distributed process management - part 2 (12)

distributed process management - part 2 (12) - COP 4600...

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COP 4600: Intro To OS (Distributed Process Management – Part 2) Page 1 © Dr. Mark Llewellyn COP 4600 – Summer 2011 Introduction To Operating Systems Distributed Process Management – Part 2 Department of Electrical Engineering and Computer Science Computer Science Division University of Central Florida Instructor : Dr. Mark Llewellyn [email protected] HEC 236, 407-823-2790 http://www.cs.ucf.edu/courses/cop4600/sum2011
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COP 4600: Intro To OS (Distributed Process Management – Part 2) Page 2 © Dr. Mark Llewellyn Message from branch A to branch B Message from branch B to branch A One possible scenario – in this case reported account balance is correct Example Scenarios For Bank Example Process/Event Graph
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COP 4600: Intro To OS (Distributed Process Management – Part 2) Page 3 © Dr. Mark Llewellyn If at the time of balance determination, the balance from branch A is in transit to branch B. In this case balance determined at 3:00 pm is incorrect. All messages in transit must be examined at time of observation. The correct total consists of balance at both branches and amount in any message in transit. Example Scenarios For Bank Example
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COP 4600: Intro To OS (Distributed Process Management – Part 2) Page 4 © Dr. Mark Llewellyn Example Scenarios For Bank Example If the clocks at the two branches are not perfectly synchronized a problem can arise. Suppose that a transfer message is initiated at branch A at local time 3:01 pm. This message arrives at branch B at 2:59 local time. The balance calculated at 3:00 pm will now show the incorrect amount of $200. The amount is incorrectly counted twice.
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COP 4600: Intro To OS (Distributed Process Management – Part 2) Page 5 © Dr. Mark Llewellyn Some Terms Channel Exists between two processes if they exchange messages. State Sequence of messages that have been sent and received along channels incident with the process. Snapshot Records the state of a process. Global state The combined state of all processes. Distributed Snapshot A collection of snapshots, one for each process.
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(Distributed Process Management – Part 2) Page 6 © Dr. Mark Llewellyn Global States and Distributed Snapshots The problem with a distributed system is that a true global state cannot be determined because of the time lapse associated with message transfer. We can attempt to define a global state by collecting snapshots from all processes. For example, in the figure on the next page, at the time of taking the snapshot, there is a message in transit on the <A,B> channel (message 2), one in transit on the <A,C> channel (message 3), and one in transit on the <C,A> channel (message 4). Messages 2 and 4 are properly represented, however, message 3 is not.
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This note was uploaded on 10/03/2011 for the course COP 4600 taught by Professor Montagne during the Summer '08 term at University of Central Florida.

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distributed process management - part 2 (12) - COP 4600...

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