Status of this document

This is a W3C Note published on 15 February 1999 as a deliverable of the XML Schema Working Group, which is part of the W3C XML Activity. It lists a base set of agreed requirements for an XML schema language.

This document represents a compromise that leaves many design questions open, creating opportunity for decision-making in the design phase. As the XML Schema work continues, the concrete implications of these requirements for the design will be worked out and documented. This document is a living document; it will be reviewed regularly by the Working Group, and may be revised to reflect changes in the Working Group's understanding. The Working Group does not anticipate substantial changes, but may decide to refine existing requirements or add new ones.

Comments about this document should be addressed to the XML Schema Requirements Comments list at www-xml-schema-comments@w3.org. Comments accumulated by 1 March 1999 will be reviewed in March 1999.

A list of current W3C technical reports and publications, including working drafts and notes, can be found at http://www.w3.org/TR.

Abstract

This document specifies the purpose, basic usage scenarios, design principles, and base requirements for an XML schema language.

Table of Contents

1. Overview
2. Purpose
3. Scenarios
4. Principles
5. Requirements
   • Structural schemas
   • Datatypes
   • Conformance

1. Overview

The XML 1.0 specification defines the concepts of well-formedness and validity; it is very simple to check a document for well-formedness, while validation requires more work but allows the user to define more powerful constraints on document structure. XML validity requires that a document follow the constraints expressed in its document type definition, which provides the rough equivalent of a context-free grammar for a document type.

For some uses, applications may need definitions of markup constructs more informative, or constraints on document structure tighter than, looser than, or simply different from those which can be expressed using document type definitions as defined in XML 1.0. There is also a widespread desire to allow markup constructs and constraints to be specified in an XML-based syntax, in order to allow tools for XML documents to be used on the specifications.

By charter, the XML Schema Working Group is assigned to address the following issues:

structural schemas

a mechanism somewhat analogous to DTDs for constraining document structure (order, occurrence of elements, attributes). Specific goals beyond DTD functionality are

• integration with namespaces
• definition of incomplete constraints on the content of an element type
• integration of structural schemas with primitive data types
• inheritance: Existing mechanisms use content models to specify part-of relations. But they only specify kind-of relations implicitly or informally. Making kind-of relations explicit would make both understanding and maintenance easier

primitive data typing

integers, dates, and the like, based on experience with SQL, Java primitives, etc.; byte sequences ("binary data") also need to be considered

conformance

The relation of schemata to XML document instances, and obligations on schema-aware processors, must be defined. The Working Group will define a process for checking to see that the constraints expressed in a schema are obeyed in a document (schema-validation); the relationship between schema-validity and validity as defined in XML 1.0 will be defined.

The XML Schema work is interdependent with several other areas of W3C activity. These are listed below under Design Principles.

In the design of any language, trade-offs in the solution space are necessary. The following design principles should guide the working group in making these trade-offs. Design principles are desirable, but not fully measurable, characteristics.

The XML schema language shall be:

1. more expressive than XML DTDs;
2. expressed in XML;
3. self-describing;
4. usable by a wide variety of applications that employ XML;
5. straightforwardly usable on the Internet;
6. optimized for interoperability;
7. simple enough to implement with modest design and runtime resources;
8. coordinated with relevant W3C specs (XML Information Set, Links, Namespaces, Pointers, Style and Syntax, as well as DOM, HTML, and RDF Schema).

The XML schema language specification shall:

1. be prepared quickly;
2. be precise, concise, human-readable, and illustrated with examples.

Structural requirements

The XML schema language must define:

1. mechanisms for constraining document structure (namespaces, elements, attributes) and content (datatypes, entities, notations);
2. mechanisms to enable inheritance for element, attribute, and datatype definitions;
3. mechanism for URI reference to standard semantic understanding of a construct;
4. mechanism for embedded documentation;
5. mechanism for application-specific constraints and descriptions;
6. mechanisms for addressing the evolution of schemata;
7. mechanisms to enable integration of structural schemas with primitive data types.

Datatype requirements

The XML schema language must:

1. provide for primitive data typing, including byte, date, integer, sequence, SQL & Java primitive data types, etc.;
2. define a type system that is adequate for import/export from database systems (e.g., relational, object, OLAP);
3. distinguish requirements relating to lexical data representation vs. those governing an underlying information set;
4. allow creation of user-defined datatypes, such as datatypes that are derived from existing datatypes and which may constrain certain of its properties (e.g., range, precision, length, mask).

Conformance

The XML schema language must:

1. describe the responsibilities of conforming processors;
2. define the relationship between schemas and XML documents;
3. define the relationship between schema validity and XML validity;
4. define the relationship between schemas and XML DTDs, and their information sets;
5. define the relationship among schemas, namespaces, and validity;
6. define a useful XML schema for XML schemas;