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

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

3. 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

4. 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

5. 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

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

7. 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

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

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

10. 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

11. 2 Tier Client server PCs Network Database Server PC or UNIX
Presentation
Application
PCs
Network
Database Server
PC or UNIX
Database
File system

12. 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

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

14. 3 Tier Client/Server PCs Network Servers Presentation Database
File system
Application

15. 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

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

17. 4 Tier Client/Server PCs Network Servers Browser Database File system
Web
Application

18. 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

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

21. 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

22. 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

23. 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)

24. 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

25. 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

26. 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

27. 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

28. 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)

29. 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

30. 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

31. Next week
Basic concepts in computers communicating with each other.