Outline: Distributed Applications

Lecture notes for 15.564: Information Technology IOutline: Distributed Applications Types of Distributed Systems The Client/Server ModelPeer to Peer ModelThe Web as a Client/Server System1Networks Enable Distribution Remote access Resource Sharing Application partitioning Client/ServerNew kinds of applications emailEDI Groupware 2Chrysanthos Dellarocas.Page 1

Lecture notes for 15.564: Information Technology IRemote access Centralized computer power Several remote “dumb” terminals Example: Early airline reservation on3Resource sharing Several stand-alone computers share expensiveperipherals e.g. printers, plotters, scannersExample: Office LANsIBM CompatibleIBM CompatibleIBM CompatibleToken RingLaser PrinterDisk ArrayPlotter4Chrysanthos Dellarocas.Page 2

Lecture notes for 15.564: Information Technology IApplication Partitioning Split the application functionality in severalpieces Place each piece in the machine where it can behandled most efficiently5The Client/Server Model Split application functionality into two pieces: CLIENT Sends requests to server to access network resourcesUsually (but not always) is the piece that interfaces withuserUsually a medium-end PCSERVER-Furnishes clients with application-specific resources- Databases- Huge disk drives- Connections to network-Accepts and responds to requests from several clientsUsually a high-end PC, minicomputer, or mainframe6Chrysanthos Dellarocas.Page 3

Lecture notes for 15.564: Information Technology IExample: Database ServersSQL QUERYDATABASE SERVERDESKTOP PC (CLIENT)QUERY RESULTS- Contains all data- Runs queries, updatestables, and returns results- Runs user interface- Formulates queries anddisplays results7Example: File ServersWorkstationMinicomputerAthena NetworkMinicomputerWorkstationDiskless Athena workstations:- Provide user interface- Execute programsAthena file servers:- Store all programs- Store all user files8Chrysanthos Dellarocas.Page 4

Lecture notes for 15.564: Information Technology IExample: ATM machines(Transaction Servers)TRANSACTIONBANK COMPUTERATMTRANSACTION RESULTS- Checks account balances- Credits or debits customeraccount- Executes local dialogwith customer- Control mechanicalparts of ATM9How E-mail worksMail server ofgates@microsoft.comChris Dellarocas’s PCBill Gates’ PCSender composesmessage and presses“Send”InternetMail server of ChrisDellarocas's emailaddress10Chrysanthos Dellarocas.Page 5

Lecture notes for 15.564: Information Technology IHow E-mail worksMail server ofgates@microsoft.comMessage is sent tosender’s mail serverChris Dellarocas’s PCBill Gates’ PCInternetMail server of ChrisDellarocas's emailaddress11How E-mail worksMail server ofgates@microsoft.comChris Dellarocas’s PCBill Gates’ PCMessage is transferredto recipient’sInternetmail serverand stays there untilrecipient retrieves itChrysanthos Dellarocas.Mail server of ChrisDellarocas's emailrightaddress12Page 6

Lecture notes for 15.564: Information Technology IHow E-mail worksWhen recipient checkshis email, mail clientcontacts mail serverMail server ofgates@microsoft.comBill Gates’ PCChris Dellarocas’s PCInternetMail server of ChrisDellarocas's emailaddress13How E-mail worksMail server ofgates@microsoft.comChris Dellarocas’s PCBill Gates’ PCInternetMailserver sends allunread messagesback to clientMail server of ChrisDellarocas's emailaddress14Chrysanthos Dellarocas.Page 7

Lecture notes for 15.564: Information Technology IHow the Web worksOpen b serverInternetWeb client(Netscape,Explorer, etc.)Domain Name Server15How the Web worksOpen .30.0.22Looking up host:web.mit.edu .Web serverWeb client(Netscape,Explorer, etc.)InternetWhat is theIP address ofweb.mit.edu?Domain Name ServerChrysanthos Dellarocas.16Page 8

Lecture notes for 15.564: Information Technology IHow the Web worksOpen Location:http://web.mit.edu /sloan/www/index.htmlHere it isContacting host:web.mit.edu .Web serverWeb client(Netscape,Please send me fileExplorer, etc.) /sloan/www/index.htmlInternetDomain Name Server17How the Web worksWeb serverWeb client(Netscape,Explorer, etc.)InternetDomain Name ServerChrysanthos Dellarocas.18Page 9

Lecture notes for 15.564: Information Technology IThe WWW as a Client/Server System Web Clients Web Servers Use HTTP protocol to connect to serversRequest and display Web pages stored in serversTypical clients: Web browsersListen for incoming connections from clientsUse HTTP protocol to converse with clientsStore and transmit Web pages to clientsAny machine connected on the Internet can be aWeb client and/or a Web server all It takes is the right software19Client/Server Advantages Price PC networks much cheaper than mainframes ofequivalent computing powerScalability Easy to grow/modernize system as needs change Add clientsUpgrade/add serversVendor-Independence Different system components can come from differentvendors20Chrysanthos Dellarocas.Page 10

Lecture notes for 15.564: Information Technology IClient/Server Advantages (cont’d) Availability If one machine goes down,your business stays upSuperior User Interfaces Since user interface code is executed locally,interfaces can be arbitrarily elaborateEnd users can customize their interfaces to fit theirindividual needs/preferences21Client/Server Disadvantages Maintenance Support tools lacking Parts don’t always work togetherChanges must propagate to all clientsThere are several possible culprits when somethinggoes wrongWith the client/server architecture, you locate or buildtools yourselfEnd User Education Required End users need to know enough to customize theirenvironment22Chrysanthos Dellarocas.Page 11

Lecture notes for 15.564: Information Technology ITypes of Client/Server Systems Issues: How much processing to do locally vs. in the server-PRESENTATION LAYER: User InterfaceAPPLICATION LAYER: Application-specific processingDATA MNG LAYER: Actual storage of data-THIN CLIENTS: Only presentation layerIn how many pieces to split the application-2-tier, 3-tier and multi-tier architectures23Types of Client/Server Systems24Chrysanthos Dellarocas.Page 12

Lecture notes for 15.564: Information Technology IFat Client Systems Client implements presentation and applicationlayer local processing at client side Example: Lotus Notes, Quicken Advantages better server scalability -- server needs to do less workless network trafficDisadvantages client is more complex; difficult to port to differentplatformschanges in server architecture are more likely torequire changes in client25Thin Client Systems Client only implements presentation layer All processing is done at server side Example: WWW Advantages easy to port client to different architectures client is decoupled from changes in the applicationDisadvantages server does all the work; might get easily saturatedpotentially long network delays26Chrysanthos Dellarocas.Page 13

Lecture notes for 15.564: Information Technology I2-tiered vs. multi-tiered architectures Limitations of 2-tiered client/server single serverserver location fixed (otherwise clients need tochange)What if we want to add a second server to share the loadwe want to move the network location of a serverwe want to change our database from Sybase toOracle but do not want to modify all clients27Enter Middleware Set of technologies that “glue” together clientsand servers Examples: Name serversLoad anthos Dellarocas.Page 14

Lecture notes for 15.564: Information Technology IInternet Middleware: Domain Name System Internet hosts are referenced by name but, in reality, IP addresses are numbers florin.mit.edu18.171.0.30Internet has a set of Domain Name Servers thatmap names to IP addresses Each server keeps “authoritative” information for itsassigned domain only (e.g. Australia)Name queries go to server most local to requestor firstLocal server queries remote servers if name does notfall under its “jurisdiction”29How DNS works30Chrysanthos Dellarocas.Page 15

Lecture notes for 15.564: Information Technology IOriginal Web model (Client/Server)Web clientsWeb serverInternet31Peer-to-Peer (P2P) web model2. Bob has itAlice1. Lookingforpicture3. DatatransferBob32Chrysanthos Dellarocas.Page 16

Lecture notes for 15.564: Information Technology IApplications of P2P model File sharing Napster, GnutellaUtilization of spare computing power Auctioning of machine cyclesSETI@home Better information search Highly robust distributed computing No single point of failure33P2P Algorithms (1) Centralized directory model-peers connect to a central directory to publish whatinformation they offer for sharingChrysanthos Dellarocas.Page 17

Lecture notes for 15.564: Information Technology IP2P Algorithms (2) Flooded requests model a peer floods the request to directly connected peers (no centraldirectory) which they further flood their peers etc.until the request is answered or a number of flooding steps areperformedIssues with P2P Everybody wants your files but few people want to give you theirs! Possible solution: Reputation mechanismsWhat is the business model? Who can make money through P2P?36Chrysanthos Dellarocas.Page 18

IBM Compatible IBM Compatible IBM Compatible Laser Printer Plotter Disk Array Chrysanthos Dellarocas. . -THIN CLIENTS: Only presentation layer In how many pieces to split the application-2-tier, 3 -tier and multi-tier architectures 23 24 Types of