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# Logical clocks and logical time

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### Transcript

• 1. Logical Time and Clocks1 | Internal use only
• 2. The need for logical time • Unlike conventional sequential programs. the computations performed by distributed systems – do not yield a linear sequence of events – system inherently define a partial ordering – genuinely concurrent events have no influence on one another. another2 | Internal use only
• 3. The relation “happened before,” • Condition 1: Sequential behaviour. If events e and f occur in the same process instance p, and f occurs after e, – then e → f ( e happened before f ) • Condition 2: Process creation . If event e and process instance q occur in process instance p. – event f occurs in q, – and q begins after e, – then e → f3 | Internal use only
• 4. Happened before ... • Condition 3: Process termination. If event e and process instance q occur in process instance p. – event f occurs in q. – and e occurs after q terminates. – then f → e4 | Internal use only
• 5. Condition 4 • : Synchronous (un-buffered) message-passing. – If event e is a synchronous input (output) – and event f is the corresponding output (input), – and there is an event g such that • e → g, • then f → g. – If there is an event h such that h → e, then • h→f5 | Internal use only
• 6. Condition 5 • Asynchronous (buffered) message-passing. – If event e is an asynchronous send and event f is the corresponding receive, • then e → f6 | Internal use only
• 7. Condition 6 • Transitivity : –e→f –f→g – Then e → g7 | Internal use only
• 8. Concurrent Events • a -/-> e , e -/-> a since they occur at different processes • Such events are called concurrent • Therefore a || e8 | Internal use only
• 9. 9 | Internal use only
• 10. Lamport Logical Clocks10 | Internal use only
• 11. Vector time11 | Internal use only
• 12. May be Continued ... So any12 | Internal use only
• 13. Thank You !!13 | Internal use only