UNIT 2: SYSTEM Models

Software and hardware service layers in distributed systems

Applications, services

Computer and network hardware

Platform

Operating system

Middleware

PLATFORM

•The bottom layers (os, computer , network hardware layers) serve as platform for distributed applications.•The service provided by these bottom layers is utilized by higher layers.•Important examples are

•Intel X86/linux•Intel X86/Solaries•Intel X86/Windows

•Powe PC/Mac OS X

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MIDDLEWARE

•A piece of software placed between the application and operating system•The objects or processes in a set of computers that interact with each other to carryout communication and resource sharing support for distributed application•Object oriented middleware products are

•CORBA•JAVA RMI

•DCOM•WEB SERVICES

•ISO/ITU-T REFERENCE MODEL FOR OPEN DISTRIBUTED PROCESSING

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Figure 2.1Generations of distributed systems

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Figure 2.2Communicating entities and communication paradigms

Figure 2.3Clients invoke individual servers

Server

Client

Client

invocation

result

Serverinvocation

result

Process:Key:

Computer:

Figure 2.4aPeer-to-peer architecture

Figure 2.4bA service provided by multiple servers

Server

Server

Server

Service

Client

Client

Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012

Figure 2.5Web proxy server

Client

Proxy

Web

server

Web

server

serverClient

Figure 2.6Web applets

a) client request results in the downloading of applet code

Web server

ClientWeb serverApplet

Applet code

Client

b) client interacts with the applet

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Figure 2.8Two-tier and three-tier architectures

MOBILE DEVICES

• These are hardware computing components that carry software and are freely movable between physical locations and networks.

• Examples• Laptops,handheld devices, mobile phones,PDA,digital cameras,smart

watches

• Spontaneous Interoperation refers to the variation on the client server model, where the creation and destruction of connection among devices take place regularly.

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Performance issues

• Responsiveness• Interactive application user always expect fast and

consistent response

• Throughput• It is the rate of completing the computational task

• Balancing computational loads• It should enable applications and service processes.

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Quality of service provided in distributed system

• Adaptability and resource availability are characteristics of service quality.

• QOS gets affected by non functional properties such as reliability , security and performance.

• Service quality can be achieved based on availability

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Dependebility issues

• Fault Tolerance• Ability to continue its function even in presence of faults

that may arise in software, hardware and networks • Reliability can be obtained by means of redundancy

• Security• Securing confidentials data from attacks

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SYSTEM MODELS

• IT IS A MODEL USED TO CREATE ACCURATE ASSUMPTIONS REGARDING THE SYSTEMS THAT ARE TO BE MODELED.

• FACTORS TO BE CONSIDERED• SECURITY• SYSTEM THAT IS VULNERABLE TO ATTACKS

• INTERACTION• COMMUNICATION AND COORDINATION BETWEEN PROCESS

• FAILURE• VULNERABLE TO FAILURE WITH THE OCCURANCE OF FAULT

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INTERACTION MODEL

• Interacting processes are responsible for the execution of activities in the distributed systems.

• Each process has a state that cannot be accesses or updated by another process.

• A state consists of variables and a set of data that can be updated and accessed by its state.

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Factors that effect the interacting process

• Performance of communication channels the communication in models takes the place by implementation of streams by the transmission of message over the network

• There are three performance characteristics– Latency- delay that occurs when message is transmitted from

one process to another.– Jitter-it is the difference in time during the transmission of a

message.– Bandwidth- it is the amount of data that can be transmitted

over a network in a given time period.

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Factors that effect the interacting process

• Variation in time– The processes running on distinct computers can relate

the timestamps with their events.– Clock drift rate is the relative rate at which a computer

clock drifts from an accurate clock that is to be referred.

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Variants of Interaction Model

• Synchronous distributed systems– Based on assumptions of time– System which has known lower and upper bounds

associated with execution of each and every phase of a process

– Transmitted messages are received within bounded time

• Asynchronous distributed systems– No assumptions about time– There are no bounds on execution speed of a process– They donot have timingconventions.

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Failure Model

• It defines methods of failure occurences to facilitate an understanding of failure effects.

• Omission failure• Arbitrary failures• Timing failures

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