribute data to an element's list of attributes (i.e., changing the XML info set), while W3C XML Schema does. Again, this design is intentional and is to separate validation and augmentation.<ref>While annotations in RELAX NG can support default attribute values, the RELAX NG specification does not mandate that a validator provide this ability to modify an XML infoset as part of validation. The WXS specification does mandate this behavior. An additional specification associated with RELAX NG does provide this ability. See [http://www.oasis-open.org/committees/relax-ng/compatibility.html#default-value Relax NG DTD Compatibility (default value)].</ref>

W3C XML Schema has a rich "simple type" system built-in (xs:number, xs:date, etc., plus derivation of custom types), while RELAX NG has an extremely simplistic one because it is meant to use type libraries developed independently of RELAX NG, rather than grow its own. This is seen by some as a disadvantage. In practice it is common for a RELAX NG schema to use the predefined "simple types" and "restrictions" (pattern, maxLength, etc.) of W3C XML Schema.

In W3C XML Schema a specific number or range of repetitions of patterns can be expressed whereas it is practically not possible to specify at all in RELAX NG (<oneOrMore> or <zeroOrMore>).

==== Disadvantages ====

W3C XML Schema is complex and hard to learn, although that is partially because it tries to do more than mere validation (see [[PSVI]]).

Although being written in XML is an advantage, it is also a disadvantage in some ways. The W3C XML Schema language, in particular, can be quite verbose, while a DTD can be terse and relatively easily editable.

Likewise, WXS's formal mechanism for associating a document with a schema can pose a potential security problem. For WXS validators that will follow a [[URI]] to an arbitrary online location, there is the potential for reading something malicious from the other side of the stream.<ref name="clark">James Clark (co-creator of RELAX NG). [http://www.imc.org/ietf-xml-use/mail-archive/msg00217.html RELAX NG and W3C XML Schema] {{webarchive |url=https://web.archive.org/web/20070927235958/http://www.imc.org/ietf-xml-use/mail-archive/msg00217.html |date=September 27, 2007 }}</ref>

W3C XML Schema does not implement most of the DTD ability to provide data elements to a document.

Although W3C XML Schema's ability to add default attributes to elements is an advantage, it is a disadvantage in some ways as well. It means that an XML file may not be usable in the absence of its schema, even if the document would validate against that schema. In effect, all users of such an XML document must also implement the W3C XML Schema specification, thus ruling out minimalist or older XML parsers. It can also slow down the processing of the document, as the processor must potentially download and process a second XML file (the schema); however, a schema would normally then be cached, so the cost comes only on the first use.

==== Tool Support ====

WXS support exists in a number of large XML parsing packages. [[Apache Xerces|Xerces]] and the [[.NET Framework]]'s [[Base Class Library]] both provide support for WXS validation.