Lecture 04 Client Server

8/8/2019 Lecture 04 Client Server

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Lecture 4Middleware: Client-Server

Computing

Wireless and Mobile Systems Design



Middleware: Client-Server Computing2

Lecture Objectives

● Differentiate and exemplify client-server,client-agent-server, and client-intercept-server

models of middleware

● Understand functionality provided by middleware for

client, agent, and server software development● Understand how the client-server computational

model middleware is able to deal with heterogeneous

wireless devices and scalability

● Characterize the design requirements of exampleclient-server applications and rationalize the choice

of middleware and design for application

development




Resources

● H. Gunzer, “Introduction to Web Services,” March2002, http://bdn.borland.com/article/images/28818/

webservices.pdf

● Microsoft Developer Network, “Web Services

Basics,” http://msdn.microsoft.com/webservices/understanding/webservicebasics/




Agenda

● Client-server and middleware overview● Client-server computational models

■ Client-server

■ Client-agent-server

■ Client-intercept-server ● Web services overview

● Web services based on Extensible Markup Language

(XML) for mobile devices

■ Small Object Access Protocol (SOAP)■ Web Service Description Language (WSDL)




Middleware (Review)

● Middleware is a layer between the operating systemand applications that provides a set of services■ Abstract interface to the application

■ Uniform view of operating systems, networks, and hardware

platforms

Operating System

and Networking Services

Middleware

Application ProgramsAPI




Client-Server Computing

Client-Server vs. Peer-to-Peer

Server

• Client initiates

communications

• Client issues requestto a server

• Server replies or

performs some service

Peer-to-Peer Computing

• Any participant can

initiate communication

• Any device cangenerate a request

• Any device may

provide a response




Client-Server Models for Middleware

● Client-server ● Client-agent-server

● Client-intercept-server models




Client-Server Model (1)

● Communication models■ Message passing with queuing

■ Remote procedure call (RPC)

● Client-server synchronization

■ Synchronous○ Inadequate – Client is blocked if disconnected

■ Asynchronous

○ Client requests are queued locally and control is returned

immediately to the client

○ When the mobile is connected, the queue is drained in

the background and any queued messages or calls are

forwarded to the server




Mobile Host

Client-Server Model (2)

● Optimization for mobile/wireless involves changes at

both the client and server ● Example (worst-case)

■ Standard web browser accessing an HTTP server over a

wireless link

ApplicationClient

Application

Server

FixedNetworkWirelessNetwork




Client-Agent-Server Model (1)

● Computation is split into two units■ Client-agent – optimized for mobile environment

■ Agent-server –traditional (non-mobile) design

● Dealing with disconnection

■ Clients submit requests to the agent and retrieve responsesfrom the agent upon reconnection

■ The agent can employ optimization and transformation

techniques to the data to be transmitted depending, based

on

○ Type of data○ Type of devices

○ Application-specific requirements




Client-Agent-Server Model (2)

● Optimization for mobile/wireless involves client and

agent

● Single application server to support both fixed

network clients and mobile clients

Mobile Host

ApplicationClient

Application

Server

FixedNetwork

WirelessNetwork

Agent




Client-Agent-Server Example

● Wireless Application Protocol (WAP)■ Mobile device runs the client

■ The gateway acts as the agent

■ Standard HTTP server can be used

Client

Web

Browser

Server

HTTP

Server

CGI,

etc.

Gateway

Encoders

Decoders

encodedrequest

encoded

response

request

responseProtocol

Gateways

wireless




Client-Intercept-Server Model (1)

● Inter-agent optimization protocols can be usedbetween the client-side agent and the server-side

agent■ Minimize communication cost via…

○

Caching○ Differencing

○ Header reduction

○ One-HTTP connection per session

● Caching/prefetching at the client agent to allow

disconnected operations■ For example, ADO .NET/SQL server CE



Middleware: Client-Server Computing 14

Client-Intercept-Server Model (2)

●

Example – IBM Web Express■ Web Browser (client)↔ HTTP↔ Client-side agent↔ TCP/IP

connection↔ Server side agent↔ HTTP↔ Web server

Mobile Host

Application

Server

FixedNetwork

WirelessNetwork

AgentClient

Client

Side

Agent




Web Services

● Web Services are encapsulated, loosely coupledcontracted functions offered via standard protocols

where:■ Encapsulated – implementation of the function is never seen

from the outside

■ Loosely coupled – changing the implementation of one

function does not require change of the invoking function

■ Contracted – there are publicly available descriptions of the

function’s behavior, how to bind to the function, and its

input and output parameters

From webservices.org, quoted in H. Gunzer, “Introduction to Web Services




Web Services Architecture (1)

Service

Broker

Service

Provider

Service

Requester

inquire publish

bind




Web Services Architecture (2)

● Service broker ■ Look up service between a service provider and a service

requestor

● Service provider ■

Publishes its services to the service broker ● Service requester

■ Asks the service broker where to find a suitable service

provider and then binds itself to the provider




Web Services Components

● XML – Extensible

Markup Language

● SOAP – Simple Object

Access Protocol● WSDL – Web Services

Definition Language

● UDDI – Universal

Discovery Description

Integration

UDDI

WSDL

SOAP

XML

HTTP FTP SMTPWebServ

i ces

XML normally

carried using

HTTP

UDDI is the

“yellow pages”of web services




Extensible Markup Language (XML)

● XML is a platform-neural data standard for describing, storing and transporting data across the

web

● XML is a core technology for web services

■ XML is the basis for separately developed web services tointeract with each other

■ XML facilitates component development

○Package application functionality as small, easily

connected web services

○Compose an application using “web services”components




XML and Microsoft’s .NET

● .NET is a Microsoft technology to develop XML webservices on many platforms■ .NET Active Server Page (ASP) – used to develop web

services

■ .NET Compact Framework (CF) – used to develop web

services on small devices

■ Microsoft Internet Toolkit (MIT) – used to develop server-

side proxies to deal with heterogeneous mobile devices




XML and Wireless Web Access (1)

● The synergy between XML and wireless can beconsidered from two perspectives:■ XML as a data representation technology

■ XML as a meta-language (a set of rules to define domain-

specific data definition languages)

● XML as a data representation technology■ Facilitates content delivery (via XML transformation) to a

wide variety of mobile devices and platforms




XML and Wireless Web Access (2)

● XML as a meta-language■ Key to the success of XML-based wireless web access

initiatives

■ WML – XML-compliant language used by WAP 1.0

■ XHTML – XML-compliant language adopted by WAP 1.2 and

future i-mode

■ Web Service Description Language (WSDL) – XML-compliant

language normally used in a WSDL document to describe a

web service’s interface

○ Provides clients with information on how to invoke a web

service




XML Transformation

● XML separates the datafrom a fixed description

of how the data should

be displayed

●

Extensible Style-SheetLanguage (XLS)

describes how to

display content

●

XSLT is a scheme totransform XML data into

device-specific formats

XML

WML

XHTML

cHTML

XSLT




XSLT to Target XML to Different Devices

WML

cHTML

PDA-specific

<?xml version=‘1.0’?>

<response id=“Result”>

<classECE/CS 4984

</class>

<price>

Invaluable

</price>

</response>

XSLT

for WAP

XSLT

for i-mode

XSLT

for PDA

XSLT

Engine

http://wapsight.com/info/2002/06/17/203731.html

http://wapsight.com/info/2002/06/17/203731.html




SOAP for Invoking Web Services (1)

● Small Object Access Protocol (SOAP) is a standardfor invoking XML web services through remote

procedure calls over HTTP

● Used by Microsoft’s .NET■

.NET Compact Frame implements SOAP 1.1 to allow smartdevices to establish connections and move data to other

devices

UDDI

WDSLSOAP

XML




SOAP for Invoking Web Services (2)

● To make a method call to a web service…■ Client first learns the service interface via the server’s Web

Services Description Language (WSDL) document

■ Client then sends a request to the URL where the service is

running in a SOAP message in XML format for the RPC

UDDI

WDSLSOAP

XML




SOAP and WSDL

● WSDL document specifies■ The name of the method to be invoked

■ The namespace Universal Resource Identifier (URI) for the

method name

○ A namespace is a unique string to fully qualify XML

element and attribute names

■ The parameter names and their data types

■ The SOAPAction

○ Placed in the HTTP header that the client sends when it

invokes the service■ The URL at which the service is running




Accessing Web Services

From http://www.learnxmlws.com/tutors/wsdl/wsdl.aspx

Middleware: Client-Server Computing




WSDL for Defining Web Services

● WSDL is an XML-based language used to…■ Define a web service

■ Describe how to access methods provided by the service

UDDI

WDSL

SOAPXML




Example: WSDL of a Weather Service

● <definitions> element is the root element of the

WSDL document

● Declare the WSDL name space as the default name

space for the document■ All elements and attributes belong to this name space

unless they have another name space prefix

● Each service is defined using a service element

<definitions name ='weatherservice'xmlns='http://schemas.xmlsoap.org/wsdl/'>

<service name='WeatherService' >

…

</service>

</definitions>




31

Port Definitions in WSDL

● Within a service element, you specify the different“ports” on which this service is accessible

● A port specifies the service URL

● Each port has a unique name and a binding attribute

● When using SOAP, the port element contains a<soap:address/> element with the service address

URL




32

Example: WSDL Port Definitions

● If the URL is http://www.ws/demos/wsdl/devxpert/weatherservice.asp, the port definition would look

like …

<port name=‘WeatherSoapPort’

binding=‘wsdlns:WeatherSoapBinding’><soap:address location=

‘http://www.ws/demos/wsdl/devxpert/weatherservice.asp’ />

</port>




33

WSDL Message Element

● To use web services in an RPC model, WSDLdescribes the message structures by using the

<message> element■ Each <message> contains zero or more <part> elements.

■ A <part> corresponds to a parameter or a return value in the

RPC call




34

Example: Messages Elements

● The GetTemperature method would include one inputmessage and one output message

<message name='Weather.GetTemperature'>

<part name='zipcode' type='xsd:string'/>

<part name='celsius' type='xsd:boolean'/></message>

<message name='Weather.GetTemperatureResponse'>

<part name='Result' type='xsd:float'/>

</message>

xsd: float is predefined in http://www.w3.org/2001/XMLSchema




35

WSDL Operation Element

● The WSDL <operation> element is used to specifywhich message is the input and which message is

the output

● The parameterOrder attribute is optional and may be

used to specify a space-delimited list of part namesto indicate the order of parameters when making the

RPC call

● <input> and <output> elements are specified within

an <operation> element■ Each refers to the corresponding message by its fully

qualified name




36

Example: Operation Element

● Example: wsdlns:Weather.GetTemperature

<portType name='WeatherSoapPort'>

<operation name='GetTemperature'

parameterOrder='zipcode celsius'>

<input message='wsdlns:Weather.GetTemperature' />

<output message='wsdlns:Weather.GetTemperatureResponse' />

</operation>



</portType>




37

WSDL Binding Element (1)

● The WSDL <binding> element is used to bind a <port>

● Inside the <binding>, element the <soap:binding>

extension element can specify the transport protocol

and style of the request

■ SOAP can be used over HTTP, SMTP, or, possibly, any other transport

■ RPC and document are the two styles

<binding name='WeatherSoapBinding'

type='wsdlns:WeatherSoapPort' >

…

</binding>




38

WSDL Binding Element (2)

● For each operation that this service exposes, a valueof the SOAP Action is specified in the

<soap:operation> element■ The client must put the SOAP Action value in the HTTP

header when it invokes the service




39

Example: Binding Element

<binding name='WeatherSoapBinding'

type='wsdlns:WeatherSoapPort' >

<soap:binding style='rpc'

transport='http://schemas.xmlsoap.org/soap/http' />

<operation name='GetTemperature' >

<soap:operation

soapAction='http://tempuri.org/action/

Weather.GetTemperature' />

…

</operation>

</binding>




40

WSDL Example I (1)

<binding name='WeatherSoapBinding' type='wsdlns:WeatherSoapPort' ><soap:binding style='rpc'transport='http://schemas.xmlsoap.org/soap/http' />

<operation name='GetTemperature' >

<soap:operation

soapAction='http://tempuri.org/action/Weather.GetTemperature' />

<input>

<soap:body use='encoded' namespace='http://tempuri.org/message/'encodingStyle='http://schemas.xmlsoap.org/soap/encoding/' />

</input>

<output>

<soap:body use='encoded' namespace='http://tempuri.org/message/'

encodingStyle='http://schemas.xmlsoap.org/soap/encoding/' />

</output>

</operation></binding>




41

WSDL Example I (2)

● The <operation> element can…■ Include <input> and an <output> element

■ Use a <soap:body> element within each to specify how the

data is encoded

● The URI http://schemas.xmlsoap.org/soap/encoding/

indicates the SOAP encoding style as described in

the SOAP 1.1 specification




42

WSDL Example II (1)

● WSDL of XMethods, a delayed stock quote XML webservice




43

WSDL Example II (2)

<service name="net.xmethods.services.stockquote.StockQuoteService">

<documentation>net.xmethods.services.stockquote.StockQuote webservice</documentation>

<port name="net.xmethods.services.stockquote.StockQuotePort"binding="tns:net.xmethods.services.stockquote.StockQuoteBinding">

<soap:address location="http://66.28.98.121:9090/soap" />

</port>

</service>

<message name="getQuoteResponse1"><part name="Result" type="xsd:float" />

</message>

<message name="getQuoteRequest1">

<part name="symbol" type="xsd:string" />

</message>

<portType name="net.xmethods.services.stockquote.StockQuotePortType"><operation name="getQuote" parameterOrder="symbol">

<input message="tns:getQuoteRequest1" />

<output message="tns:getQuoteResponse1" />

</operation>

</portType>




Summary

●

Examined client-server computational model,including models for client-server middleware■ Client-server

■ Client-agent-server

■ Client-intercept-server

● Discussed web services

● Examined web services based on XML

UDDI

WSDLSOAP

XML

HTTP FTP SMTP

Documents

Lecture 04 Client Server