Distributed Systems 2006

Web Services

(With material from [Møller & Schwartzbach, 2003])

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Plan

  Basics– SOAP– WSDL– UDDI

  Related specifications

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A Web Service

  A software system designed to support interoperable machine-to-machine interaction over a network– It has an interface described in a machine-

processable format (specifically WSDL)– Other systems interact with the Web service in a

manner prescribed by its description using SOAP messages

– These are typically conveyed using HTTP with an XML serialization in conjunction with other Web-related standards

  [W3C]

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Motivation

  Heterogeneous, distributed systems– Scalability– Interoperability– Modifiability

  The main motivation is interoperability– The ability for separately developed applications to

work in together

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SOAP, SOAP, UDDI

  SOAP– A framework for exchanging XML-based information in a network– SOAP used to be an acronym: Simple Object Access Protocol– "This is no longer the case." (it is neither simple nor has anything to do

with objects)– The currently most hyped XML/Web service technology

  WSDL (Web Service Description Language)– An XML-based language for describing network services– WSDL descriptions of capabilities and locations of services– Like an Interface Description Language for Web services– Often communication using SOAP + HTTP

  UDDI (Universal Description, Discovery, and Integration)– Provides a registry mechanism for clients and servers to find each other– Uses SOAP for communication

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Backupserver

Primary server Legacy

system

web services

Externalsystem

web services

(web services)

(web services)

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An Example

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A Result

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SOAP RequestMessage transport

Message intent

Message contents

Application data

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SOAP Response

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org.apache.axis.utils.tcpmon

  Part of Apache Axis toolset

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Anatomy of SOAP

  Lightweight, stateless protocol framework for exchanging XML messages– Applications can create complex interaction patterns

• Request/response• Request/multiple responses• ...

– by combining one-way exchanges with features provided by an underlying protocol and/or application-specific information

  Silent on the semantics of any application-specific data it conveys

  Message contents– xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/”

• soapenv:Envelope– soapenv:Header

» Application-specific control information» May be processed by intermediaries

– soapenv:Body» Information exchanged between sender and receiver

  Typical protocols– HTTP– SMTP

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  (Really, the Google search API is a bad example of a web service design– E.g., http://www.prescod.net/rest/googleapi/

  But has its merits, e.g., in interaction with other web services)

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WSDL

  WSDL is an XML-based language that allows formal descriptions of the interfaces of Web services– which interactions does the service provide?– which arguments and results are involved in the interactions?– which network addresses are used to locate the service?– which communication protocol should be used?– which data formats are the messages represented in?

  So what are the benefits?– an interface description is a contract between the server

developers and the client developers– having formal descriptions allows tool support, e.g. code

template generators

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A WSDL Example (1)

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A WSDL Example (2)Data types defined using XML Schema

Message types definedfrom data types

Port type defining operations

Binding to SOAP RPCover HTTP

Port where the runningservice may be accessed

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WSDL Layers

  The language can be described as having two layers– the service definition layer describes abstract

properties:• data types• message types• operations• services

– The binding layer describes concrete properties:• Protocols• Data formats

(using SOAP, HTTP, MIME)

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WSDL Elements

  An actual WSDL document consists of a set of definitions of the following kinds:– types - containing XML Schema element and type definitions – message - consisting of either

• a number of named parts typed by XML Schema elements, or • a single part typed by a XML Schema type

– portType - describing a set of operations, each being either • one-way: receiving an input message, • request-response: receiving an input message and then responding with an

output message (like Remote Procedure Calls), • solicit-response: sending an output message and then receiving an input

message, or • notification: sending an output message

– binding - selects communication protocol and data formats for each operation and message

– service - describes a collection of named ports, each associated with a binding and a network address

  An import mechanism allows modularization of definitions.

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SOAP in WSDL

  The SOAP binding in WSDL:– selects document or rpc style (rpc wraps message

parts)– selects HTTP/SMTP/... protocol– selects encoding type

• Encoded – specifies data formats• Literal

– places message parts in header or body parts of the envelope

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Examples of style versus encoding

  public void myMethod(int x, float y); – RPC/Encoded

• <soap:envelope> <soap:body> <myMethod> <x xsi:type="xsd:int">5</x> <y xsi:type="xsd:float">5.0</y> </myMethod> </soap:body> </soap:envelope>

– RPC/Literal• <soap:envelope> <soap:body> <myMethod> <x>5</x> <y>5.0</y>

</myMethod> </soap:body> </soap:envelope> – (Document/Encoded)– Document/Literal

• <soap:envelope> <soap:body> <xElement>5</xElement> <yElement>5.0</yElement> </soap:body> </soap:envelope>

  WS-I Compliance– Tries to clean up to web service mess so that web services may actually

interoperate– Recommends RPC/Literal or Document/Literal

  (Why not use Document/Literal all the time?)

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Problems with WSDL

  The HTTP and MIME bindings are too loosely specified (so not all WSDL documents "make sense")

  WSDL is closely tied with SOAP– SOAP is big and complicated without offering very

much

  Complex interaction patterns cannot be described (e.g. transactions)

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Using Web Services with Java

  JAX-RPC (JSR-101)– Java API for XML-Based RPC

• Including WSDL <-> Java mapping – Apache AXIS (Apache Extensible Interaction System)

• http://ws.apache.org/axis/ • a Java-based implementation of SOAP + WSDL

– largely allows the programmer to forget these technologies• WSDL <-> Java• Used together with a servlet container such as Tomcat or Jetty

– Java Web Services Developer Pack  JAX-WS 2.0 (JSR-224)

– Java API for XML Web Services – was JAX-RPC 2.0– Heavy use of Java 5.0 annotations to customize mapping from WSDL and at

runtime– @WebService

@BindingType(value="http://www.w3.org/2003/05/soap/bindings/HTTP/")public class AddNumbersImpl {

– Similar to .NET, will (probably) be built into Java 6.0

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UDDI - Universal Description, Discovery, and Integration

  A Web service registry mechanism– "a meta service for locating Web services by enabling robust queries

against rich metadata"

  UDDI business registration: XML files used to describe business entities and their Web services– white pages - business address, contact info, etc.– yellow pages - industrial categorizations based on standard taxonomies

(allows search within particular industry, product category, geographical region, ...)

– green pages - more technical information, for instance WSDL descriptions

  UDDI also provides a SOAP+WSDL-based registry API for registering ("publish") and discovering ("inquire") Web services

  A UDDI browser: http://www.soapclient.com/uddisearch.html

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An Example

  <businessEntity businessKey="ba744ed0-3aaf-11d5-80dc-002035229c64"   operator="www.ibm.com/services/uddi"   authorizedName="0100001QS1">  <discoveryURLs>  <discoveryURL

useType="businessEntity">http://www.ibm.com/services/uddi/uddiget?businessKey=BA744ED0-3AAF-11D5-80DC-

002035229C64</discoveryURL>

  </discoveryURLs>  <name>XMethods</name>  <description xml:lang="en">Web services resource site</description>  <contacts>  <contact useType="Founder">  <personName>Tony Hong</personName>  <phone useType="Founder" />  <email useType="Founder">thong@xmethods.net</email>  </contact>  </contacts>  ...

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An Example

  <businessServices>  <businessService serviceKey="d5921160-3e16-11d5-98bf-002035229c64"   businessKey="ba744ed0-3aaf-11d5-80dc-002035229c64">  <name>XMethods Delayed Stock Quotes</name>  <description xml:lang="en">20-minute delayed stock quotes</description>  <bindingTemplates>  <bindingTemplate bindingKey="d594a970-3e16-11d5-98bf-002035229c64"   serviceKey="d5921160-3e16-11d5-98bf-002035229c64">  <description xml:lang="en">SOAP binding for delayed stock quotes service</description>  <accessPoint URLType="http">http://services.xmethods.net:80/soap</accessPoint>  <tModelInstanceDetails>  <tModelInstanceInfo tModelKey="uuid:0e727db0-3e14-11d5-98bf-002035229c64" />  </tModelInstanceDetails>  </bindingTemplate>  </bindingTemplates>  </businessService>  </businessServices>  </businessEntity>

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UDDI Elements

  A UDDI businessEntity describes a business and its services– Key attributes locate various data structures – a businessService describes a particular service (or family of services) – a bindingTemplate describes where and how a service is accessed – a tModel describes compliance with a specification (e.g. a WSDL description)

  This tModel for the single service described above refers to bindings in a WSDL description:

  <tModel tModelKey="uuid:0e727db0-3e14-11d5-98bf-002035229c64"  operator="www.ibm.com/services/uddi"  authorizedName="0100001QS1">  <name>XMethods Simple Stock Quote</name>  <description xml:lang="en">Simple stock quote interface</description>  <overviewDoc>  <description xml:lang="en">wsdl link</description>  <overviewURL>http://www.xmethods.net/tmodels/SimpleStockQuote.wsdl</overviewURL>  </overviewDoc>  <categoryBag> <keyedReference tModelKey="uuid:c1acf26d-9672-4404-9d70-

39b756e62ab4” ... </categoryBag>  </tModel>

  the WSDL document has no service part - the service address is specified in the bindingTemplate

  UDDI is much more general (and more complicated) than these examples suggest...

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SOAP to UDDI

  The following SOAP message could be sent to a UDDI registry to inquire about services named "delayed stock quotes":

  <?xml version="1.0" encoding="UTF-8"?>  <Envelope

xmlns="http://schemas.xmlsoap.org/soap/envelope/">  <Body>  <find_service businessKey="*" generic="1.0"

xmlns="urn:uddi-org:api">  <name>delayed stock quotes</name>  </find_service>  </Body>  </Envelope>

– Alternatively, we could search by category codes of various kinds– The find_service meta-service operation is specified in the WSDL for the UDDI

API– Of course, the UDDI registries are also registered services :-)

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Sidetrack: SOA

  Service-Oriented Architecture– Practices and technology that enable application

functionality to be provided and consumed as services

– Services can be invoked, published and discovered, and are abstracted away from the implementation using a single, standards-based form of interface.

  Resources– http://www.cbdiforum.com– http://www.google.com/search?q=oasis+soa

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SOA and Web Services

  SOA is not equivalent to web services– Other possible underlying technologies

J2EE/CORBA, .NET, …

  Concepts map well to web services

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SOA Concepts

  Service– Contractually defined behavior– Implemented and provided by a component,

used by another component– Web services

• Maps to a web service

  Service descriptions– Each service should include a service

description in standardized format– Technical parameters, constraints, and

policies that define the terms to invoke the service

– Web services• Maps to a WSDL description of a web service

  Advertising and discovery– Services should be able to be discovered– Services need to be advertised to be

discovered– Web services

• May be provided by a UDDI repository

  Data model– Invoker and invokee need to have a common

conception of data– Web services

• XML Schemas may define data as used by services

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Internal: Service Layer

  Defines an application’s boundary with a layer of services that establishes a set of available operations and coordinates the application’s response in each situation

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Internal: Services

  Service guidelines– Self-contained – coarse-grained functionality– “Stateless” – should not retain client state between

invocations– Context-independence – should not depend on other

services

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External: Architecture

  Service-Oriented Architecture does not define a software architecture per se– ”… the structure or structures of [a] system, which

comprise software elements, the externally visible properties of those elements, and the relationships among them” [Bass et al., 2003]

  Interaction among services still need to be designed at an architectural level

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External: Deutsch’s Eight Fallacies of Distributed Computing

  1. The network is reliable  2. Latency is zero  3. Bandwidth is infinite  4. The network is secure  5. Topology doesn't change  6. There is one administrator  7. Transport cost is zero  8. The network is homogeneous

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Seven Fallacies of SOA

  Fallacy #1– There's Nothing New Under the Sun, and SOA Is No Exception

  Fallacy #2– SOA is a Revolutionary Paradigm Shift

  Fallacy #3– SOAs are All Hype, No Substance

  Fallacy #4– SOA is a Panacea

  Fallacy #5– The Overhead from SOA Leads to Unacceptably Poor Performance

  Fallacy #6– A Bottom-Up Approach to SOA is Good Enough

  Fallacy #7– SOA is Optional

  http://www.zapthink.com/report.html?id=ZAPFLASH-08052004

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FastWS

  Application are unaware of which protocol they use– Implementations may serve XML and Fast concurrently

– http://java.sun.com/developer/technicalArticles/WebServices/fastWS/

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The Web Service Stack

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Status of the Stack

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Summary

  Web services– Machine-to-machine interaction over a network– Typically WSDL + SOAP + HTTP

  A large number of supporting specifications

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References

  [Møller & Schwartzbach, 2003]– Møller, A., and Schwartzbach, M. (2003). Interactive

Web Services with Java. http://www.brics.dk/~amoeller/WWW, accessed 2006-02-14