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Company

LOGO

MBA C003: Organisational Behaviour

Unit 4: LeadershipSession 1: Oct 16, 2010

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Distributed systems

Distributed system is an application that executes a

collection of protocols to coordinate the actions of

multiple processes on a network.

A distributed system is a collection of independent

computers that appear to the users of the system as a

single system.

Examples:

Network of workstations

Distributed manufacturing system (e.g., automated assembly line)

Network of branch office computers

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Advantages of Distributed Systemsover Independent PCs

Data sharing: allow many users to access to acommon data base

Resource Sharing: expensive peripherals like

color printers

Communication: enhance human-to-humancommunication, e.g., email, chat

Flexibility: spread the workload over theavailable machines

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Disadvantages of Distributed Systems

Software: difficult to develop software fordistributed systems

Network: saturation, lossy transmissions

Security: easy access also applies to secretedata.

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Design Issues of DistributedSystems

Transparency Flexibility

Reliability

Performance

Scalability

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Transparency

How to achieve the single-system image, i.e.,how to make a collection of computersappear as a single computer.

Hiding all the distribution from the users aswell as the application programs can beachieved at two levels:1. Hide the distribution from users

2. At a lower level, make the system looktransparent to programs.

1) and 2) requires uniform interfaces such asaccess to files, communication.

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Flexibility

Make it easier to change Monolithic Kernel: systems calls are trapped

and executed by the kernel. All system callsare served by the kernel, e.g., UNIX.

Microkernel: provides minimal services.1) IPC2) some memory management3) some low-level process management and

scheduling4) low-level i/oE.g., Mach can support multiple filesystems, multiple system interfaces.

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Reliability

Distributed system should be more reliablethan single system. Example: 3 machineswith .95 probability of being up. 1-.05**3

probability of being up. Availability: fraction of time the system is

usable. Redundancy improves it.

Need to maintain consistency

Need to be secure

Fault tolerance: need to mask failures,recover from errors.

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Performance

Without gain on this, why bother withdistributed systems.

Performance loss due to communication

delays: fine-grain parallelism: high degree of

interaction

coarse-grain parallelism Performance loss due to making the

system fault tolerant.

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Scalability

Systems grow with time or become obsolete.Techniques that require resources linearly in terms of thesize of the system are not scalable. e.g., broadcastbased query won't work for large distributed systems.

Examples of bottleneckso Centralized components: a single mail server

o Centralized tables: a single URL addressbook

o Centralized algorithms: routing based oncomplete information.

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Handling Failure

Is an important theme in distribrutedsystem design.

It falls in two categories.

Hardware failure Software failure

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Hardware Failure

Hardware failure where dominant utill late80s.

Decreased heat production and power

consumption of small circuits played apositive role in improving hardware failure.

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Software Failure

Software failures are a significant issues inthe distributed systems.

Even with regress testing software bugs

account for a substantial fraction of un-planned down-time (Estimated at 25 to 35percent)

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Distributed System Security

Hardware Vs Software: Hardware security: keys, locks, cards, visitor

monitoring, etc. (not main focus)

Software security:

(in terms of user requirements) integrity: message transmitted over network must be identical to

the original

confidentiality: message transmitted must not be readable tounintended entities

availability: the system must not be unavailable due to securityattacks

accountability: user actions that is security-critical must be

traceable.

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Best of Luck

Thank You