Distributed System: A distributed system is A collection of independent computers that appears to its users as a single coherent system. Distributed computing is a field of computer science that studies distributed systems. A distributed system consists of multiple autonomous computers that communicate through a computer network. The computers interact with each other in order to achieve a common goal.

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 computing also refers to the use of distributed systems to solve computational problems. In distributed computing, a problem is divided into many tasks, each of which is solved by one or more computers.

Some examples of distributed systems :

• Local Area Network and Intranet


• Database Management System


• Automatic Teller Machine Network


• Internet/World-Wide Web


• Mobile and Ubiquitous Computing

Centralized System: Centralized systems are systems that use client/server architecture where one or more client nodes are directly connected to a central server. This is the most commonly used type of system in many organisations where client sends a request to a company server and receives the response.

Centralized System Characteristics:

• One component with non-autonomous parts


• Component shared by users all the time


• All resources accessible


• Software runs in a single process


• Single point of control


• Single point of failure

Distributed System Characteristics

• Multiple autonomous components


• Components are not shared by all users


• Resources may not be accessible


• Software runs in concurrent processes on different processors


• Multiple points of control


• Multiple points of failure

Advantages of Distributed Systems

• Economics: a collection of microprocessors offer a better price/performance than mainframes. Low price/performance ratio: cost effective way to increase computing power.


• Speed: a distributed system may have more total computing power than a mainframe.


• Inherent distribution: Some applications are inherently distributed. Ex. a supermarket chain.


• Reliability: If one machine crashes, the system as a whole can still survive. Higher availability and improved reliability.


• Incremental growth: Computing power can be added in small increments.


• Modular expandability


• Another deriving force: the existence of large number of personal computers, the need for people to collaborate and share information.

Advantages of Centralized System:

• Easy to physically secure. It is easy to secure and service the server and client nodes by virtue of their location


• Smooth and elegant personal experience – A client has a dedicated system which he uses(for example, a personal computer) and the company has a similar system which can be modified to suit custom needs


• Dedicated resources (memory, CPU cores, etc)


• More cost efficient for small systems upto a certain limit – As the central systems take less funds to set up, they have an edge when small systems have to be built


• Quick updates are possible – Only one machine to update.


• Easy detachment of a node from the system. Just remove the connection of the client node from the server and voila! Node detached.

Issues of Distributed Systems

• Transparency


• Flexibility


• Reliability


• Performance


• Scalability

Issues of Centralized System

• Highly dependent on the network connectivity – System can fail if the nodes lose connectivity as there is only one central node.


• No graceful degradation of system – abrupt failure of the entire system


• Less possibility of data backup. If the server node fails and there is no backup, you lose the data straight away


• Difficult server maintenance – There is only one server node and due to availability reasons, it is inefficient and unprofessional to take the server down for maintenance. So, updates have to be done on-the-fly(hot updates) which is difficult and the system could break.