The basic characteristics of client/server architectures 
are:

1) combination of a client or front-end portion that 
interacts with the user,and a server or back-end portion 
that interacts with the shared resource. 
The client process contains solution-specific logic and 
provides the interface between the user and the rest of the 
application system. The server process acts  as a software 
engine that manages shared resources such as databases, 
printers, modems, or high powered processors. 

2) the front-end task and back-end task have fundamentally 
different requirements for computing resources such as 
processor speeds, memory, disk speeds and capacities, and 
input/output devices. 

3) the environment is typically heterogeneous and 
multivendor. The hardware platform and operating system of 
client and server are not usually the same.Client and 
server processes communicate through a well-defined set of 
standard application program interfaces (API's) and 
RPC's.

4) An important characteristic of client-server systems is 
scalability. They can be scaled horizontally or vertically. 
Horizontal scaling means adding or removing client 
workstations with only a slight performance impact. 
Vertical scaling means migrating to a larger and faster 
server machine or multiservers.

Advantages of client-server architecture:-


In most cases, client-server architecture enables the roles
and responsibilities of a computing system to be distributed
among several independent computers that are known to each
other only through a network. This creates an additional
advantage to this architecture: greater ease of maintenance.
For example, it is possible to replace, repair, upgrade, or
even relocate a server while its clients remain both unaware
and unaffected by that change. This independence from change
is also referred to as encapsulation.
All the data is stored on the servers, which generally have
far greater security controls than most clients. Servers can
better control access and resources, to guarantee that only
those clients with the appropriate permissions may access
and change data.
Since data storage is centralized, updates to those data are
far easier to administer.
It functions with multiple different clients of different
capabilities.




Disadvantages of client-server architecture :-


Traffic congestion on the network has been an issue since
the inception of the client-server paradigm. As the number
of simultaneous client requests to a given server increases,
the server can become severely overloaded.
The client-server paradigm lacks the robustness. Under
client-server, should a critical server fail, clients’
requests cannot be fulfilled.

The explosion of the World Wide Web is due to the world-
wide acceptance of a common transport (TCP/IP), server 
standard (HTTP), and markup language (HTML).  Many 
corporations have discovered that these same
technologies can be used for internal client/server 
applications with the same ease that they are used on the 
Internet.  Thus was born the concept of the "Intranet": the 
use of Internet technologies for implementing internal 
client/server applications.

One key advantage of Web-based intranets is that the 
problem of managing code on the client is greatly reduced.  
Assuming a standard browser on the desktop, all changes to 
user interface and functionality can be done by changing 
code on the HTTP server.  Compare this with the cost of
updating client code on 2,000 desktops.

A second advantage is that if the corporation is already 
using the Internet, no additional code needs to be licensed 
or installed on client desktops.  To the user, the internal 
and external information servers appear integrated.

A rapidly-disappearing disadvantage is that there is 
limited ability to provide custom coding on the client.  In 
the early days of the Web, there were limited ways of 
interacting with the client.  The Web was essentially "read-
only".  With the release of code tools such as Java
and _JavaScript, this limitation is no longer a major issue.

The distribution of applications and business logic across 
multiple processing platforms.  Distributed processing 
implies that processing will occur on more than one 
processor in order for a transaction to be completed. In 
other words, processing is distributed across two or more 
machines and the processes are most likely not running at 
the same time, i.e. each process performs part of an 
application in a sequence. Often the data used in a 
distributed processing environment is also distributed 
across platforms.

Cooperative processing is computing which requires two or 
more distinct processors to complete a single transaction.  
Cooperative processing is related to both distributed and 
client/server processing. It is a form of distributed 
computing where two or more distinct processes are required 
to complete a single business transaction. Usually, these 
programs interact and execute concurrently on different 
processors. Cooperative processing can also be considered 
to be a style of client/server processing if communication 
between processors is performed through a message 
passing architecture. 

Middleware is distributed software needed to support 
interaction between clients and servers. In short, it is 
the software that is in the middle of the Client/Server 
systems and it acts as a bridge between the clients and 
servers. It starts with the API set on the client side that 
is used to invoke a service and it covers the transmission 
of the request over the network and the resulting response. 
It neither includes the software that provides the actual 
service - that is in the servers’ domain nor the user 
interface or the application login - that's in client’s 
domain

•	General Middleware
•	Service-specific middleware.

A three-tier architecture introduces a server (or 
an "agent") between the client and the server.  The role of 
the agent is manyfold.  It can provide translation services 
(as in adapting a legacy application on a mainframe to a 
client/server environment), metering services (as in
acting as a transaction monitor to limit the number of 
simultaneous requests to a given server), or intellegent 
agent services (as in mapping a request to a number of 
different servers, collating the results, and returning a 
single response to the client.

Three-tier architecture also known as MVC architecture in 
which software design is split into three groups as Model, 
View and Controller part of UI goes in model section while 
Business logic goes in controller section and data access 
layer goes in view layer. 
Now consider the view or data access layer(DAL) in which 
operations such as data access from database insertion such 
type of operations are kept in this section. In Controller 
known as business logic layer in which all logic related to 
UI and DAL is implemented here and accessed from UI using 
instance of objects. Finally in UI only UI related code is 
written here.

A two-tier architecture is where a client talks directly to 
a server, with no intervening server.  It is typically used 
in small environments (less than 50 users).

A common error in client/server development is to prototype 
an application in a small, two-tier environment, and then 
scale up by simply adding more users to the server.  This 
approach will usually result in an ineffective system, as 
the server becomes overwhelmed.  To properly scale to 
hundreds or thousands of users, it is usually necessary to 
move to a three-tier architecture.

A server process (program) fulfills the client request by 
performing the task requested. Server programs generally 
receive requests from client programs, execute database 
retrieval and updates, manage data integrity and dispatch 
responses to client requests. Sometimes server programs 
execute common or complex business logic. The server-based 
process "may" run on another machine on the network. This  
server could be the host operating system or network file 
server; the server is then provided both file system 
services and application services. Or in some cases,
another desktop machine provides the application services. 
The server process acts as a software engine that manages 
shared resources such as databases, printers, communication 
links, or high powered-processors. The server process 
performs the back-end tasks that are common to similar 
applications.  

The client is a process (program) that sends a message to a 
server process (program), requesting that the server 
perform a task (service).Client programs usually manage the 
user-interface portion of the application, validate data 
entered by the user, dispatch requests to server programs, 
and sometimes execute business logic. The client-based
process is the front- end of the application that the user 
sees and interacts with. The client process contains 
solution-specific logic and provides the interface between 
the user and the rest of the application system. The client 
process also manages the local resources that the user 
interacts with such as the monitor, keyboard, workstation 
CPU and peripherals. One of the key elements of a client 
workstation is the graphical user interface (GUI).   
Normally a part of operating system i.e. the window manager 
detects user actions, manages the windows on the display 
and displays the data in the windows.

Client/server is a computational architecture that involves 
client processes requesting service from server processes.