“ A type of system where all users connect to a central server, which is the acting agent for all communications.”
The centralized device is normally known as the server that is responsible of managing all the processing and controlling.
>   Only the administrator has access to the server. All the other clients on the network request the services from the server.
>   The administrator is in charge to assign the rights to these different users/clients that request the services.
>   It is run on a single computer system and do not interact with other computer systems.
>   In short “distributed systems: physically separate computers working together.”

Example Of centralized :

Simple LAN network.

Cloud system.

Distributed System:

                                 A collection of independent computers that appears to its users as a single coherent system”

A distributed system consists of a collection of autonomous computers, connected through a network and distribution middleware, which enables computers to coordinate their activities and to share the resources of the system.

A computer program that runs in a distributed system is called a distributed program, and distributed programming is the process of writing such programs.

Distributed system may have a common goal, such as solving a large computational problem.

Alternatively, each computer may have its own user with individual needs, and the purpose of the distributed system is to coordinate the use of shared resources or provide communication services to the users.

Each computer has its own local memory, and information can be exchanged only by passing messages from one node to another by using the available communication links.
Advantages of Distributed Computing
The important advantage of distributed computing system is reliability. It is more reliable than a single system. If one machine from system crashes, the rest of the computers remain unaffected and the system can survive as a whole.
Incremental Growth
In distributed computing the computer power can be added in small increments i.e. new machines can be added incrementally as per requirements on processing power grow.
Sharing of Resources
Shared data is required to many applications such as banking, reservation system and computer-supported cooperative work. As data or resources are shared in distributed system, it is essential for various applications.
As the system is very flexible, it is very easy to install, implement and debug new services. Each service is equally accessible to every client remote or local.
A distributed computing system can have more computing power than a mainframe.  Its speed makes it different than other systems.
Open system
As it is open system, it can communicate with other systems at anytime. Because of an open system it has an advantage over self-contained system as well as closed system.
It is yet another advantage of distributed computing system. The collection of processors in the system can provide higher performance than a centralized computer.
Disadvantages of Distributed Computing
Troubleshooting and diagnosing problems are the most important disadvantages of distributed computing system. The analysis may require connecting to remote nodes or checking communication between nodes.

Less software support is the main disadvantage of distributed computing system. Because of more software components that comprise a system there is a chance of error occurring.
The underlying network in distributed computing system can cause several problems such as transmission problem, overloading, loss of messages. Hence, the problems created by network infrastructure are the disadvantages of distributed computing.
The easy distributed access in distributed computing system which increases the risk of security. The sharing of data creates the problem of data security.


Ø  INTERNET: It is interconnected collection of computer network of many different types like WAN,
MAN, LAN e.g.

Ø  INTRANET: An intranet is a portion of the internet that is separately administered and has boundary that can be configured to enforce the local security policies. It is composed of several local area network linked by back bone connection. The intranet is connected to the internet via a router, which allows the user inside the intranet to access the services elsewhere such as web or email.

It is an integration of small and portable computing device into distributed system.
Examples of various mobile computing devices are:
a) Laptop computers
b) Handheld device like PDA Personal Digital Assistant, Mobile phone pagers, video camera and digital camera.

This is a computer network that connects several computer workstations together with special software forming a cluster.

In teller machine, the data is shared by the server on different machine of Banks. A concept of distributed system in which all the data of a server in a bank will be stored and also be a shared data to the network.

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