Difference between centralized and distributed computing

Centralized

A system with centralized multiprocessor parallel architecture.In the late 1980 s Centralized systems have been progressively replaced by distributed systems.

characteristics of centralized system

• Non autonomous components
• usually homogeneous technology
• Multiple users share the same resources at all time
• single point of control
• single point of failure

Distributed

set of tightly coupled programs executing on one or more computers which are interconnected through a network and coordinating their actions. These programs know about one another and carry out tasks that none could carry out in isolation

characteristics of distributed system

• autonomous components
• Mostly build using heterogeneous technology
• System components may be used exclusively
• Concurrent processes can execute
• Multiple point of failure

Requirement of distributed system

1. Scalability- possibility of adding new hosts
2. openness- easily extended and modified
3. Heterogeneity-supports various H/W S/w platforms
4. Resource sharing- H/w, S/W and data
5. fault tolerance- ability to function correctly even if faults occur

Main basic differences are:

• distrib-systems have no global state
  • no shared memory
  • no shared variables
• distrib-systems have no shared time clock
  • therefore order of events is difficult
• distrib-systems can have race conditions
  • race conditions see http://en.wikipedia.org/wiki/Race_condition

So "computing" in a distrubuted environment is very difficult. Do you have concret question about programing models or whatever?

Centralized Systems

"In Centralized Systems,several jobs are done on a particular central processing unit(CPU)"

Distributed Systems

"In Distributed Systems,jobs are distributed among several processor.The Processor are interconnected by a computer network"

(Sheheryar ,NUML)

Briefly, Centralized computing, as the name itself depicts, is concerned with just a single server. The particular operation is being held at this server location and nowhere else.

Distributed computing is held where the system requirement is quite large, and the job is distributed to several processors and the solutions are then combined together, keeping in mind that the processors are interconnected by a computer network.

centralized system:is a system which computing is done at central location using terminals attached to central computer in brief (mainframe and dump terminals all computation is done on the mainframe through terminals )

distributed system:is a collection of independent computers that appear to its users as single coherent system where hardware is distributed consisting of n processing elements (processor and memory )also software is distributed where no centralized os each processing element has its own os ,no physically centralized file system and inter-process communication via message passing at lowest level

Big Note:the main differences is reliability. in distributed system if one machine crashes,the system as a whole can still survive

METHOD OF ARBITRATION In all but the simplest systems, more than one module may need control of the bus.

In a centralized scheme, a single hardware device, referred to as a bus controller or arbiter, is responsible for allocating time on the bus.

In a distributed scheme, there is no central controller. Rather, each module contains access control logic and the modules act together to share the bus.

in centralized system in case the server fails it affects the whole system because the server controls the whole operation in D.S system incase a system fails it doesn't affect the operations of the other computers because they are independent and distributed in operations

Let us try to understand this with an example.

Say you are carrying a large amount of money. You are in a crowded train, where your pocket may be picked and you might lose money. What is the ideal strategy for carrying money?

Put all money in a single pocket: In this case, it is easy for you to just put the money in the pocket and be done. When you go back home, you can simply take out money from the pocket and count it. But wait. What if your pocket is picked? You lose ALL the money (bankrupt? eh!). Seems like it is not the best idea to store all the money in a SINGLE pocket. Let us think what else we can do Divide your money: Put some of it in the left pocket, put some in the right pocket and maybe put some in your bag (which has a limited capacity). You need to devise a strategy to divide the money with you. Also, when you go back home, you will have to spend time collecting money from different pockets and collecting it at one place. However, we are in a better situation now. If one of our pocket (or bag) is picked, we do not lose ALL of the money. The chances of your bag, left pocket and the right pocket, all being picked is fairly low. With a little overhead of dividing money, you can now avoid losing all of your money. Isn’t that better? This is how distributed systems work. They divide the information (money in your case) and keep it on different machines (pockets and bags for us). This way if one of the machine goes down, we are not at a big loss. That is, we do not have a single point of failure

Another important thing that distributed systems implement is data replication. They put replicas of same information in multiple machines. This way, if one of the machines goes down, we do not lose the information. So, we now have something called as fault tolerance.