3-Tier Client Server Model

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Fig Showing 2 Tier Client Server Model

Through the appearance of Local-Area-Networks, PCs came out of their isolation, and
were soon not only being connected mutually but also to servers. Client/Server-
computing was born. A

two-tiered application is an application whose functionality can

only be segmented into two logical tiers, presentation services and data

services. The

presentation services of a two-tiered application are responsible for gathering information
from the user, interacting with the data services to perform the application's business
operations, and presenting the results of those operations to the user.

The Presentation

services are also called the presentation layer because it presents information to the user.
Things you might find in a presentation layer include a Web browser, a terminal, a
custom-designed GUI, or even a character-based user interface

Client-Server

architecture was a major buzzword in the early 90's, taking initially dumb terminal
applications and giving them a fancy windows-like front end, using PCs with terminal
emulators which presented pretty GUIs (Graphical user interface) or later Visual Basic
etc front-ends. A web browser talking to a web server is an example of a client talking to
a server. Here there is presentation logic (presentation tier) happening at the client, and
data/file access (data access tier) and logic happening at the server.

One reason why the

2-tier model is so widespread is because of the quality of the tools and middleware that
have been most commonly used since the 90's: Remote-SQL, ODBC, relatively
inexpensive and well-integrated PC-tools (like Visual Basic, Power-Builder, MS Access,
4-GL-Tools by the DBMS manufactures). In comparison the server side uses relatively
expensive tools. In addition the PC-based tools show good Rapid-Application-
Development (RAD) qualities i.e. simpler applications can be produced in a
comparatively short time. The 2-tier model is the logical consequence of the RAD-tools'
popularity.

3 Tier: Client Server

Browser based Interface
html /xml
J

Win32 Client
Applications

(Vi

l B i f

)

Sql

Server

Oracle

RDBMS

Mail

Server

File

System

Business Layer

Data Service Layer

Presentation Layer

COM /
COM +

COM /
ASP

IIS / Apache

Business Rules and Process

HTTP