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

Characteristics of the 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 that 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 a distributed system:

• autonomous components


• Mostly build using heterogeneous technology


• System components may be used exclusively


• Concurrent processes can execute


• Multiple points of failure

 

The requirement of distributed system:

1. Scalability- the 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- the 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 the order of events is difficult
  
  
  
  


• Distrib-systems can have race conditions
  
  
  
  • Race conditions