Transcript Slides for Week 1

Distributed Information
Systems
- The Client server model
Week 1 – Lecture 2
Client server model
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Any computer connected to a network
is called a host – (a host for a process which
provides or requests a service)
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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
Operating system
Proc
A
Proc
B
Proc
C
Server
process
Proc
X
Proc
Y
Host 2
Proc
Z
Proc. Z asks Proc. C to
Perform a service
Host 1 does not ask
Host 2 to perform a
Service
Operating system
Server
process
Proc. A asks Proc. X to
Perform a service.
Client
process
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
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Presentation
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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
Basic presentation
Dumb
terminals
Network
Mainframe or
Minicomputer
Form definition
Application
Database
File system
Centralised mainframe or
Mini computer
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Weaknesses
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Constrained by single server
Expensive hardware at the time
Strengths
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Low bandwidth network
Architecturally simple
Most hardware & system software from the
one supplier
PC Lan
Presentation
Application
Database
PCs
Network
File system
PC Server
PC Lan
Operating system
PCs
Application
Network
Operating system
File Server
File system
PC LAN
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Weaknesses
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Very high bandwidth – LAN only
Constrained by single server
Software is distributed
Strengths
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Cheap hardware
Provides low cost systems to small
organisations with up to 20 users
2 Tier
Client server
Presentation
Application
PCs
Network
Database Server
PC or UNIX
Database
File system
2 Tier
Client server
Operating system
PCs
Application
Proc
C
Network
Operating system
Database Server
Windows or UNIX
File system
Database
Two Tier Client Server
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Weaknesses
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Constrained by single server
Distributed software
Not scalable
Strengths
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Relatively simple architecture
Cheap hardware
3 Tier
Client/Server
Presentation
PCs
Network
Servers
Application
Database
File system
3 Tier
Client/Server
Operating system
Presentation Proc
Process
C
PCs
Network
Operating system
Application Proc
Process
C
Operating system
File system
Database
Process
Three Tier Client Server
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Weaknesses
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Presentation layer still distributed
Complex architecture
Strengths
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Scalable
Consistent Performance
Lower bandwidth than 4 tier
4 Tier
Client/Server
Browser
PCs
Network
Servers
Web
Application
Database
File system
4 Tier
Client/Server
Operating system
Browser
Process
Proc
C
PCs
Network
Operating system
WEB
Server
Operating system
Application Proc
Process
C
Operating system
File system
Database
Process
Four Tier Client Server
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Weaknesses
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Browser not designed for TP
Perhaps higher bandwidth than 3 tier
Complex architecture
Strengths
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Software not distributed
Scalable
Centralised
Presentation
PC
Lan
Client server
2 Tier
3 Tier
Presentation
Presentation
Presentation
Presentation
Presentation
Presentation
Application
Application
4 Tier
Presentation
Database
Network – LAN and WAN
Presentation
Presentation
Application
Application
Application
Database
File system
File system
Database
Database
Database
File system
File system
File system
The Advantages of distribution
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Scalable
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Open
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Software components can be replaced or added
Heterogenous
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Additional servers can be added as needed
Software can be in different languages, need
different operating systems, run on different
hardware
Fault tolerant
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While multiple servers is inherently more likely to
result in errors, they also provide redundancy
The advantages of being centralised
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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
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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
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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
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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
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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
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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
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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
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Users often come from many different
countries
How does the system adapt to the specific
needs of each user
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Language
Character set
Legal system
Tax laws
Currency
Week 12
Case study
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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
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Basic concepts in computers
communicating with each other.