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

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

2 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

3 Middleware: Client-Server Computing 3 Resources ● H. Gunzer, “Introduction to Web Services,” March 2002, webservices.pdf ● Microsoft Developer Network, “Web Services Basics,” understanding/webservicebasics/

4 Middleware: Client-Server Computing 4 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)

5 Middleware: Client-Server Computing 5 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 System and Networking Services Middleware Application Programs API

6 Middleware: Client-Server Computing 6 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

7 Middleware: Client-Server Computing 7 Client-Server Models for Middleware ● Client-server ● Client-agent-server ● Client-intercept-server models

8 Middleware: Client-Server Computing 8 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

9 Middleware: Client-Server Computing 9 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 Application Client Application Server Fixed Network Wireless Network

10 Middleware: Client-Server Computing 10 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

11 Middleware: Client-Server Computing 11 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 Application Client Application Server Fixed Network Wireless Network Agent

12 Middleware: Client-Server Computing 12 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 encoded request encoded response request response Protocol Gateways wireless

13 Middleware: Client-Server Computing 13 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

14 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 Fixed Network Wireless Network Agent Client Side Agent

15 Middleware: Client-Server Computing 15 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, quoted in H. Gunzer, “Introduction to Web Services

16 Middleware: Client-Server Computing 16 Web Services Architecture (1) Service Broker Service Provider Service Requester inquirepublish bind

17 Middleware: Client-Server Computing 17 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

18 Middleware: Client-Server Computing 18 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 HTTPFTPSMTP Web Services XML normally carried using HTTP UDDI is the “yellow pages” of web services

19 Middleware: Client-Server Computing 19 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

20 Middleware: Client-Server Computing 20 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

21 Middleware: Client-Server Computing 21 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

22 Middleware: Client-Server Computing 22 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

23 Middleware: Client-Server Computing 23 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

24 Middleware: Client-Server Computing 24 XSLT to Target XML to Different Devices WML cHTML PDA-specific { "@context": "", "@type": "ImageObject", "contentUrl": "", "name": "Middleware: Client-Server Computing 24 XSLT to Target XML to Different Devices WML cHTML PDA-specific

25 Middleware: Client-Server Computing 25 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

26 Middleware: Client-Server Computing 26 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

27 Middleware: Client-Server Computing 27 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

28 Middleware: Client-Server Computing 28 Accessing Web Services From Middleware: Client-Server Computing

29 Middleware: Client-Server Computing 29 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

30 Middleware: Client-Server Computing 30 Example: WSDL of a Weather Service ● 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

31 Middleware: Client-Server Computing 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 element with the service address URL

32 Middleware: Client-Server Computing 32 Example: WSDL Port Definitions ● If the URL is weatherservice.asp, the port definition would look like …

33 Middleware: Client-Server Computing 33 WSDL Message Element ● To use web services in an RPC model, WSDL describes the message structures by using the element ■ Each contains zero or more elements. ■ A corresponds to a parameter or a return value in the RPC call

34 Middleware: Client-Server Computing 34 Example: Messages Elements ● The GetTemperature method would include one input message and one output message xsd: float is predefined in

35 Middleware: Client-Server Computing 35 WSDL Operation Element ● The WSDL 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 ● and elements are specified within an element ■ Each refers to the corresponding message by its fully qualified name

36 Middleware: Client-Server Computing 36 Example: Operation Element ● Example: wsdlns:Weather.GetTemperature

37 Middleware: Client-Server Computing 37 WSDL Binding Element (1) ● The WSDL element is used to bind a ● Inside the, element the 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 …

38 Middleware: Client-Server Computing 38 WSDL Binding Element (2) ● For each operation that this service exposes, a value of the SOAP Action is specified in the element ■ The client must put the SOAP Action value in the HTTP header when it invokes the service

39 Middleware: Client-Server Computing 39 Example: Binding Element …

40 Middleware: Client-Server Computing 40 WSDL Example I (1)

41 Middleware: Client-Server Computing 41 WSDL Example I (2) ● The element can… ■ Include and an element ■ Use a element within each to specify how the data is encoded ● The URI indicates the SOAP encoding style as described in the SOAP 1.1 specification

42 Middleware: Client-Server Computing 42 WSDL Example II (1) ● WSDL of XMethods, a delayed stock quote XML web service

43 Middleware: Client-Server Computing 43 WSDL Example II (2) web service

44 Middleware: Client-Server Computing 44 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 WSDL SOAP XML HTTPFTPSMTP

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