Distributed Information Systems - The Client server model Week 1 – Lecture 2

Client server model Any computer connected to a network is called a host – (a host for a process which provides or requests a service) The process on a client host requests a service from a process on the server host A process is a program running within a host

Host 1 Client process Server process Server process Client process Proc. A asks Proc. X to Perform a service. Proc. Z asks Proc. C to Perform a service Host 1 does not ask Host 2 to perform a Service Client process Operating system Server process Proc A Proc B Proc C Operating system Server process Proc X Proc Y Proc Z Client process Host 2

Main frame or mini with dumb terminals A brief history of IS 1970’s 1980’s 1990’s 2000’s Main frame or mini with dumb terminals LAN server & PCs 2 tier Client Server 3 & 4 tier Client server

Application layers Presentation Application – the business logic Formatting & displaying data Displaying a form & accepting data Checking the validity of the data entered Application – the business logic Database – storing and locating data File system - recording and reading records on disk

Centralised Dumb terminals Network Mainframe or Minicomputer Basic presentation Dumb terminals Network Form definition Application Database File system Mainframe or Minicomputer

Centralised mainframe or Mini computer Weaknesses Constrained by single server Expensive hardware at the time Strengths Low bandwidth network Architecturally simple Most hardware & system software from the one supplier

PC Lan PCs Network PC Server Presentation Application Database File system PC Server

PC Lan PCs Network File Server Application File system Operating system PCs Application Network Operating system File Server File system

PC LAN Weaknesses Strengths Very high bandwidth – LAN only Constrained by single server Software is distributed Strengths Cheap hardware Provides low cost systems to small organisations with up to 20 users

2 Tier Client server PCs Network Database Server PC or UNIX Presentation Application PCs Network Database Server PC or UNIX Database File system

2 Tier Client server PCs Network Database Server Windows or UNIX Operating system PCs Proc C Application Network Operating system Database Server Windows or UNIX File system Database

Two Tier Client Server Weaknesses Strengths Constrained by single server Distributed software Not scalable Strengths Relatively simple architecture Cheap hardware

3 Tier Client/Server PCs Network Servers Presentation Database File system Application

3 Tier Client/Server PCs Network Presentation Process File system Operating system Presentation Process Proc C PCs Network Operating system Operating system File system Application Process Proc C Database Process

Three Tier Client Server Weaknesses Presentation layer still distributed Complex architecture Strengths Scalable Consistent Performance Lower bandwidth than 4 tier

4 Tier Client/Server PCs Network Servers Browser Database File system Web Application

4 Tier Client/Server PCs Network Browser Process File system Operating system Browser Process Proc C PCs Network Operating system Operating system Operating system File system Application Process Proc C WEB Server Database Process

Four Tier Client Server Weaknesses Browser not designed for TP Perhaps higher bandwidth than 3 tier Complex architecture Strengths Software not distributed Scalable

The Advantages of distribution Scalable Additional servers can be added as needed Open Software components can be replaced or added Heterogenous Software can be in different languages, need different operating systems, run on different hardware Fault tolerant While multiple servers is inherently more likely to result in errors, they also provide redundancy

The advantages of being centralised Architecturally simpler All software and hardware from one supplier Eliminates the overhead of distribution Easier to control security On average, requires less bandwidth

Communications & Networks How does the client host find the server host ? Having found the server host, how are messages reliably passed between them, over telephone lines, wireless links, optical fibre cables? This is the subject matter of communications and networks (weeks 2-5)

Distributed Components How does the client know what server can perform the service? What format does the client use to pass the parameters to the server? Week 6

Integrating disparate systems Organisations have many separate systems Systems in one organisation have to integrate with systems in another Systems are usually developed in complete isolation from each other – often many years apart. They use different data definitions, different coding structures, different data values These systems still have to work together. Week 7 - 9

Security How do we prevent unauthorised users accessing the system? How do we prevent authorised users doing things they should not, or seeing data they must not? How do we identify and authenticate a user? How do we pass messages between systems without those messages being intercepted? Week 10

Server platform There are many different operating systems, computer suppliers, types of middleware etc What are the critical differences? How do we select the right software and hardware to meet our system needs? Week 11

Performance, scalability & reliability Most DIS have many concurrent users How do we design a system to give the required level of performance The number of users and the quantity of data will increase, how do we add hardware and change the software to accommodate those extra users and maintain the required levels of performance & reliability? (week 11)

International differences Users often come from many different countries How does the system adapt to the specific needs of each user Language Character set Legal system Tax laws Currency Week 12

Case study Rory Collins – ex CIO of the ASX Will discuss a major project to implement a new system for the Singapore Stock Exchange Week 12

Next week Basic concepts in computers communicating with each other.