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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