hi
2-tier - also called client/server applications which 
consist of two separate applications, one server 
application (typically a database server like Oracle, MSSQL 
or any other standalone database) and a client application.
suppose  you want to have many different clients for front-
end would be unwise to copy all the business logic (all the 
rules of your application) to each client app.,. so its 
better to create a middle layer containing the business 
logic, and then let the clients talk with this middle 
layer. Thus the clients can relatively easily be changed 
for other types of clients, without need for remembering 
how was the business logic put together.

3-tier architeture where one tier is the backend database 
engine, one is the front end client and one is just in 
between them, also named the application server or business 
logic node.
its uses mainly Load balancing and Fault tolerance.

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my knowledge.
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Thanks Ravi, for a short but a very effective explanation.

very good Ravi
thanks

is a web application 2 tier or three tier?

thanks ravi

very good ravi,thanks

2 tier architecture is a client-server application...
3 tier architecture is web application.

I Agree with the point one but not agreed with the next one

"its uses mainly Load balancing and Fault tolerance."

It is not like all the tier 3 system will use the above.

Loadbalancing and fault tolerance will increase the 
availablility and scalablity of the System

2 tier is client server application or in 2 tier one layer 
acts as web+App and another as data layer.
in 3 tier one is web layer 2nd is app layer and third is 
data layer.
web layer = where client end code resides
app layer = where business logic of application resides
data layer  = database layer where database resides

In 2 tier architecture the application is splitted in to 
two parts which is located in two machines called as client 
and server that is why it is also known as client server 
architecture..in this architecture the business logic 
resides either with the client or with the server where 
database resides, so if client machine  contains the 
business logic then is termed as Fat client coz it has to 
look after the client related as well as business related 
issues where as if business logic resides with the database 
server then it is termed as Fat Server since it has to 
manage the database related as well as business related 
issues so some how either the client or the server 
performance hampers.


in 3 tier architecture the application is physically 
divided and located in  to three different machine one 
takes care only about clientrelated issues known as client 
tier, one second takes care about all the business logic 
related issus and input out taken from the client tier 
known as middle tier and the third contains the database 
and responsible for managing and manipulating all the 
database related issues callled as database tier so this 
architecture is basically used for load balancing and fault 
tolerence.

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 Thanks.