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cop5615fa10quiz2Key

# cop5615fa10quiz2Key - [Note that Vci[i only changes...

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Quiz 2 Key COP 5615 Fall 2010 1. In a system with N processes using vector clocks, (a) what does VCi[j] represent? (b) When and how does it change? 2. In a system with N processes using matrix clocks, (a) what does MCi[j,k] represent? (b)When and how does it change? KEY 1. (a) (2) VCi[j] represents Pi's knowledge about the number of events that have occurred at Pj. [VCi[i] represents exactly the number of events that have happened at Pi (since Pi knows this perfectly).] (b) (1) It changes only whenever Pi receives a message with a timestamp that has a larger value for its jth entry (i.e., TS[j] > VCi[j]), since the local vector clock is element wise merged with the timestamp by taking the maximum.
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Unformatted text preview: [Note that Vci[i] only changes whenever an event happens at Pi (local event, send message, or receive message). 2. (a) (1) MCi[j,k] represents Pi's knowledge of Pj's knowledge of the number of events that have happened at Pk. That is, Pi knows that Pj knows that at least MCi[j,k] events have happened at Pk. (b) (1) It changes only whenever Pi receives a message with a matrix timestamp that has a larger value for TS[j,k]. This indicates that the sender knows that Pj knows about more events at Pk than Pi knew that Pj knew about. Clear? [Note that MCi[i,*] is the same at Pi's vector clock.]...
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