slides for week 1 - lec 2
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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
Operating system
Proc A
Proc B
Proc C
Operating system
Proc X
Proc Y
Proc Z
Clientprocess
Clientprocess
Serverprocess
Serverprocess
Host 1
Host 2
Proc. A asks Proc. X toPerform a service.
Proc. Z asks Proc. C toPerform a service
Host 1 does not ask Host 2 to perform aService
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 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
Network
Dumbterminals
Centralised
Mainframe orMinicomputer
Basic presentation
Form definitionApplicationDatabaseFile system
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
Network
PCsPC Lan
PC ServerFile system
PresentationApplicationDatabase
Network
PCsPC Lan
File Server
Operating system
Application
Operating system
File system
PC LAN
Weaknesses 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
Network
PCs
2 TierClient server
Database Server PC or UNIX
DatabaseFile system
PresentationApplication
Network
PCs
2 TierClient server
Database Server Windows or UNIX
Proc C
Operating system
File system
Operating system
Application
Database
Two Tier Client Server
Weaknesses Constrained by single server Distributed software Not scalable
Strengths Relatively simple architecture Cheap hardware
Network
PCs
ServersDatabaseFile system
Presentation
3 TierClient/Server
Application
Network
PCs
3 TierClient/Server Proc
C
Operating system
File system
Operating systemPresentatio
n
Process
DatabaseProcess
Proc C
Operating system
Application
Process
Three Tier Client Server
Weaknesses Presentation layer still distributed Complex architecture
Strengths Scalable Consistent Performance Lower bandwidth than 4 tier
Network
PCs
Servers
DatabaseFile system
Browser4 TierClient/Server
ApplicationWeb
Network
PCs
4 TierClient/Server Proc
C
Operating system
Browser
Process
Proc C
Operating system
Application
Process
Operating system
File system
DatabaseProcess
Operating system
WEBServer
Four Tier Client Server
Weaknesses Browser not designed for TP Perhaps higher bandwidth than 3 tier Complex architecture
Strengths Software not distributed Scalable
Network – LAN and WAN
Centralised PC Lan
2 Tier 3 Tier 4 Tier
Client server
Presentation Presentation Presentation
Presentation Presentation
Presentation Presentation
Presentation
Presentation
ApplicationApplication
Application Application
Application
Presentation
DatabaseDatabase
Database
Database DatabaseFile system File system File system File system File system
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.
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