Introduction%20to%20Distributed%20Computing

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Lecture 6 Introduction to Distributed Computing
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Distribute Systems and Distributed Computing
Wei Chen, Professor Tennessee State University

• Contents
• Introduction of Distributed Systems
• Distributed Computation Models
• Synchronous Network Algorithms
• Asynchronous Network Algorithms
• Clock in Asynchronous Distributed Systems and
  Snapshot algorithms
• Fault-Tolerant Algorithms

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Construction of Distributed Systems

• Units of A Distributed System
• Units/Nodes (Computers, sensors, )
• Network
• Software

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What Is Distribute Systems?

• There is no unique definition for distributed
  systems. Similar terms are
• Networked system
• parallel systems
• Concurrent systems
• Decentralized systems

Enslows Definition Distributed System
Distributed hardware Distributed control
Distributed data
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Distributed Hardware
Physically shared/distributed memory and
logically shared/distributed memory
Logically shared Logically distributed
Physically shared Common memory Simulated message passing
Physically distributed Distributed shared memory message passing
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• Distributed System Software
• Cooperating the actions of units (computers,
  sensors, )
• Supporting system resources (hard ware and
  software) sharing.
• Supporting data sharing.

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• Distributed Control
• Master/Slave (fixed/dynamic)
• Multi control points (homogenous/heterogeneous)

• Distributed Data
• Copies Processors hold data copies.
• Divided data Data are divided and distributed
  to processors.

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Features of Distributed Systems

• Resource sharing hardware, software, data
• Openness openness of main interface of the
  system, scalability of the current system
• Concurrency concurrent execution of the
  processes, high performance, nice rate of price
  and performance (PC cluster poor mans
  supercomputer )
• Fault tolerance ability to tolerate the fault
  of system units, availability( using potential
  redundancy to overcome the system fault).
• Transparency a distributed system can be looked
  as one computer (access transparency, position
  transparency, parallel transparency,.)

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Distributed Computational models

• Processes never mind in which unit they are.
• Communicating links communication channels

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• Process
• Generalized RAM (Random Access Machine)
• CPU with program
• Infinite memory
• Message changeable with adjacent processes
• Each process has a distinct ID.
• Without fault (it will be considered in the
  model with fault).

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• Communication Links
• Bidirectional
• Asynchronous
• Infinite FIFO queue
• Without fault (no massage losing, alteration)

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• Synchronism
• Running speed of each process
• Transmission delay
• Speed of the local clock in each process
  constant upper and lower bounds

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Local Clock of Synchronous Distribute Systems
p(t) the value of the local clock of process p
at moment t
Typical synchronous distributed model Round Model

• Processes execute each round synchronously
• Each round consists of message receiving,
  computation, and
• message sending
• Processes accept the message sent from the
  previous round.

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• Complete asynchronous model
• No assumption for the speed of processes.
• No assumption for the transmission delay of
  communication links.
• No assumption for the speed of local clocks.