CSC 7103 20090903

CSC 7103 20090903 - Knowing C = synchronized global time...

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CSC 7103 9/3/2009 Page 1 of 4 Timing diagram Arrows are messages, spikes are events. Time services Distributed algorithms have: Scattered resources All decisions are local Lack of global time Avoid single points of failure All these algorithm characteristics have an effect on time services. Figure 1 Ideally t will change at the same rate as C. We can only guarantee 1 – Ρ ≤ dc/dt ≤ 1 + Ρ. The clocks will never be farther apart than 2ΡT. (one is +ΡT and one is –ΡT). Therefore T ≤ x/2Ρ.
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CSC 7103 9/3/2009 Page 2 of 4 If the local time is t and the global time is T… T < T t = T is okay T > T t= T is not okay because events may appear to occur in the future, which violates causality When the latter occurs, you must adjust your skew to gradually meet up with the correct time. Message delivery How can we ensure a delivery of at most once? Figure 2
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Unformatted text preview: Knowing C = synchronized global time And: Maximum message lifetime Maximum clock skew A table can be created, shown in Figure 3 Figure 3 Sender ID Seq. No/C If the timestamp of a message is less than C it is guaranteed to be a duplicate. If the system crashes it can discard anything before C in the table since that happened before the crash. Example 1 Logical clock synchronization Lamports algorithm Figure 4 CSC 7103 9/3/2009 Page 3 of 4 Lamports algorithm only guarantees partial ordering. Figure 5 How do we detect this problem? Vector Logical Clock Vector Logical Clock P 1 will store information about its time and other processors time. When a message is sent, it will send its time as well as its state information about other processors in the message. Figure 5 CSC 7103 9/3/2009 Page 4 of 4...
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CSC 7103 20090903 - Knowing C = synchronized global time...

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