Essential Guide XML PDF

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THE ESSENTIAL GUIDE TO

BY SHARON L. HOFFMAN AUGUST 2005

XMLXML

is a key technology for sharing data

between business entities because it

bridges different ways of storing and

referencing data. Although XML can be described as a

language, the extensible nature of XML means that its

more correctly classified as a standard.Many interrelated standards (for a list, see Essential

XML Standards on page 4) complement XML and expand

its capabilities. XML is also a fundamental building block

for other standards. For example, many Web-services

standards, such as Simple Object Access Protocol (SOAP)

and Web Services Description Language (WSDL), are based

on XML. To give you a sense of how you might use XML

in your own applications, lets start with a quick look at

SUPPLEMENT TO iSeries NEWS 2005

XML syntax and how XML compares with languages

used for related tasks.

XML in ContextAn XML document is made up of XML elements. Each

element contains a starting tag, an ending tag, and (usually)

data nested between the two tags. By choosing descriptive

names for elements, you can make your XML documents

more human-readable and therefore self-documenting. InFigure 1, the highlighted line is a single element called

product_code. If a document contains more than one element

of the same type, the tags will be repeated for each element

as shown for the product_code and requested_qty elements

in Figure 1. For more information about XML syntax see

Essential XML Syntax and Terminology on page 3.

Repeating the data description for every element means

that XML documents are entirely self-contained you

wont need to refer to a database layout, for example.

However, the overhead of repeating all

the element-description information

quickly becomes unwieldy. As a result,

most developers prefer using data-description languages (e.g., SQL, DDS)

to define databases. However, XML shines

in data-transfer applications that involve

relatively small amounts of data (these are

typically single transactions such as an

inventory inquiry or a purchase order).

Data transfer is by far the most common

XML application in iSeries environments.

However, you can also use XML to add

meaning to text within documents. Used

in this way, XML becomes a powerful

Figure 1:Sample XML document

bike component availability9/1/2005

Acme CompanySharon [email protected]

1234556789225


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THE ESSENTIAL GUIDE TO SOFTWARE XML



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1 XML is case sensitive.

2 Generally, white space (e.g., indents, blank lines) in anXML document is ignored.

3 You can choose any element names you like aslong as they conform to a few basic rules:

Element names cannot contain spaces. Element names must begin with a letter or

an underline.

After the first character, element names cancontain numbers, hyphens, periods, colons,letters, and underscores. (Colons are usuallyavoided in element names because they havespecial meaning within XML.) Element names cannot begin with the lettersxml, regardless of case (i.e., xml, XML, xMl,and Xml are all invalid).

4 Elements can contain one or more attributes. Inmany cases, the XML designer may choose whether touse elements or attributes to define a particular structure.As a rule of thumb, attributes should be used forinformation that is not integral to the element.

5 An element cannot contain more than one attribute withthe same name.

6 Both starting and ending tags are required for allelements except empty elements. Empty elements occurmost often when an element is completely defined by itsattributes.

7 Elements must be properly nested (i.e., once an innerelement tag is opened, it must be closed before anyouter tags).

The following nesting is correct:

Sharon

Hoffman

The following nesting is syntactically correct, although itdoesnt make much sense:

Sharon

The following nesting is syntactically incorrect:

Sharon

Hoffman

8 The outermost element in any XML document isreferred to as the root element.

9 The root element may be preceded by a documentdeclaration and processing instructions.

10 Built-in XML entities are used to include a characterthat has special meaning in XML (e.g., a greater-thansign) within XML content. You can also defineadditional entities as short-hand for text and structuresthat you use repeatedly.

11 An XML document that has correct syntax is well formed.

12 An XML document that conforms to the structure definedby its Document Type Definition (DTD) or schema isvalid. It is possible for an XML document to be wellformed but invalid, but the reverse is not possible.

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tool for organizing information and improving search

capabilities. To understand the benefits of an XML-

encoded document, you should consider the differences

between XML and HTML.

Although the two languages are syntactically similar

because they have the same antecedents (see Essential XML

History on page 5 for information), they have different

strengths. HTML is best used to format information for

display, while the descriptive information in XML tagsmakes it easier to deal with document content. For example,

suppose you have a document containing a list of PC

printers that contains information about the features of each

printer model. If the document is stored in HTML, its

difficult to create a search that finds all printers that support

color printing, duplex printing, and can print at least 10

pages per minute. Conversely, if you store the same document

using XML, you would probably create separate elements

for each important feature (e.g., maximum_print_speed)

and could easily develop an application that searches for

all printers that meet your criteria. Of course, a database

is ideal for such a search, but XML provides database-like

search capabilities for information that is stored in documents

such as user manuals or marketing brochures. As youll

see in the following section, the XML data can easily be

converted into HTML for display purposes.

Because XML documents are plain text, you can write

XML using any text editor (e.g., Notepad). However, as you

begin working with XML, youll quickly find that an XML-

aware editor is a big time-saver. An XML editor shouldhelp you write XML by providing syntax-checking and

document-generation capabilities. For example, if you begin

to create a new element, some editors will automatically

generate the ending tag for you.

An XML document can stand entirely on its own, without

any related documents. More often, though, an XML

document is part of a larger application architecture that

includes components that define the structure required for

a particular type of XML document, solutions that reformat

XML data (e.g., create an HTML document for display using

data from an XML document), and applications that process

ESSENTIAL XML SYNTAX AND TERMINOLOGY



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ESSENTIAL XML STANDARDS

XLINK is a standard for defining hyperlinks in XML. XML Namespaces make it possible to create unique

element names. XML Schemas define the rules for the specialized

XML documents used to define the structure ofother XML documents.

XPATH addresses each part of an XML documentvia a hierarchical structure (e.g., first_name withincustomer_name within quote_request).

XQUERY is a relatively new standard that providesSQL-like query capabilities for XML documents.

Extensible Stylesheet Language (XSL) formatsXML documents for display. There are twocomponents of the XSL standard: XSLTransformations (XSLT) and XSL FormattingObjects (XSL FO).

XML itself is a standard, but it also involves many related standards. Here are

some of the most widely used XML standards.

XML documents. Understanding how these pieces work

together is vital to understanding XML.

The Big PictureAn XML document is almost always associated with a

second document that defines the valid structure for a

particular type of documents. For example, an XML

document might contain a particular inventory inquiry from

XYZ Company, but the structural-definition document would

define the format for all inventory inquiry documents.

There are two standards for these structural-definition

documents: DTD is the older and simpler standard, whereas

XML schema is the newer standard. DTDs and schemas

serve the same purpose, but their complexity and capabilities

vary significantly.

Figure 2 contains a DTD that you could use to define the

XML document in Figure 1, and Figure 3 contains the schema

for the same document. Both the DTD and the schema were

generated using an XML editor (WebSphere Development

Studio Client for iSeries WDSc, in this case). Youll find

that creating a sample document (e.g., an inventory inquiry)and using it to generate an initial version of the DTD or

schema is often the simplest way to create a structural-

definition document. While you may need to clean up the

generated code, it will give you a good starting point for

developing the DTD or schema.

Whether you use a DTD or a schema, there is

typically a one-to-many relationship between the

DTD or schema and the XML documents. For

example, you could publish a DTD or a schema

(or both) specifying the format for incoming

inventory inquiries and, hopefully, many of your cus-

tomers would then begin to send you inventory

inquiries in XML format. DTDs and schemas forexternal documents (versus documents that are inter-

nal to a particular company) are usually published

online so that they can be shared more easily.

Ideally, everybody would use the same structure

for the same type of document (e.g., inventory inquiries), but

thats not always the case not even within a single industry.

Fortunately, many industry groups are working on standards

that should help alleviate some of the Tower-of-Babel aspects

of XML. Youll find the latest information on industry-specific

XML structures online at xml.org.

In addition to DTDs and schemas, other components can

be associated with XML documents. For example, if you

plan to display an XML document in a Web page, youll

probably want to first convert the XML document into

an HTML document. Similarly, you often might need to

create multiple XML documents that contain the same

general information but use slightly different structures.

If you need to convert lots of documents between the same

two structures, it makes sense to automate the process. The

simplest way to do this is via an Extensible Stylesheet

Language Transformations (XSLT) document that defines

how input elements should be formatted in the output (XML

or HTML) document. For example, if several of your vendors

accept inventory inquiries in XML, but each uses a slightly

different schema, you could develop a generic XMLinventory inquiry, then create the variations using XSLT.

As with DTDs and schemas, your XML editor should include

tools to help you create XSLT documents.

An XSLT document works in conjunction with an XSLT

Figure 2:A DTD generated by WDSc

for the XML document in Figure 1

(customer_reference,date_required,customer,requested_products)>


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Although most XML editors include an XML parser, youllalso need an XML parser for production applications. XMLparsers may be part of a Web application server, or theymay be available as separate software options. There are

two general standards for XML parsers: DocumentObject Model (DOM) and Simple API for XML (SAX).The only functional difference between DOM parsers

and SAX parsers is that DOM parsers can modify anXML document, while SAX parsers are read-only (ofcourse, an application that uses a SAX parser can alwayswrite out a new XML document in a different formatthan the incoming XML document). The other differ-ences between DOM and SAX parsers dont affect theircapabilities, but they can have an impact on ease-of-use,and in some cases, performance.

SAX parsers are event-driven and are best suited forapplications that need to choose specific elements from alarger XML document. Youll find the SAX parsers moreintuitive if your programming background includes languages

that have event-driven capabilities (e.g., Visual Basic, Java).DOM parsers read an entire XML document into anapplication where the elements can be referenced, muchas an RPG program might reference fields in a recordformat. Therefore, DOM parsers have an advantage overSAX parsers when you need to process a high percentageof the elements in an XML document. In addition,DOM parsers generally feel more natural than SAXparsers if your programming background includes procedurallanguages such as RPG and Cobol.

Essential XML History

The Essential XML Resources

The histories of individual computer languages are mostly just curiosities, but XMLs history provides a glimpse into

its syntax as well. XML is part of the same family of languages as HTML and is based on Standard Generalized Markup

Language (SGML). SGML is a direct descendent of Generalized Markup Language, which was developed by IBM

researchers in the 1960s.

The concept behind markup languages is to separate document content from document structure and display. Thus

in both XML and HTML, the tags contain information about data formatting information in HTML, and contextinformation in XML.

SGML became an ISO standard in 1986. HTML, which evolved somewhat independently but incorporates many SGML

concepts, is slowly being brought back into compliance with the larger SGML standard.

In 1996, developers began working on a simplified version of SGML that focuses on document structure rather than

document format. That project is the basis for XML, which became a Worldwide Web Consortium standard in 1998.

ESSENTIAL XML PARSER CONCEPTS


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Charles F. Goldfarbs All the XML Books in PrintGoldfarb, one of the developers of SGML, attempted to

list all the XML books in print. Although the list was last

updated in early 2004, its still a useful resource.xmlbooks.com

The CoverPagesThe XML CoverPages include XML news,

background material, and technical tips.

xml.coverpages.org

DevX.comXML FAQs, articles, discussion groups and more.

devx.com/xml

World Wide Web Consortium XML pagew3.org/XML

XML.comOReilly Media, Inc., a premier technical book publisher,

maintains this XML information site.

xml.com

IBM RESOURCES

Developerworks XML site

www-106.ibm.com/developerworks/xmliSeries XML information home page

www-1.ibm.com/servers/enable/site/xml/iseries/index.html

Two IBM white papers illustrate how to processXML documents using RPG or Cobol:

Parsing XML documents using the newV5R3 ILE COBOL syntaxwww-1.ibm.com/servers/enable/site/education/abstracts/3db2_abs.html

XML Interface for RPG maps XMLinto DB2 UDB for iSerieswww-1.ibm.com/servers/enable/site/education/ibo/record.html?xmlface
http://xmlbooks.com/http://xmlbooks.com/http://xml.coverpages.org/http://devx.com/xmlhttp://www.w3.org/XML/http://xml.com/http://localhost/var/www/apps/conversion/tmp/scratch_7/www-106.ibm.com/developerworks/xmlhttp://www-1.ibm.com/servers/enable/site/xml/iseries/index.htmlhttp://www-1.ibm.com/servers/enable/site/education/abstracts/3db2_abs.htmlhttp://localhost/var/www/apps/conversion/tmp/scratch_7/www-1.ibm.com/servers/enable/site/education/ibo/record.html?xmlfacehttp://localhost/var/www/apps/conversion/tmp/scratch_7/www-1.ibm.com/servers/enable/site/education/ibo/record.html?xmlfacehttp://www-1.ibm.com/servers/enable/site/education/abstracts/3db2_abs.htmlhttp://www-1.ibm.com/servers/enable/site/xml/iseries/index.htmlhttp://localhost/var/www/apps/conversion/tmp/scratch_7/www-106.ibm.com/developerworks/xmlhttp://xml.com/http://www.w3.org/XML/http://devx.com/xmlhttp://xml.coverpages.org/http://xmlbooks.com/




processor software that applies the rules defined in the

XSLT document to an incoming XML document and pro-

duces an output document in HTML, XML, or text format.

An XSLT processor is typically bundled into a Web appli-

cation server such as WebSphere Application Server (WAS)

and can be accessed by calling APIs in an application.

Most XML editors also include an XSLT processor for

testing purposes.

From XML to the Database

and Vice-VersaIn an iSeries environment, XML projects

almost invariably involve extracting

data from DB2 UDB for iSeries or

moving data from XML documents

into the database. While its possible

to store entire XML documents in

iSeries files, more often youll need to

separate the data for one or more elements

from its tags and store the data itself as a

field or fields within existing iSeries databaserecords. Youll also find lots of requirements for

the opposite task creating XML documents using data

from one or more database records.

The underlying software that is used to separate an

XML document into data and data-description components

is an XML parser. An XML parser understands the rules

of XML syntax, just as the parser that is part of the RPG

compiler understands RPG syntax. For more about XML

parsers, see Essential XML Parser Concepts on page 5.

As you begin developing in XML, you might not even

realize that youre using an XML parser. For example,

when an XML editor validates an XML document against

its associated DTD or schema, an XML parser is invoked

to perform the validation. XML parsers, including those

for iSeries, are typically free. The iSeries-specific XML

parser support is packaged in the no-charge licensed program

product, XML Toolkit for iSeries (5733-XT1).If youre working with very low document

volumes, it may be possible to assemble and

disassemble XML documents using the tools

built into an XML editor. However, for

production processing of XML documents,

youll usually need to develop code that

moves data back and forth between a par-

ticular type of XML document (e.g., an

inventory inquiry) and the associated data-

base records.

You can create an XML document using a

variety of techniques. At one end of the spectrum,

you could write an RPG program that creates an XML

document as an iSeries database file by hand-coding the

tags and their contents. Then, you could convert the database

file to a stream file using the CPYTOSTMF (Copy to Stream

File) CL command. Other options include using APIs to

output a stream file from an RPG program, generating an

XML document using the results of an SQL query, or

writing a Java application that builds an XML document.

Although you can write custom code to extract data from

an XML document, its simpler to leverage the capabilities

of an XML parser. For example, you might write code that

invokes specific parser functions such as reading the data

for a particular type of element (e.g., product_code).Java is the language of choice for working with XML

because it includes extensive support for accessing parser

APIs. However, you can also invoke parser APIs using

RPG or Cobol, and products are available that will auto-

mate part of the process of assembling or disassembling

XML documents.

Explore XMLXML is a powerful tool for communicating data between

applications using different databases and running on different

platforms, and it is rapidly becoming the medium of choice for

transaction-level data transfer. XML can also organize infor-

mation within a document, thus making it easier to modifyand search large amounts of text. For all its strengths, XML is

still a relatively new technology with a maze of confusing,

and sometimes competing, standards. To take advantage of

XML, it helps to have a clearly defined goal and the flexi-

bility to experiment with various tools and techniques. Its

also useful to understand how other businesses are using XML.

To explore the opportunities XML offers, visit the Web

sites listed in Essential XML Resources on page 5.

Sharon L. Hoffman is a senior technical editor foriSeries NEWS.

Figure 3:An XML schema generated by WDSc

for the XML document in Figure 1

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Essential Guide XML PDF