CS6601 DISTRIBUTED SYSTEMS NOTES UNIT I

CS6601 DISTRIBUTED SYSTEMS

UNIT I INTRODUCTION

Definition:

A distributed system is one in which components located at networked computers

communicate and coordinate their actions only by passing messages.

Eg:

Internet and Intranet

Significant Consequences Of Distributed Systems:

Concurrency:

The capacity of the system to handle shared resources can be increased by adding more resources (for example. computers) to the network.

No global clock:

-          When programs need to cooperate they coordinate their actions by exchanging messages. Close coordination often depends on a shared idea of the time at which the programs’ actions occur.

-           But it turns out that there are limits to the accuracy with which the computers in a network can synchronize their clocks – there is no single global notion of the correct time.

Independent failures:

-          Faults in the network result in the isolation of the computers that are connected to it, but that doesn’t mean that they stop running. In fact, the programs on them may not be able to detect whether the network has failed or has become unusually slow.

-          the failure of a computer, or the unexpected termination of a program somewhere in the system (a crash), is not immediately made known to the other components

-          Each component of the system can fail independently, leaving the others still running.

Need for Distributed system:

-          To share resources

-          Sharing of hardware components such as disks and printers to software-defined entities such as files, databases and data objects of all kinds.

-          It includes the stream of video frames that emerges from a digital video camera and the audio connection that a mobile phone call represents.

Tightly coupled system :

-          Parrallel Computing

-          Single primary memory that is shared by all the processors

-          Communication between the processors take place through the shared memory

Loosely coupled system:

-          Processors do not shared memory and each processor has its own local memory

-          Communication between the processors is done by passing messages across network