Effective XML

1
Effective XML

• Elliotte Rusty Harold
• elharo_at_metalab.unc.edu
• http//www.cafeconleche.org/

2
Part I Syntax
3
Stay with XML 1.0

• XML 1.1
• New name characters
• C0 control characters
• C1 control characters
• NEL
• Undeclare namespace prefixes
• Incompatible with
• Most XML parsers
• W3C and RELAX NG schema languages
• XOM, JDOM

4
Part II Structure
5
The XML Stack
6
Allow All XML syntax

• CDATA sections
• Entity references
• Processing instructions
• Comments
• Numeric character references
• Document type declarations
• Different ways of representing the same core
  content not different information

7
Distinguish text from markup

• A DocBook element
• ltprogramlistinggtlt!CDATAltvaluegt
  ltdoublegt28657lt/doublegtlt/valuegtgtlt/programlisting
  gt
• The content isltvaluegt ltdoublegt28657lt/doublegtlt
  /valuegt
• This is the sameltprogramlistinggtltvaluegt
  ltdoublegt28657lt/doublegt
  lt/valuegtlt/programlistinggt

8
The reverse problem

• Tools that create XML from strings
• Tree-based editors like ltOxygen/gt or XML Spy
• WYSIWYG applications like OpenOffice Writer
• Programming APIs such as DOM, JDOM, and XOM
• The tool automatically escapes reserved
  characters like lt, gt, or .
• Just because something looks like an XML tag does
  not mean it is an XML tag.

9
White space matters

• Parsers report all white space in element
  content, including boundary white space
• An xmlspace attribute is for the client
  application only, not the parser
• White space in attribute values is normalized
• Parsers do not report white space in the prolog,
  epilog, the document type declaration, and tags.

10
Make structure explicit through markup

• Bad
• ltTransactiongtWithdrawal 2003 12 15
  200.00lt/Transactiongt
• Better
• ltTransaction type"withdrawal"gt
• ltDategt2003-12-15lt/Dategt
• ltAmountgt200.00lt/Amountgt
• lt/Transactiongt

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Store metadata in attributes

• Material the reader doesnt want to see
• URLs
• IDs
• Styles
• Revision dates
• Authors name
• No substructure
• Revision tracking
• Citations
• No multiple elements

12
Remember mixed content

• Narrative documents
• Record-like documents
• The RSS problem
• ltitemgt
• lttitlegtXerlin 1.3 releasedlt/titlegt
• ltdescriptiongt
• Xerlin 1.3, an open source XML Editor written
  in
• Java, has been released. Users can extend the
  
• application via custom editor interfaces for
• specific DTDs. New features in version 1.3
  include
• XML Schema support, WebDAV capabilities, and
• various user interface enhancements. Java 1.2
  
• or later is required.
• lt/descriptiongt
• ltlinkgthttp//www.cafeconleche.org/news2003April7lt
  /linkgt
• lt/itemgt

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What you really want is this
ltdescriptiongt ltpgtlta href"http//www.xerlin.or
g"gtltstronggtXerlin 1.3lt/stronggtlt/agt,an open
source XML Editor written in Java, has been
released. Users can extend the application via
custom editor interfaces for specific DTDs. New
features in version 1.3 includelt/pgt ltulgt
ltligtXML Schema supportlt/ligt ltligtWebDAV
capabilitieslt/ligt ltligtVarious user interface
enhancementslt/ligt lt/ulgt ltpgtJava 1.2 or later
is required.lt/pgt lt/descriptiongt
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What people do is this
ltdescriptiongtltpgtlta href"http//www.xerlin.o
rg"gtltstronggtXerlin 1.3lt/stronggtlt/agt, an
open source XML Editor written in Java, has been
released. Users can extend the application via
custom editor interfaces for specific DTDs. New
features in version 1.3 includelt/pgt
ltulgt ltligtXML Schema supportlt/ligt
ltligtWebDAV capabilitieslt/ligt
ltligtVarious user interface enhancementslt/ligt
lt/ulgt ltpgtJava 1.2 or later is
required.lt/pgt lt/descriptiongt
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Prefer URLs to unparsed entities and notations

• URLs are simple and well understood
• Notations and unparsed entities are confusing and
  little used
• URLs dont require the DTD to be read
• Many APIs dont even support notations and
  unparsed entities

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Part III Semantics
17
Use processing instructions for process-specific
content

• For a very particular, even local, process
• Describes how a particular process acts on the
  data in the document
• Does not describe or add to the content itself
• A unit that can be treated in isolation
• Content is not XML-like.
• Applies to the entire document

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Processing instructions are not appropriate when

• Content is closely related to the content of the
  document itself
• Structure extends beyond a single processing
  instruction
• Needs to be validated

19
Include all information in instance documents

• Not all parsers read the DTD
• Especially browsers
• Beware
• Default attribute values
• Parsed entity references
• XInclude
• ID type dependence (XPath, DOM, etc.)

20
Encode binary data using quoted printable and/or
Base64

• Quoted printable works well for mostly text
• Base-64 for non-text data
• Can you link to the data with a URL instead?
• Can you bundle the data with XML using zip, jar,
  XOP, or MIME?

21
Use namespaces for modularity and extensibility

• Not hard simple cases can use one default
  namespace
• http URIs are normally preferred
• DTD validation is tricky
• Code to namespace URIs, not prefixes
• Avoid namespace prefixes in element content and
  attribute values

22
Reuse XHTML for generic narrative content

• lt!ENTITY xhtml1 SYSTEM "http//www.w3.org/TR/xht
  ml1/DTD/strict.dtd"gtxhtml1
• lt!ELEMENT description Blockgt

23
Choose the right schema language for the job

• DTDs
• The W3C XML Schema Language
• RELAX NG
• Schematron

24
Use only what you need

• You need
• Well-formed XML 1.0
• A parser
• You probably need
• Namespaces
• You may not need
• DTDs
• Schemas
• XInclude
• SOAP
• WS-Kitchen-Sink
• etc.

25
Always use a parser

• Cant use regular expressions
• Detecting encoding
• Comments and processing instructions that contain
  tags
• CDATA sections
• Unexpected placement of spaces and line breaks
  within tags
• Default attribute values
• Character and entity references
• Malformed documents
• Internal DTD Subset
• Why not?
• Unfamiliarity with parsers
• Too slow

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Layer Functionality
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Program to standard APIs

• Easier to deploy in Java 1.4/1.5
• Different implementations have different
  performance characteristics
• SAX is fast
• DOM interoperates
• Semi-standard
• JDOM
• XOM
• Bleeding edge
• StAX
• JAXB

28
Read the complete DTD

• Be conservative in what you generate liberal in
  what you accept
• Important content from DTD
• Default attribute values
• Namespace declarations
• Entity references
• ID types

29
Navigate with XPath

• More robust against unexpected structure
• Allow optimization by engine
• Easier to code enhanced programmer productivity
• Might be slower

30
Validate inside your program with schemas
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Part IV Implementation
32
Write documents in Unicode

• Prefer UTF-8
• Smaller in English
• ASCII compatible
• Normalization
• É, ü, ì and so forth
• NFC
• ICU

33
Avoid Vendor Lockin Beware

• Opaque, binary data used in place of marked up
  text.
• Over-abbreviated, inobvious names like F17354 and
  grgyt
• APIs that hide the XML
• Products that focus on the "Infoset
• Alternate serializations of XML
• Patented formats

34
Hang on to your relational database
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Document Namespaces with RDDL
lt!DOCTYPE html PUBLIC "-//XML-DEV//DTD XHTML RDDL
1.0//EN"
"http//www.rddl.org/rddl-xhtml.dtd"gt lthtml
xmlns"http//www.w3.org/1999/xhtml"
xmlnsxlink"http//www.w3.org/1999/xlink"
xmlnsrddl"http//www.rddl.org/"gt ltheadgt
lttitlegtMegaBank Statement Markup Language
(MBSML)lt/titlegt lt/headgt ltpgt This is the XML
namespace for the lta href"http//developer.megaba
nk.com/xml/"gtMegaBank Statement Markup
Languagelt/agt. lt/pgt ltrddlresource
xlinktype"simple" xlinkhref"http//develope
r.megabank.com/xml/spec.html"
xlinkrole"http//www.w3.org/TR/html4/"
xlinkarcrole "http//www.rddl.org/purposes
normative-reference" gt ltpgt The lta
href"http//developer.megabank.com/xml/spec.html"
gtMegaBank Statement Markup Language
Specification 1.0lt/agt lt/pgt lt/rddlresourcegt lt/bo
dygtlt/htmlgt
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Pick the correct MIME type

• application/xml
• Not text/xml!
• Don't use charset
• application/mathmlxml
• image/svgxml
• application/xsltxml

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TagSoup Your HTML
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Catalog common resources

• lt?xml version"1.0"?gt
• ltcatalog xmlns
• "urnoasisnamestcentityxmlnsxmlcatalog"
• gt
• ltpublic publicId
• "-//OASIS//DTD DocBook XML V4.2//EN"
• uri
• "file///opt/xml/docbook/docbookx.dtd"/gt
• lt/cataloggt

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Compress if space is a problem
//output OutputStream fout new
FileOutputStream("data.xml.gz") OutputStream
out new GZipOutputStream(fout)
OutputFormat format new OutputFormat(document)
XMLSerializer output new XMLSerializer(out,
format) output.serialize(doc) // input
InputStream fin new FileInputStream("data.xml.gz
") InputStream in new GZipInputStream(fin)
DocumentBuilderFactory factory
DocumentBuilderFactory.newInstance()
DocumentBuilder parser factory.newDocumentBuilde
r() Document doc parser.parse(in) // work
with the document...
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To Learn More

• This Presentation http//cafeconleche.org/slides/
  sd2007west/effectivexml
• Effective XML 50 Specific Ways to Improve Your
  XML Documents
• Elliotte Rusty Harold
• Addison-Wesley, 2003
• ISBN 0-321-15040-6
• 44.99
• http//cafeconleche.org/books/effectivexml