WSDL Web Services Description Language

1
WSDLWeb Services Description Language
2
Goals of WSDL

• Describes the formats and protocols of a Web
  Service in a standard way
• The operations the service supports
• The message(s) needed to invoke the operations
• The binding of the messages to a communication
  protocol
• The address to which messages should be sent
• Serves the same function as an Interface
  Description Language (IDL)

3
WSDL Description

• A Web Service is described at both the abstract
  and concrete levels
• Abstract Level
• What are the operations that are supported?
• What messages are needed to invoke the
  operations?
• Concrete Level
• How are the messages bound to a transport
  protocol?
• What is the Web address to which the messages
  should be sent?

4
WSDL Abstract Level

• At the abstract level, obtaining a service is
  like executing a method of an object
• WSDL defines the following elements
• An interface is like an object it consists of a
  set of operations
• An operation is like a method it is invoked by
  messages
• A message is composed of parts
• A part is like a parameter and has an associated
  type

5
Example

• ltinterface name GetQuoteIfgt
• ltoperation name getQuoteOp
• patternhttp//www.w3.org/
  2003/06/wsdl/in-outgt
• ltinput message gsgetQuoteOpReq/gt
  
• ltoutput message gsgetQuoteOpResp
  /gt
• ltoutfault name invalidSymbolFault
  
• messageReferencegsgetQuoteOpResp
• message
  gsinvalidSymbolFaultMsg/gt
• lt/operationgt
• lt!-- other operations go here --gt
• lt/interfacegt

gs is the target namespace of the document
containing this declaration and the message
declarations
6
Patterns

• The messages exchanged when an operation is
  invoked conform to a pattern
• WSDL is considering a number of patterns, each
  identified by a URI. The most commonly used are
• In-Out
• Input sent by requestor, output produced by
  service
• Might be synchronous (requestor waits for
  response e.g. RPC) or asynchronous
• In-Only
• Input sent by requestor, no response expected

7
Faults
ltoutput message gsgetQuoteOpResp/gt ltoutfault
name invalidSymbolFault messageReferencegs
getQuoteOpResp message
gsinvalidSymbolFaultMsg/gt

• In-Out pattern uses a Fault Replaces Message
  fault rule
• Server replaces the message referenced by
  messageReference with message if it detects a
  fault
• Server raises the named fault in the requestor
• In-Only pattern uses No Fault fault rule
• No fault message is allowed

8
Other Message Patterns (tentative)

• Robust In-Only
• Same as In-Only except that a fault message is
  allowed using Message Triggers Fault rule
• Server responds to input message with a fault
  message
• In-Multi-Out
• A single input message followed by 0 or more
  instances of output message in response
• Uses Fault Replaces Message Rule rule
• Out Only
• Out-In
• Out-Multi-In .
• and users can define their own rule

9
Example Message Definitions

• ltmessage name getQuoteOpReqgt
• ltpart name stockSymbol type
  xsdstring/gt
• lt/messagegt
• ltmessage name getQuoteOpRespgt
• ltpart name stockSymbol type
  xsdstring/gt
• ltpart name QuoteValue type
  xsdfloat/gt
• lt/messagegt
• ltmessage name invalidSymbolFaultMsggt
• ltpart name faultInfo type
  gsfaultType/gt
• lt/messagegt

10
Parts of a Message

• A message can have many parts
• Each part can be bound to a different position
  within the physical message sent by the transport
• With SOAP parts can be distributed over body and
  header blocks
• Each part can have a simple or complex type
  defined in an XML schema

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Example

• ltschemagt
• ltcomplexType name faultTypegt
• ltsequencegt
• ltelement name faultCode type
  string/gt
• ltelement name faultDetail type
  string
• minOccurs 0 maxOccurs 1/gt
• lt/sequencegt
• lt/complexTypegt
• lt/schemagt

12
Concrete Level

• The concrete level defines how interfaces and
  operations are bound to transports and addresses
• A given interface can be bound to several
  different transports and addresses
• A Web Service might support an interface using
  several different transports
• For example, the operations can be invoked using
  SOAP over either HTTP or SMTP
• The same interface might be supported by several
  different Web Services using the same or
  different transports
• In all of these cases, semantically identical
  service should be provided at each address

13
Concrete Level

• At the concrete level WSDL defines the following
  elements
• A binding describes how the operations and
  messages of an interface are mapped into the
  messages of a particular transport
• An endpoint maps a binding to a Web address
• A service is a collection of endpoints that host
  related interfaces

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Example Service and Endpoint
identifies binding

• ltservice name GetQuoteServicegt
• ltendpoint name GetQuoteRPC
  bindinggqGetQuoteSOAPBindinggt
• ltsoapaddress location
  http//www.shearson.com/quoteservice/gt
• lt/endpointgt
• lt!Other endpoints go here --gt
• lt/servicegt

15
WSDL Extensibility

• A binding maps an interface to a particular
  transport
• It must be capable of targeting a variety of
  transports
• Each transport has its own idiosynchrosies
• WSDL is extended by introducing a different
  namespace for each transport

ltdefinitions xmlnshttp//schemas.xmlsoap.o
rg/wsdl/ xmlnsxsdhttp//www.w3.org/2001/
XMLSchema xmlnssoaphttp//schemas.xmlsoap
.org/wsdl/soap/ targetNamespacehttp//www.s
hearson.com/quoteservicegt lt!-- WSDL
declarations go here --gt lt/definitionsgt
introduce SOAP namespace
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Example RPC/encoded SOAP Binding
identifies interface

• ltbinding name GetQuoteSOAPBinding type
  tnsGetQuoteIfgt
• ltsoapbinding style rpc
• transport http//schemas.xmlsoap.
  org/soap/http//gt
• ltoperation name getQuoteOpgt
• ltinputgt
• ltsoapbody
• use encoded
• namespace
  http//www.shearson.com/quoteservice
• encodingStyle
  http//schemas.xmlsoap.org/soap/encoding/gt
• lt/inputgt
• Continued on next slide

rpc style msg
for tags used in messsage
SOAP extensions
encode parameters
encoding rules
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Binding Example - Continued

• ltoutputgt
• ltsoapbody
• use encoded
• namespace
  http//www.shearson.com/quoteservice
• encodingStyle
  http//schemas.xmlsoap.org/soap/encoding//gt
• lt/outputgt
• lt/operationgt
• lt!-- other operations go here
  --gt
• lt/bindinggt

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RPC/encoded Message
ltsEnvelope xmlnsshttp//www.w3.org/2003/05/soa
p-envelope xmlnsxsdhttp//www.w3.org/20
01/XMLSchema xmlnsxsihttp//www.w3.org/
2001/XMLSchema-instancegt ltsBodygt ltngetQuot
eOp xmlnsnhttp//www.shearson.com/quoteService
gt ltnstockSymbol xsitypexsdstringgt IBM
lt/nstockSymbolgt lt/ngetQuoteOpgt
lt/sBodygt lt/sEnvelopegt
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Encoding

• Problem serializer serializes arguments in
  accordance with rules specified by encodingStyle
  attribute
• Receiver can deserialize arguments since style is
  specified in the message
• But message has a declared type
• How can we be sure that the rules produce an
  instance of the type?
• In fact they might not!

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Example Encoding Style

• Suppose there are n arguments of the same type.
• Serializer might produce a message containing n
  instances of the type.
• But suppose in a particular invocation all
  arguments have same value.
• Serializer might produce a message containing n
  pointers to a single instance of the value
• Then value of each parameter (a pointer) is not
  an instance of the type

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Encoded Vs. Literal

• If useencoded, parameters are encoded in
  accordance with the specified encoding style
• If useliteral, parameters are instances of
  types specified in the message declaration
• Yields two distinct styles for invoking a remote
  procedure
• rpc/encoded
• rpc/literal

22
Encoded Vs. Literal

• If useencoded, parameters are encoded in
  accordance with the specified encoding style
• If useliteral, parameters are instances of
  types specified in the message declaration
• Yields two distinct styles for invoking a remote
  procedure
• rpc/encoded
• rpc/literal

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Example RPC/literal SOAP Binding
identifies interface

• ltbinding name GetQuoteSOAPBinding type
  tnsGetQuoteIfgt
• ltsoapbinding style rpc
• transport http//schemas.xmlsoap.
  org/soap/http//gt
• ltoperation name getQuoteOpgt
• ltinputgt
• ltsoapbody
• use literal
• namespace
  http//www.shearson.com/quoteservice/gt
• lt/inputgt
• ltoutputgt . lt/outputgt
• lt/operationgt
• lt/bindinggt

rpc style msg
dont encode parameters
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RPC/literal SOAP Binding
lttypesgt ltschemagt ltcomplexType
namecomptypgt lt/complexTypegt
ltschemagt lt/typesgt ltmessage namemsggt ltpart
namepart1 typecomptyp /gt lt/messagegt ltsoap
Envelopegt ltsoapBodygt ltnmyProc
xmlnsn gt ltnpart1gt
instance of comptyp lt/npart1gt
lt/nmyProcgt lt/soapBodygt lt/soapEnvelopegt
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RPC/encoded and RPC/literal

• RPC style specified for both bindings
• There is no schema describing the message body
• Child of body uses name of procedure
• Each grandchild corresponds to a parameter and
  uses parameter name
• Might be a grandchild for result returned
• Validation not possible

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Sending Documents

• Increasingly, Web communication is
• Asynchronous
• Web Services are loosely coupled (as opposed to
  tightly coupled, object-oriented systems that are
  developed in a more integrated fashion and are
  more oriented towards rpc)
• More appropriate for delay prone/failure prone
  environments
• Messages contain XML documents (instead of
  procedure arguments)
• A wide variety of communication patterns (as
  opposed to simply in-out) are useful

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Document Style Messaging
ltmessage name sendInvoiceMsggt ltpart name
invoice type invinvoiceType/gt lt/messagegt
ltinterface name invoiceIfgt ltoperation
name sendInvoiceOp pattern
http//www.w3.org/2003/06/wsdl/in-onlygt
ltinput message invsendInvoiceMsg/gt
lt/operationgt lt/interfacegt
one-way message
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Document/literal Binding
SOAP body contains XML documents
ltbinding name sendInvBinding type
inginvoiceIfgt ltsoapbinding style
document transport
http//schemas.xmlsoap.org/soap/http/gt
ltoperation name invsendInvoiceOpgt
ltinputgt ltsoapbody use
literal namespace
http//www.invoicesource.com/invoice/gt
lt/inputgt lt/operationgt lt/bindinggt
body is an instance of part type
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Document/literal SOAP Binding
Alternative 1 one part specified by a type
lttypesgt ltschemagt ltcomplexType
namecomptypgt lt/complexTypegt
ltschemagt lt/typesgt ltmessage namemsggt ltpart
namepart1 typencomptyp /gt lt/messagegt ltsoa
pEnvelopegt ltsoapBodygt instance of
comptyp lt/soapBodygt lt/soapEnvelopegt
n is target namespace of this document
message can have only one part in this case since
the schema of the body can have only one type
specification
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Document/literal SOAP Binding
Alternative 2 part specified by an element
lttypesgt ltschemagt ltelement nameelem
typencomptyp /gt ltcomplexType
namecomptypgt lt/complexTypegt
ltschemagt lt/typesgt ltmessage namemsggt ltpart
namepart1 elementnelem /gt lt/messagegt ltsoa
pEnvelopegt ltsoapBodygt ltnelem
xmlns target ns of WSDL doc gt
instance of comptyp
lt/nelemgt lt/soapBodygt lt/soapEnvelopegt
Part is identified as an element. An instance
of element is a child of body, named with
elements name, typed with elements type
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Sending Multiple Documents
ltelement namefirstInvoice typeinvinvoiceTyp
e /gt ltelement namesecondInvoice
typeinvinvoiceType /gt ltcomplexType
nameinvoiceTypegt lt!-- the complex type
definition goes here --gt lt/complexTypegt ltmessage
name sendInvoiceMsggt ltpart name
invoicei element invfirstInvoice /gt
ltpart name invoice2 element
invsecondInvoice /gt lt/messagegt
element specification must be used since message
has multiple parts
ltsoapBodygt ltinvfirstInvoicegt lt!--
instance of invoiceType goes here --gt
lt/invfirstInvoicegt ltinvsecondInvoicegt
lt!-- instance of invoiceType goes here --gt
lt/invsecondInvoicegt lt/soapBodygt
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Sending Messages By Email Simple Mail Transfer
Prototol

• ltservice name GetQuoteSMTPServicegt
• ltendpoint name GetQuoteSMTP
• bindinggqGetQuoteSMTPBin
  ding/gt
• ltsoapaddress
• location
  mailtostockquote_at_shearson.com/gt
• lt/endpointgt
• lt/servicegt

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Mail Example (continued)

• ltbinding name GetQuoteSMTPBinding
• type tnsGetQuoteIfgt
• ltsoapbinding style document
• transporthttp//schemas.xmlsoap
  .org/soap/smtp /gt
• ltoperation name getQuoteOpgt
• ltinputgt
• ltsoapbody useliteral/gt
• lt/inputgt
• ltoutputgt
• ltsoapbody useliteral/gt
  
• lt/outputgt
• lt/operationgt
• lt/bindinggt

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Complete WSDL Document
ltdefinitions targetNamespace.
xmlns gt lttypesgt lt!
specification of XML Schema types used in this
document --gt lt/typesgt ltmesssagegt
lt/messsagegt lt! specification of other
messages goes here--gt ltinterfacegt
lt/interfacegt lt! specification of other
interfaces goes here--gt ltbindinggt
lt/bindinggt lt! specification of other
bindings goes here--gt ltservicegt
ltendpointgt lt/endpointgt lt!
specification of other endpoints goes here--gt
lt/servicegt lt/definitionsgt
35
What WSDL Cannot Do

• WSDL describes how one operation can be invoked
• getQuoteOp
• Many services require a sequence of operations
• Send this message, receive that message, if this
  happens send this other message to another
  endpoint, etc
• Cannot be described in WSDL
• BPEL describes the logic of a Web Service
• How it is impemented
• How it communicates with other busienss processes
• Sometimes called an orchestration language