[Tutor] client server apps

[Alan Gauld]

"Noufal Ibrahim" <noufal at airtelbroadband.in> wrote

>> What's the point in having a server app between the user frontend 
>> and
>> a database backend?

> Off the cuff,  I think the layer would decouple your store (the db) 
> and
> the UI.

That's part of it but not the main reason. N-Teir was being touted 
long
before web clients had been heard of!

> On the negative side, it's one more thing that needs to be written 
> and
> maintained and is a point of failure especially when large number of
> clients connect simultaneously.

Actually the latter scenario is the main reason for going 3-teir
client server. The middle teir can be replicateed and thus a 3 teir
(or more generally an N-Teir) application will scale much better.
Most 2-teir C/S apps runout of steam with a few hundred
concurrent users due to problems with the database locking
and other types of contention. With a mid teir doing the business
rules andcalculations you can generally wscale 3-teir up to 10's
of thousands of concurrent users.

Also because you have multiple servers for 10,000 users - say each
handles 1000 users - then if one server goes down the rest keep
functioning and you only lose service to 10% of your users. Even
better if you have a layer of middleware doing the load balancing
the users won't even know the server failed because their requests
will be bounced to another server instance. The worst that happens
is a time-out and an error message for the transaction that was
running at the point of failure.

Multi-teir also gives you much more flexibility over where to put the
processing. Databases are very good at searching and sorting, but
not usually very good at high intensity calculations. So by moving
the math into the server and keeping the data access in the
database you optimise performance. You can also create services
that are usable across a range of applications using multiple
databases - eg an image processing server. But that takes us
into the realms of Service Oriented Architectures(SOA)

Further, where an application is dispersed geographically an N-Teir
solution can dramatically improve performance by reducing
network delays. For example I worked on a customer service app
for a global company with offices in Asia, Africa, the Middle East,
Europe and North America. The main database was on a mainframe
in Europe but we had intermediate app servers in South Africa,
Australia (for all Asia) , two in the USA,  and two in Europe(UK & 
Italy).
During the trials (with only one server in the UK) we were getting
5 second plus transaction times in the US and Asia but after
distributing the servers we got 1-2 seconds everywhere. Of course
to make that work you have to design the transaction sequences to
maximise caching etc, it's not automatic, but done right it can be
a major boost to performance.

Finally, by using intelligent caching, intermediate servers can
partially protect from a database failure, since many transactions
(especially read-only) can be carried out using most recently used
data in a cache. This will typically be adequate to cover the
downtime between the database going down and a cold
standby being booted into life (say 10-20 minutes) and the
network reconfigured.

HTH,

-- 
Alan Gauld
Author of the Learn to Program web site
http://www.freenetpages.co.uk/hp/alan.gauld