Lec12 - Today: Logical Clocks Last class: clock...

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Computer Science Lecture 12, page CS677: Distributed OS Today: Logical Clocks Last class: clock synchronization Logical clocks Vector clocks Global state and distributed snapshots 1 Computer Science Lecture 12, page CS677: Distributed OS Logical Clocks For many problems, internal consistency of clocks is important Absolute time is less important Use logical clocks Key idea: Clock synchronization need not be absolute If two machines do not interact, no need to synchronize them More importantly, processes need to agree on the order in which events occur rather than the time at which they occurred 2
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Computer Science Lecture 12, page CS677: Distributed OS Event Ordering Problem: define a total ordering of all events that occur in a system Events in a single processor machine are totally ordered In a distributed system: No global clock, local clocks may be unsynchronized Can not order events on different machines using local times Key idea [Lamport ] Processes exchange messages Message must be sent before received Send/receive used to order events (and synchronize clocks) 3 Computer Science Lecture 12, page CS677: Distributed OS Happened Before Relation If A and B are events in the same process and A executed before B , then A -> B If A represents sending of a message and B is the receipt of this message, then A -> B
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Lec12 - Today: Logical Clocks Last class: clock...

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