Lecture 4 Middleware: Client-Server Computing Wireless and Mobile Systems Design

Lecture 4Middleware: Client-Server

Computing

Wireless and Mobile Systems Design

Middleware: Client-Server Computing 2

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 example client-server applications and rationalize the choice of middleware and design for application development


Resources

● H. Gunzer, “Introduction to Web Services,” March 2002, 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 system and applications that provides a set of services■ Abstract interface to the application■ Uniform view of operating systems, networks, and hardware

platforms

Operating Systemand Networking Services

Middleware

Application ProgramsAPI


Client-Server Computing

Client-Server vs. Peer-to-Peer

Server

• Client initiates communications

• Client issues request to a server

• Server replies or performs some service

Peer-to-Peer Computing

• Any participant can initiate communication

• Any device can generate 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

ApplicationServer

FixedNetwork

WirelessNetwork


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 responses

from 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

ApplicationServer

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

WebBrowser

Server

HTTPServer

CGI,etc.

Gateway

EncodersDecoders

encodedrequest

encodedresponse

request

responseProtocol

Gateways

wireless


Client-Intercept-Server Model (1)

● Inter-agent optimization protocols can be used between 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


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

ApplicationServer

FixedNetwork

WirelessNetwork

AgentClient

ClientSide

Agent


Web Services

● Web Services are encapsulated, loosely coupled contracted 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)

ServiceBroker

ServiceProvider

ServiceRequester

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 SMTP

Web

Ser

vice

s

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 to

interact 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 web services 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 be considered 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 data from a fixed description of how the data should be displayed

● Extensible Style-Sheet Language (XLS) describes how to display content

● XSLT is a scheme to transform 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”> <class ECE/CS 4984 </class> <price> Invaluable </price></response>

XSLTfor WAP

XSLTfor i-mode

XSLTfor PDA

XSLTEngine


SOAP for Invoking Web Services (1)

● Small Object Access Protocol (SOAP) is a standard for invoking XML web services through remote procedure calls over HTTP

● Used by Microsoft’s .NET■ .NET Compact Frame implements SOAP 1.1 to allow smart

devices to establish connections and move data to other devices

UDDI

WDSL

SOAP

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

WDSL

SOAP

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

SOAP

XML


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>


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


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>


WSDL Message Element

● To use web services in an RPC model, WSDL describes 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


Example: Messages Elements

● The GetTemperature method would include one input message 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


WSDL Operation Element

● The WSDL <operation> element is used to specify which 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 names to 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


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>


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>


WSDL Binding Element (2)

● For each operation that this service exposes, a value of 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


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>


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>


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


WSDL Example II (1)

● WSDL of XMethods, a delayed stock quote XML web service


WSDL Example II (2)

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

service</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

UDDIWSDLSOAPXML

HTTP FTP SMTP

Documents

Lecture 4 Middleware: Client-Server Computing Wireless and Mobile Systems Design