WSDL, UDDI, SOAP, REST: SOA Acronym Soup

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WSDL, UDDI, SOAP, REST SOA Acronym Soup

• Matthew J. Dovey
• Oxford University

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WebService Soup The confusion
WS-Addressing
WSDL
WS-Security
WS-Eventing
WS-N
XACML
REST
SOAP
WS-I
SOA
WS-Events
SAML
BPEL
UDDI
WSRF
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Firstly

• What is a WebService?
• Goal is interoperability between enterprises
• Were getting there but it hasnt been an
  uneventful journey!

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REST

• Representational State Transfer
• SRU, OpenSearch etc. are NOT REST WebServices
• But rather REST-like in that they use XML and
  HTTP but not the underlying philosophy of REST
• Underlying philosophy of mapping HTTP verbs to
  the Create/Retrieve/Update/Delete philosophy of
  APIs
• Discussed in a PhD thesis by Roy Fielding

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True REST

• URLs are nouns, not verbs
• The basic API (verbs) is C.R.U.D
• Create (HTTP POST)
• Retrieve (HTTP GET) no side effects
• Update (HTTP PUT)
• Delete (HTTP DELETE)

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SRU REST style

• Noun http//www.server/database/myResultSet
• Verbs
• Search create result set
• POST /database/myResultSet HTTP/1.1
• Content-Type text/xml charsetutf-8
• ltquerygttitle fishlt/querygt
• Sort update result set
• PUT /database/myResultSet HTTP/1.1
• Content-Type text/xml charsetutf-8
• ltsortBygtauthorlt/sortBygt
• Retrieve retrieve result set
• GET /database/myResultSet HTTP/1.1
• Content-Type text/xml charsetutf-8
• ltrecordSetgtltstartRecordgt1lt/startRecordgtltendRecordgt
  10lt/endRecordgtlt/recordSetgt
• Finish delete result set

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SOAP

• Originally Simple Object Access Protocol but now
  SOAP stands for SOAP!
• (Called XP for XML Protocol for a while)
• Originates from doing Remote Procedural Calls
  over XML and HTTP
• Microsoft initiated but moved to W3C
• Originally a number of competitors
• XML-RPC etc

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Remote Procedural Style

• myVar foo(hello world, 6)
• String foo(String s, Integer i)
• return result

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Remote Procedural Style

• myVar foo(hello world, 6)
• String foo(String s, Integer i)
• return result

LAN
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Remote Procedural Style

• myVar foo(hello world, 6)
• String foo(String s, Integer i)
• return result

Internet
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More SOAP

• ltSOAPEnvelope xmlnsSOAP..."gt
• ltSOAPHeadergt
• lt/SOAPHeadergt
• ltSOAPBodygt
• ltSRWsearchRetrieveRequest xmlnsSRW..."gt
• ltSRWversiongt1.1lt/SRWversiongt
• ltSRWquerygt(dc.author exact "jones" and
  dc.title gt
• "smith")lt/SRWquerygt
• ltSRWstartRecordgt1lt/SRWstartRecordgt
• ltSRWmaximumRecordsgt10lt/SRWmaximumRecordsgt
• ltSRWrecordSchemagtinfo.../modslt/SRWrecordsS
  chemagt
• lt/SRWsearchRetrieveRequestgt
• lt/SOAPBodygt
• lt/SOAPEnvelopegt

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SOAP Components

• SOAPHeader
• Not well used, but designed for messaging and
  relaying purposes
• e.g. telling proxies to rewrite srubaseUrl
• SOAPBody
• XML payload
• SOAPFaults
• Error conditions

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WSDL 1.1

• Web Service Description Language
• Describes the SOAP Messages
• Components
• Types
• XML representations of the types (typically XML
  Schema language)
• e.g. searchRetrieveRequestType
• Messages
• XML messages built from types passed from client
  to server and server to client
• e.g. searchRetrieveRequest, searchRetrieveResponse
  
• portTypes
• Which messages are sent in which direction, and
  what the response is
• e.g. searchRetrieveRequest is sent client to
  server, and a searchRetrieveResponse sent back
• Bindings
• How to encode (e.g. SOAP document/literal)
• How to transport (e.g. HTTP, SMTP)
• Services
• Endpoint
• e.g. http//voyager.loc.gov7090/

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Invisible WSDL

• String foo(String s, Integer i)
• myVar foo(hello world, 6)

WSDL Document
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Not so invisible WSDL

• WSDL not well defined/described
• The original spec uses definition by example
• Early WSDL tools highly incompatible
• Different interpretations
• e.g. when to use xsdimport when to use
  wsdlimport
• Not so bad today (if using the major toolkits)

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Early SOAP/WSDL Toolkits Issues

• SOAP Encoding 1
• rpc versus document
• RPC Style
• String foo(String s, Integer i)
• Document Style
• Class foo
• String s
• Integer I
• String foobar(foo)

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SOAP Encoding 2 SOAP versus Literal styles

• Literal just as plain XML
• ltstuffgthellolt/stuffgt
• ltnumbers"gt  ltitemgt10lt/itemgt  ltitemgt20lt/itemgt  lt
  itemgt30lt/itemgt  ltitemgt40lt/itemgt  ltitemgt50lt/itemgt
  
• lt/numbersgt
• SOAP use particular SOAP defined XML constructs
  for arrays etc.
• ltstuff xsitypexsdstringgthellolt/stuffgt
• ltnumbers xsitype"SOAP-ENCArray"
  SOAP-ENCarrayType"xsdinteger5"gt  ltitemgt10lt/i
  temgt  ltitemgt20lt/itemgt  ltitemgt30lt/itemgt  ltitemgt4
  0lt/itemgt  ltitemgt50lt/itemgt
• lt/numbersgt

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And it gets worse

• Literal
• ltDistancegt
• ltp1gt
• ltxgt10lt/xgt
• ltygt20lt/ygt
• lt/p1gt
• ltp2gt
• ltxgt10lt/xgt
• ltygt20lt/ygt
• lt/p2gt
• ltDistancegt
• SOAP Encoding
• ltDistancegt
• ltp1 HREF"id1"/gt
• ltp2 HREF"id1"/gt
• lt/Distancegt
• ltPoint id"id1"gt
• ltxgt10lt/xgt

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XML Schema

• Officially any XML Schema construct is valid in
  WSDL
• In practice
• e.g. for a long time Java tools were defined by
  JAX-RPC which defined a subset of valid XML
  Schema
• e.g. in early stages of SRU the following caused
  headaches
• xsdpositiveInteger, xsdnonNegativeInteger etc.

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XML Schema funniessequences and choice

• ltsequencegt
• ltelement namefoo typestring /gt
• ltelement namebar typestring /gt
• lt/sequencegt
• Often generates
• String foo
• String bar
• ltchoicegt
• ltelement namefoo typestring /gt
• ltelement namebar typestring /gt
• lt/choicegt
• Also generates
• String foo
• String bar
• (Although will raise an exception if both are
  non-null at serialization)

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Consider

• ltchoicegt
• ltelement namefoo typestring /gt
• ltsequencegt
• ltelement namefoo typestring /gt
• ltelement namebar typestring /gt
• lt/sequencegt
• lt/choicegt
• Early Axis (until 2005!) generated
• String foo
• String foo
• String bar

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XML Schema funniesExtension of all groups

• ltcomplexTypetype1gt
• ltcomplexContentgt
• ltallgt
• ltelement namefoo typestring /gt
• ltelement namebar typestring /gt
• ltallgt
• lt/complexContentgt
• lt/complexTypegt
• ltcomplexTypetype2gt
• ltcomplexContentgt
• ltextension basetype2"gt
• ltallgt
• ltelement namefoo1 typestring /gt
• ltelement namebar2 typestring /gt
• ltallgt
• lt/extensiongt
• lt/complexContentgt
• lt/complexTypegt

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XML Schema funnies Qualified Dublin Core case

• Health warning complex XML Schema follows!!!
• ltxscomplexType name"SimpleLiteral"gt
• ltxscomplexContent mixed"true"gt
• ltxsrestriction base"xsanyType"gt
• ltxssequencegt
• ltxsany processContents"lax"
  minOccurs"0" maxOccurs"0"/gt lt/xssequencegt
• ltxsattribute ref"xmllang"
  use"optional"/gt
• lt/xsrestrictiongt
• lt/xscomplexContentgt
• lt/xscomplexTypegt
• ltxscomplexType name"W3CDTF"gt
• ltxssimpleContentgt
• ltxsrestriction base"dcSimpleLiteral"gt
• ltxssimpleTypegt
• ltxsunion memberTypes"xsgYear
  xsgYearMonth xsdate xsdateTime"/gt
• lt/xssimpleTypegt
• lt/xsrestrictiongt

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Enter WS-I

• Web Service Interoperability profile
• Key components SOAP, WSDL, UDDI
• Defines a set of rules for interoperable
  WebServices
• e.g. use SOAP document/literal style
• Rules for wsdlimport versus xsdimport
• Still defines any XML Schema as valid etc.
• Compliant toolsets much better at interoperating
  today

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UDDI (the oft forgotten third part of WS-I)

• Universal Description, Discovery and Integration
  (of WebServices)
• Provides a model of organisations and webservices
• Provides a WebService API for interacting with
  registries of WebServices
• publishing,
• searching,
• retrieving,
• replicating,
• subscribing
• Template based searching (still issues to be
  resolved)
• Not well used outside of enterprise due to out of
  band contractual negotiations needed for
  inter-enterprise web services
• Poor open source implementations for version 3.0

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And the rest

• WSDL 2.0
• WS-Addressing indirection via server EPR (End
  Point References)
• WS-N, WS-Events, WS-Evening Notification
  publish/subscribe
• BPEL WebService Workflow/Choreography
• WSRF Representing resources through WebServices
• WS-Security SAML, XACML, etc. for encryption,
  signing/validation and credential delegation
• ebXML-BP, OWL-S, Semantic Framework etc. for
  policy/contractural negotiations
• etc
• Compatibility declines as you go further up the
  stack
• Straying from WS-I can be dangerous

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And finally retro-fitting

• Service Oriented Architecture Reference Model
• Attempting to retro fit a definition onto SOA
• Service Oriented Architecture Blueprints
• Attempting to retro fit a methodology

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Last word

• Interoperability of WebServices have been a
  difficult road
• For more complex tasks (security, messaging,
  asynchronous etc.) there is still some
  difficulties ahead
• But we do have interoperability at the basic
  level and it is getting better