copyright © curt hill, 2004-2011 client – server computing an important paradigm

Copyright © Curt Hill, 2004-2011

Client – Server Computing

An important paradigm


Historical Perspective• Before the 60s computers were not

usually interconnected• In the mid 1960s time sharing was

developed– One computer– Many terminals– Terminals could be hardwired or

communicate over a modem– This has some of the form of client

server


History II• The 1970s saw the advent of many

important computing innovations– UNIX and many other time sharing

systems– Personal computers– Computer to computer communication

• In the 1980s the Internet starts to emerge

• In the late 1980s the web browser and server are developed– This is a model client server


The Paradigm• Server provides one service• Waits for requests for that service• Client initiates a request• Server replies to the request• Information may flow in both

directions• Many clients may make requests

with just the one server


Why Client - Server?• Specialization

– The client handles the user interface using local computational ability

– The server manages the data and communication

• Sharing– One server may support many clients– Services provided are effectively

shared– Data may be maintained in only one

location


Clients• An application program that runs

on the local computer• Able to use the local computer for

doing work• When invoked by a user, it initiates

communication with a server• Handles all the user interface

issues• Only used for a single session

Example• The ubiquitous client is the web

browser• Several functions:• Communicate with server• Display HTML/XML• Interpret client-side languages,

such as JavaScript• The trend is for the web browser to

be the universal interface to every kind of server on the web



Servers• Specialized program that typically

provides only one service• May handle multiple client requests

simultaneously (or pseudo simultaneously)

• Interacts with users in many sessions– Both simultaneously and over long time

periods

• Waits for a request and then produces the reply

Example

• The web server• Waits for requests using HTTP• Produces results in HTML/XML• May handle many clients

simultaneously



Connection Diagrammed

Server

Client

RequestReply

Client Client

Request

ReplyReplyRequest


Interaction Diagrammed

Client

ServerWait Wait

Wait

• The server is not waiting if it is processing other requests

• The client may also perform other processing while waiting for the reply

Request Reply

Time

Processing


Notation• Not standardized• Is it:• A) Client Server• B) Client/Server• C) Client-Server• D) Client, Server• E) Any of the above


Server Complexity

• Servers are pretty complicated• Usually multi-threaded

– Allows concurrent execution– Often one thread generated for each

request

• Often run on multi-CPU computers• Security is also of great concern


Transport Protocols• Communication between a client

and server must conform to a particular protocol

• Each protocol determines the values of many variables that exist in communications over the Internet

• These include:– Connection type– Port– TCP or UDP


Connection Type• A connection requires the following

actions– Connecting– Several requests and replies– Disconnecting

• The session requires overhead and bookkeeping for the server

• A message passing protocol requires no session or a very abbreviated session


TCP and UDP• The Connection Protocol

– Requires the overhead of a connection

– Guarantees delivery and in the correct order of messages

• User Datagram Protocol– A no connection message– Faster but less reliable

• Both require a port number


Port

• Every internet request to a server must specify a port number

• If a machine is not listening on that port the request is denied

• A request on a valid port may be accepted if the other parts of the protocol are


Port Assignments• Ports in the range 0 – 1023 are

well known ports– These are publicly agreed upon

• Ports in the range 1024 – 49151 are registered ports– Usually registered to support

proprietary services– May be used for other things as well

• Ports in the range 49152 – 65535 are called dynamic or private ports– Available for almost anything


Some well known ports

• 20, 21 – File Transfer Protocol (FTP)

• 23 – Telnet• 25 – Simple Mail Transfer (SMTP)• 53 – Domain Name Server (DNS)• 80 – Hyper Text Transfer Protocol

(HTTP)


Registered and Private

• Registered– Groupwise uses 1677– Cisco uses 1986-1999– Lambda MOO often uses 7777 and

7000


Telnet Protocol

• Used for timesharing on systems such as UNIX– Port 23, TCP

• A connection is made• Usually followed by commands and

replies• A logoff usually forces a disconnect


Hyper Text Transmission Protocol

• A lightweight protocol on port 80• TCP but a session only lasts for one

request• Little bookkeeping on the server end• For more involved transaction

processing it requires cookies or other such processing on client side


File Transfer Protocol

• One of the oldest TCPs around and fading

• Uses 21 for command information• Uses 20 for data transfer


Domain Name Service

• A common UDP, 53• Broadcast a request to resolve a

name• Any name server may reply to the

request• If some requests are lost, so what?


Lambda MOO

• Uses Telnet protocol at port specified by the server only– Here it may be 7777 or 8888

• Lambda MOO is a server – only responds to requests

• However, it may broadcast appropriate messages to any connected client


Multiple tiers• The classic client-server is a two

tier system• The client is one tier and the server

another• Three or more tiers are also

possible• The middle server functions as a

– Server to the client– Client to other servers– Known as middleware

• Often used to distribute load


A Three Tier Approach

Server

Application Server

Client ClientClientClient

ServerServer Server


Examples of Multi-Tier• Authentication server

– Maintains a connection to verify user– Funnels all requests to appropriate

server

• Shopping cart– Records purchases– The product lookup/purchase handled

by other servers

• Distributed Database Management System


Horizontal Distribution• Multi-Tier is a vertical distribution• A Horizontal distribution may also

be effective• A single server owns the web

address: www.x.com• It then distributes all further

requests to a rotation of identical web servers– www1.x.com– www2.x.com– …– wwwN.x.com

http://www.x.com/


Horizontal Distribution

www.x www1.x wwwN.xwww2.x …

Client ClientClientClientClientClient

FirstRequest Subsequent

Request


Alternatives to Client-Server

• Group multicast• Peer to peer• Publish-Subscribe• Several types of distributed system• Among others


Peer to Peer

• An alternative to client-server• Has a symmetric relationship• Each peer provides all the same

services and makes all the same requests as any other peer

Finally

• The importance of the client-server paradigm will only grow

• This is the model the Internet and WWW is based upon

• Cloud computing is all about moving applications from the local computing paradigm to the client server paradigm


copyright © curt hill, 2004-2011 client – server computing an important paradigm

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