DCS - Tutorial1(Introduction to Distributed Systems)

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AACS5164 Distributed Computer Systems Tutorial1

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Tutorial 1

Instructions

1. Read through Chapter 1 when you answer these questions.

2. You must try to answer all the questions before or during your tutorial class.3. If you have any problems, ask the tutor or discuss with your classmates.

4. Attempting these questions yourself will help you learn the material.

IF YOU JUST COPY ANSWERS, YOU WILL NOT LEARN ANYTHING.

Q1. The construction of distributed systems produces many challenges; discuss any THREE (3) challenges.

Heterogeneityo Heterogeneous components must be able to interoperate

Distribution transparencyo Distribution should be hidden from the user as much as possible

Fault toleranceo Failure of a component (partial failure) should not result in failure of the whole system

Scalabilityo System should work efficiently with an increasing number of userso System performance should increase with inclusion of additional resources

Concurrencyo Shared access to resources must be possible

Opennesso Interfaces should be publicly available to ease adding new components

Securityo The system should only be used in the way intended

Transparencyo It hides the fact that the processes and resources are physically distributed across multiple

computers.





Q2.(a)“A distributed operating system looks to it users like an ordinary centralized operating system, but infact runs on multiple independent CPUs. In other words, the user views the system as a virtual processor,

not as a collection of distinct machines.”

(i) Identify the key concept the statements above refer to.

(1 mark)

The key concept here is transparency.

In other word, the use of multiple processors should be invisible (transparent) to the user.

(ii) List and illustrate all the types of key concept that you have identified in Q1. (a) (i).

(14 marks)

Name transparency Hides from a client the ability of a system to change the location of

an object to which the client is bound

Location transparency Hides where an object resides. Users cannot tell where hardware

and software resources such as CPUs, printers, files, data bases arelocated.

Access transparency Access to local or remote resources is identical

E.g. Network File System

Migration transparency Hides from an object the ability of a system to change that object’slocation. Resources must be free to move from one location to

another without their names changed. E.g., /usr/lee, /central/usr/leeReplication transparency Hides the fact that an object or its state may be replicated and that

replicas reside at different locations. OS can make additional copies

of files and resources without users noticing.

Concurrency and parallelism

transparency

Hides the coordination of activities between objects to achieve

consistency at a higher level. The users are not aware of theexistence of other users. Need to allow multiple users to

concurrently access the same resource. Lock and unlock for mutual

exclusion.

Failure transparency Hides failure and possible recovery of objects





(b) Briefly explain why a distributed system needs to be transparent. (2 marks)To hide from the user and the application programmer the separation/distribution of components,

so that the system is perceived as a whole rather than a collection of independent components.

(c) Illustrate FOUR (4) network characteristics on how they may affect distributed real- time

applications. ( 8 marks)

[Total: 25 marks]

Network Latency

delay experienced due to the requirement of the data to travel from the source to the destination.asynchronous (or isochronous)

Bandwidth versus cost Distributed real time applications require constant bandwidth – higher cost

Routing optimization Distributed real-time system must reside on a network and therefore are subject to its topology and

its effect on distribution and routing. ( shortest path not always the fastest path in rush hour)

Micronetwork characteristicsbuffer size on the network switches, the queue sizes, as well as the packet sizes allowed on the n/w.

Q3. Compare and contrast the peer-to-peer distributed computing model over the client/server model.

Client/Server Model Peer-to-peer Model

-Request from client and reply from server

(There are two types of computers in the

client/server, there are server and client. So the

server will handle all the request from the clientcomputer and reply to client computer)

- Request and reply perform by both side

(The computer in peer-to-peer model will be only the

one type of computer, but they might act as server by

itself and communicating between themselves)

- Centralized(The ways of communication that held inclient/server model will be centralized as all the

request concentrate to the server in the middle)

- Decentralized(The ways of communication in peer-to-peer modelwill be conducted directly between each other in the

network and they are all communicate without passing

through the server)

- Server overloaded, traffic over server side

(All the network traffic is concentrated on the serverside and the server need to handle all the request

from the client computers and this may cause theserver overburdened by its workload.)

- All computer’s workload are balanced, traffic

busy over all network(The computers are communicate in direct manner and

the workload is being distributed over the network)

- One-to-many communication(The server will need to handle the workload from

all the client computers and they all depend on theserver to process the request.)

- One-to-one communication(The communication take place only involved two

computers directly.)

- Star topology might applied(The example topology that may be shape in the star

topology manner.)

- Bus and Ring topology(The bus and ring topology may apply in this model

that the entire computer communicated with link eachother together.)

- Risk of system failure is HIGH(All the communication and processing focus on theserver side and if server is down, the

communication will wipe out entirely.)

- Risk of system failure is LOW or NONE(The processing only between two computers and therelow risk of system failure as they can recover from

other computer as well)

- Upgrade of application software is easy(When comes into upgrade process of applicationsoftware, the server will only handle the upgrade

process.)

-Upgrade of application software is hard(As the ways of communication is distributed anywayin the model and the upgrade process will cover the

entire network model.)





Q4. Explain and describe an example for each of the following:

1. soft real-time system

Consequences consist of a degradation of performance but not failure

2. firm real-time system

Tolerate a low probability of missing a deadline

3. hard real-time systemConsequences are system failure, bring to death.

4. reactive real-time system Involves a system that has constant interaction with its environment

E.g. Pilot constantly pressing control functions

5. embedded real-time system Is used to control specialized hardware that is installed in a larger system. Microprocessor

E.g. fuel to air mixture in automobile

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DCS - Tutorial1(Introduction to Distributed Systems)