Schemas:An introduction Chapter 1 This chapter provides a brief explanation of schemas and why they are important. It also discusses the basic schema design goals, and describes the various existing schema languages. 1.1 What is an XML schema? The word schema means a diagram, plan, or framework. In XML, it refers to a document that describes an XML document. Suppose you have the XML instance shown in Example 1-1. It consists of a product element that has two children (number and size) and an attribute (effDate). Example 1-2 shows a schema that describes the instance.
It contains element and attribute declarations that assign data types and element-type names to elements and attributes. Example 1-1. Product instance 557 10 Example 1-2. Product schema 1.2 The purpose of schemas 1.2.1 Data validation One of the most common uses for schemas is to verify that an XML document is valid according to a defined set of rules. A schema can be used to validate: The structure of elements and attributes.
For example, a product must have a number and a size, and may optionally have an effDate (effective date). The order of elements. For example, number must appear before size. The data values of attributes and elements, based on ranges, enumerations, and pattern matching. For example, size must be an integer between 2 and 18, and effDate must be a valid date. The uniqueness of values in an instance. For example, all product numbers in an instance must be unique. 1.
2.2 A contract with trading partners Often, XML instances are passed between organizations. A schema may act as a contract with your trading partners. It clearly lays out the rules for document structure and what is required. Since an instance can be validated against a schema, the "contract" can be enforced using available tools. 1.2.3 System documentation Schemas can provide documentation about the data in an XML instance.
Anyone who needs to understand the data can refer to the schema for information about names, structures, and data types of the items. To include further documentation, you can add annotations to any schema component. 1.2.4 Augmentation of data Schema processing can also add to the instance. It inserts default and fixed values for elements and attributes, and normalizes whitespace according to the data type. 1.2.
5 Application information Schemas provide a way for additional information about the data to be supplied to the application when processing a particular type of document. For example, you could include information on how to map the product element instances to a database table, and have the application use this information to automatically update a particular table with the data. In addition to being available at processing time, this information in schemas can be used to generate code such as: User interfaces for editing the information. For example, if you know that size is between 2 and 18, you can generate an interface that has a slider bar with these values as the limits. Stylesheets to transform the instance data into a reader-friendly representation such as XHTML. For example, if you know that the human-readable name for the content of a number element is "Product Number" you can use this as a column header. Code to insert or extract the data from a database. For example, if you know that the product number maps to the PROD_NUM column on the PRODUCTS table, you can generate an efficient routine to insert it into that column.
Tools have only just begun to take advantage of the possibilities of schemas. In the coming years, we will see schemas used in many creative new ways. 1.3 Schema design XML Schema is packed with features, and there are often several ways to accurately describe the same thing. The decisions made during schema design can affect its usability, accuracy, and applicability. Therefore, it is important to keep in mind your design objectives when creating a schema. These objectives may vary depending on how you are using XML, but some are common to all use cases. 1.
3.1 Accuracy and precision Obviously, a schema should accurately describe an XML instance and allow it to be validated. Schemas should also be precise in describing data. Precision can result in more complete validation as well as better documentation. Precision can be achieved by defining restrictive data types that truly represent valid values. 1.3.2 Clarity Schemas should be very clear, allowing a reader to instantly understand the structure and characteristics of the instance being described.
Clarity can be achieved by: appropriate choice of names, consistency in naming, consistency in structure, good documentation, avoiding unnecessary complexity. 1.3.3 Broad applicability There is a temptation to create schemas that are useful only for a specific application purpose. In some cases, this may be appropriate. However, it is better to create a schema that has broader applicability. For example, a business unit that handles only domestic accounts may not use a country element declaration as part of an address. They should consider adding it in as an optional element for the purposes of consis-tency and future usability.
There are two components to a schema''s broad applicability: reusability and extensibility. Reusable schema components are modular and well documented, encouraging schema authors to reuse them in other schemas. Extensible components are flexible and open, allowing other schema authors to build on them for future uses. Since reusability and extensibility are important, all of Chapter 21, "Extensibility and reuse," is devoted to them. 1.4 Schema languages 1.4.1 Document Type Definitions (DTDs) Document Type Definitions (DTDs) are a commonly used method of describing XML documents.
They allow you to define the basic structure of an XML instance, including: the structure and order of child elements in an element type, the attributes of an element type, basic data typing for attributes, default and fixed values for attributes, notations to represent other data formats. Example 1-3 shows a DTD that is roughly equivalent to our schema in Example 1-2. Example 1-3. Product DTD DTDs have many advantages. They are relatively simple, have a compact syntax, and are widely understood by XML implementers. When designed well, they can be extremely modular, flexible, and extensible. However, DTDs also have some shortcomings. They have their own non-XML syntax, do not support namespaces easily, and provide very limited data typing, for attributes only.
1.4.2 Enter schemas As XML became increasingly popular for data applications such as e-commerce and enterprise application integration (EAI), a more robust schema language was needed. Specifically, XML developers wanted: The ability to constrain data based on common data types such as integer and date. The ability to define their own data types in order to further constrain data. Support for namespaces. The ability to specify multiple declarations for the same element-type name in different contexts. Object oriented features such as type derivation.
The ability to express types as extensions or restrictions of other types allows them to be processed similarly and substituted for each other. A schema language that uses XML syntax. This is advantageous because it is extensible, can represent more advanced models and can be processed by many available tools. The ability to add structured documentation and application information that is passed to the application during processing. DTDs are not likely to disappear now that schemas have arrived on the scene. They are supported in many tools, are widely understood, and are currently in use in many applications. In addition, they continue to be useful as a lightweight alternative to schemas. 1.
4.3 W3C XML Schema Four schema languages were developed before work began on XML Schema: XDR (XML Data Reduced), DCD, SOX, and DDML. These four languages were considered together as a starting point for XML Schema, and many of their originators were involved in the creation of XML Schema. The World Wide Web Consortium (W3C) began work on XML Schema in 1998. The first version, upon which this book is based, became an official Recommendation on May 2, 2001. The formal Recommendation is in three parts: XML Schema Part 0: Primer is a non-normative introduction to XML Schema that provides a lot of examples and explanations. It can be found at http://www.w3.
org/TR/xmlschema-0/ XML Schema Part 1: Structures describes most of the components of XML Schema. It can be found at http://www.w3.org/TR/xmlschema-1/ XML Schema Part 2: Datatypes covers simple data types. It explains the built-in data types and the facets that may be used to restrict them. It is a separate document so that other specifications may use it, without including all of XML Schema. It can be found at http://www.w3.
org/TR/xmlschema-2/ 1.4.4 Notes on terminology 1.4.4.1 Schema "XML Schema" is the official name of the Recommendation and is also sometimes used to refer to conforming schema documents. In order to clearly distinguish between the two, this book uses the term "XML Schema" only to mean the Recommendation itself. A "schema definition" is the formal expression of a schema.
The initialism "XSDL" (XML Schema Definition Lan.