W3C XML Schema: what you might not know (and might or might not like!)

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W3C XML Schema what you might not know (and
might or might not like!)

• Noah Mendelsohn
• Distinguished Engineer
• IBM Corp.
• October 10, 2002

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Topics

• Quick review of XML concepts
• Why XML Schema?
• What is XML Schema?
• Where do schemas come from?
• A few validation tricks
• Wrapup

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Warning! To save screen
space, some examples are simplified. Namespace
decls. are omitted, only the key parts of schema
declarations are shown, etc.
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Quick review of XMLconcepts
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This is an XML document
lt?xml version1.0?gt lte1gt lte2gt
lte3 a1123 /gt lte2gt lt/e1gt

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Infoset the XML data model
lt?xml version1.0?gt lte1gt lte2gt lte3
a1123 /gt lte2gt lt/e1gt

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More on XML infosets

• XML 1.0 describes only documents with angle
  bracket syntax ltgt
• Infosets also describe DOM, SAX, and other
  representations
• XML Schema validates infosetsapplies to all of
  the representations
• XML Schema can validate from any element
  information item (e.g. e1 or e2)

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Why XML Schema?
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What are schemas for?

• Contracts agreeing on formats
• Tool building know what the data will be before
  the first instance shows up
• Database integration
• User interface tools
• Programming language bindings
• Validation make sure we got what we expected

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What is XML Schema?
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This is an XML document
lt?xml version1.0?gt ltmynse1
xmlnsmynshttp//example.org/myns
xmlnsyournshttp//example.org/yournsgt
ltmynse2gt ltyournse1 a1xyz/gt
ltmynse3 a1123 mynsa1456/gt
ltyournse1 mynsa1456/gt lt/mynse2gt
ltyournse4/gt lt/mynse1gt

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This is an XML schema
ltxsdschema targetNamespacehttp//example
.org/myns xmlnsxsd"http//www.w3.org/200
1/XMLSchema" ..namespaces ommitted to
protect innocent..gt lt!- declare element e1
-gt ltxsdelement namee1gt ltxsdsequencegt
ltxsdelement namee2/gt
ltxsdelement refyournse4/gt
lt/xsdsequencegt lt/xsdelement lt/xsdschemagt

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This is an XML schema document
ltxsdschema targetNamespacehttp//example
.org/myns xmlnsxsd"http//www.w3.org/200
1/XMLSchema" ..namespaces ommitted to
protect innocent..gt lt!- declare element e1
-gt ltxsdelement namee1gt ltxsdsequencegt
ltxsdelement namemynse2/gt
ltxsdelement refyournse4/gt
lt/xsdsequencegt lt/xsdelement lt/xsdschemagt

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This is an XML document
lt?xml version1.0?gt ltmynse1gt
xmlnsmynshttp//example.org/myns
xmlnsyournshttp//example.org/yournsgt
ltmynse2gt ltyournse1 a1xyz/gt
ltmynse3 a1123 mynsa1456/gt
ltyournse1 mynsa1456/gt lt/mynse2gt
ltyournse4/gt lt/mynse1gt

To validate this, we need gt1 schema document
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Import brings in declarations for other namespaces
ltxsdschema targetNamespacehttp//example
.org/myns.xsd xmlnsxsd"http//www.w3.org
/2001/XMLSchema" ..namespace ommitted to
protect innocent..gt ltimport namespacehttp//e
xample.org/yourns schemaLocationhttp//ex
ample.org/yourns.xsdgt ltxsdelement
namee1gt ltxsdsequencegt
ltxsdelement namemynse2/gt ltxsdelement
refyournse4/gt lt/xsdsequencegt
lt/xsdelement lt!- declare element e2 -gt
ltxsdelement namee2 type/gt lt/xsdschemagt

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Terminology
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Cool tricks with components

• In memory schemas
• Handy tools for working with schemas
• Build the components for you
• Resolve subtyping across namepaces, etc.
• Examples
• http//www.eclipse.org/xsd
• Henry Thompsons XSV
• Conformance testing

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How to read the spec.

• 3.3 Element Declarations
• 3.3.1 The Element Declaration Schema Component
• 3.3.2 XML Representation of Element Declaration
  Schema Components
• 3.3.3 Constraints on XML Representations of
  Element Declarations
• 3.3.4 Element Declaration Validation Rules
• 3.3.5 Element Declaration Information Set
  Contributions
• 3.3.6 Constraints on Element Declaration Schema
  Components
• Warning the spec. never gives any rule twice!

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Post-schema validation infoset (PSVI)

• Fearsome title, simple concept
• Infoset the data model for an XML
  documenttells you what you can know (that
  matters) after a parse.
• PSVI tells you what you can know after a
  validation
• What parts of doc are valid?
• Per which types?
• Default values
• Etc.

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Self-describing vs. schema- described docs

• You can use xsitype in your documents lte
  xsitypexsdintegergt123lt/egt
• Use xsitype with built ins (and no attributes)
• Your document is nearly self-describing
• SOAP encoding supports this
• xsitype with your own types
• Partially self-describing
• You know the type names need schema to know
  what types are
• SOAP 1.2 Encoding supports this too!

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Where do schemas come from?
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How are schema components found?

• In short, wherever you want!
• Hint from schema
• ltxsdimport ns schemaLocationyyy.xsd/gt
• Hint from instance
• ltmynse1 schemaLocation myNSUri yyy.xsd/gt
• Processor command line or config
• Compiled into application (validating HTML editor)

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Why all this flexibility?

• gt 1 schema / namespace (versions, bug fixes,
  experiments, etc.)
• Who gets control?
• Docheads want to name schema in instance
• eCommerce do you trust the schema named in a
  purchaseOrder?
• Ultimately the application chooses
• Synthetic DB builds it dynamically

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Streaming

• Most validation can be done 1 pass
• Id/idref, key/keyref require limited lookaside
• Problem
• ltmyinstancegt 10Mbytes of data here lt!-
  oops..need a new schema! --gt ltnewnsa
  schemaLocnewnsUri xxxgt lots more
  datalt/mysinstancegt
• Answer
• Assemble schema incrementally or in advance
• Result must be same cant tell which from the
  outside!

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Our language vs. your language why ltimportgt?
ltxsdschema targetNamespacehttp//example.org/ns
1 xmlnsns1http//example.org/ns1
xmlnsxsd"http//www.w3.org/2001/XMLSchema"gt
ltxsdelement nameX/gt ltxsdelement
refns1X/gt lt/xsdschemagt

A fragment of a schema document
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Our language vs. your language why ltimportgt?
ltxsdschema targetNamespacehttp//example.org/ns
1 xmlnsns1http//example.org/ns1
xmlnsns2http//example.org/ns2
xmlnsxsd"http//www.w3.org/2001/XMLSchema"gt
ltimport namespacehttp//example.org/ns2gt
ltxsdelement nameX/gt ltxsdelement
refns1X/gt ltxsdelement refns2Y/gt
lt/xsdschemagt

Add a reference to an external element
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Our language vs. your language why ltimportgt?
Unimported namespaces enhance the schema language
Imported namespaces enhance your language.
ltxsdschema targetNamespacehttp//example.org/ns
1 xmlnsns1http//example.org/ns1
xmlnsns2http//example.org/ns2
xmlnsxsd"http//www.w3.org/2001/XMLSchema
xmlnsxsd2"http//www.w3.org/2004/XMLSchemagt
ltimport namespacehttp//example.org/ns2gt
ltxsdelement nameX/gt ltxsdelement
refns1X/gt ltxsdelement refns2Y/gt
ltxsd2betterElement namenewone
/gt lt/xsdschemagt

Enhance the schema language!
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A few validation tricksModeling content
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How to validate this?
ltsoapEnvelopegt ltsoapBodygt your message
here lt/soapBodygt lt/soapEnvelopegt

• What is the content model for ltsoapBodygt?
• Can you validate the contents?

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Inside/out vocabularies (some specific SOAP
examples)
lt! SOAP PURCHASE ORDER --gt ltsoapEnvelope
xmlnssoaphttp//www.w3.org/2002/06/soap-envelop
egt ltsoapBodygt ltpopurchaseOrder
xmlnspohttp//example.org/pogt
lt/popurchaseOrdergt lt/soapBodygt lt/soapEnvelope
gt lt! SOAP INVOICE --gt ltsoapEnvelope
xmlnssoaphttp//www.w3.org/2002/06/soap-envelop
egt ltsoapBodygt ltinvinvoice
xmlnspohttp//example.org/invgt
lt/invinvoicegt lt/soapBodygt lt/soapEnvelopegt
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Schemas for envelopes
Putting skip here says dont validate the
content of the body
ltxsdcomplexType namebodyTypegt ... ltxsdsequ
encegt ltxsdany processContentsski
p/gt lt/xsdsequencegt ... lt/xsdcomplexTypegt
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Schemas for envelopes
Putting strict here says you must have
declarations and must successfully validate the
contents of body.
ltxsdcomplexType namebodyTypegt ...
ltxsdsequencegt ltxsdany
processContentsstrict/gt lt/xsdsequencegt ...
lt/xsdcomplexTypegt
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Schemas for envelopes
Putting lax here says validate only if your
schema has declarations for the contents
ltxsdcomplexType namebodyTypegt ... ltxsdsequ
encegt ltxsdany processContentslax
/gt lt/xsdsequencegt ... lt/xsdcomplexTypegt
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Versioning vocabularies schemas

• Its hard!
• Use namespaces?
• Do 50 bug fixes give you 50 namespaces?
• How much interop? Does old schema accept new
  version?
• What about Xpath?
• For better or worse schemas has no organized
  model for versioning

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Inheritance why have it?

• Allow reuse of definitions
• Model real-world inheritance and polymorphism
• Substitutability
• Mappings to programming systems w/inheritance
• Schemas provides mechanisms offering parial
  solutions to these problems

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Refinement vs. Extension

• Data inheritance is different from method
  inheritance
• No active code receiver sees everything
  order matters, e.g. for multiple inheritance
• Innovation(?) in schema
• Restriction subtype is a subset (supports
  substitutability)
• Extension subtype builds on base (supports
  modular development, some mappings to real world
  and programming languages.)
• No multiple inheritance (for now)

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Wrapup
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Some things I learned

• No such thing as a simple feature
• Big committee -gt big language
• Documents data together are cool
• But neither community gets a simple schema
  language
• Make realistic schedules we didnt make time to
  pull features

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Thank you!