Patterns: Broker Interactions for Intra- and Inter-enterprise

Chapter 7. Technology options 125

Table 7-1 Technology options across the patterns

7.2 XML

Extensible Markup Language (XML) allows you to specify your own markup

language with tags defined in a Document Type Definition (DTD) or XML

Schema. XML can be used as a means to specify the content of messages

between servers, whether the two servers are within an enterprise or represent a

business-to-business connection.

The critical factor here is the agreement between parties on the message

schema, which is specified as an XML DTD or Schema. An XML parser is used

to extract specific content from the message stream. Your design will need to

consider whether to use an event-based approach, for which the SAX API is

appropriate, or to navigate the tree structure of the document using the DOM

API.

The IBM XML4J XML parser was made available through the Apache open

source organization under the Xerces name. For open source XML frameworks,

see:

http://xml.apache.org/

Business Drivers

Direct Connection

Exposed Direct

Connection

Broker

Exposed Broker

Serial Process

Exposed Serial

Process

Parallel Process

XML üüüüüüü

Web services üüüüüüü

J2EE Connector Architecture üü

Java Message Service (JMS) üüüüüüü

Message-oriented Middleware üüüüü

Flow languages üüü

126 Broker Interactions for Intra- and Inter-enterprise

7.2.1 Defining XML documents

XML documents are defined using DTDs or XML Schemas.

DTDs are a basic XML definition language, inherited from the SGML

specification. The DTD specifies what markup tags can be used in the document

and what their structures are.

DTDs have two major problems:

򐂰 Poor data typing: In DTDs, elements can only be specified as EMPTY, ANY,

element content, or mixed element-and-text content, and there is no standard

way to specify null values for elements.

Data typing like date formats, numbers, or other common data types cannot

be specified in the DTD, so an XML document may comply with the DTD but

still have data type errors that can only be detected by the application.

򐂰 Not defined in XML: DTD uses its own language to define XML syntax that is

not compliant with the XML specification. This makes it difficult to manipulate

a DTD.

To solve these problems, the World Wide Web Consortium (W3C) specified a

new standard to define XML documents called XML Schema. XML Schema

provides the following advantages over DTDs:

򐂰 Strong typing for elements and attributes

򐂰 Standardized way to represent null values for elements

򐂰 Key mechanism that is directly analogous to relational database foreign keys

򐂰 Defined as XML documents, making them programmatically accessible

Even though XML Schema is a more powerful technology to define XML

documents, it is also a lot harder to work with, so DTDs are still widely used to

define XML documents. Additionally, simple, non-hard-typified documents can be

easily defined using DTDs with similar results to using XML Schema.

Whether to use one or the other will depend on the complexity of the messages

and the validation requirements of the application. Actually, in many cases both a

DTD and an XML Schema are provided, so they can be used by the application

depending on its requirements.

Note: Remember that the validation process of an XML document using XML

Schemas is an

expensive process. Validation should be performed only when it

is necessary.