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Time-and-Synchronization-slides

Time-and-Synchronization-slides - Time and...

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Time and synchronization (“There’s never enough time…”)
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Today’s outline Time in distributed systems A baseball example Synchronizing real clocks Cristian’s algorithm The Berkeley Algorithm Network Time Protocol (NTP) Logical time Lamport logical clocks
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Distributed time The notion of time is well-defined (and measurable) at each single location But the relationship between time at different locations is unclear e.g., packet-sending from HW 1 #6:
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A baseball example Four locations: pitcher’s mound, first base, home plate, and third base Ten events: e 1 : pitcher throws ball to home e 2 : ball arrives at home e 3 : batter hits ball to pitcher e 4 : batter runs to first base e 5 : runner runs to home e 6 : ball arrives at pitcher e 7 : pitcher throws ball to first base e 8 : runner arrives at home e 9 : ball arrives at first base e 10 : batter arrives at first base
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A baseball example Pitcher knows e 1 happens before e 6 , which happens before e 7 Home plate umpire knows e 2 is before e 3 , which is before e 4 , which is before e 8 , … Relationship between e 8 and e 9 is unclear
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Ways to synchronize Send message from first base to home? Or to a central timekeeper How long does this message take to arrive? Synchronize clocks before the game? Clocks drift million to one => 1 second in 11 days Synchronize continuously during the game?
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