Logical Time and Clocks1 | Internal use only
The need for logical time      • Unlike conventional sequential programs. the computations performed               by dist...
The relation “happened before,”      • Condition 1: Sequential behaviour. If events e and f occur               in the sam...
Happened before ...      • Condition 3: Process termination. If event e and process               instance q occur in proc...
Condition 4      • : Synchronous (un-buffered) message-passing.             – If event e is a synchronous input (output)  ...
Condition 5      • Asynchronous (buffered) message-passing.             – If event e is an asynchronous send and event f i...
Condition 6      • Transitivity :             –e→f             –f→g             – Then e → g7 | Internal use only
Concurrent Events      • a -/-> e , e -/-> a since they occur at different processes      • Such events are called concurr...
9 | Internal use only
Lamport Logical Clocks10 | Internal use only
Vector time11 | Internal use only
May be Continued ...                 So any12 | Internal use only
Thank You !!13 | Internal use only
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Logical clocks and logical time

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Logical clocks and logical time

  1. 1. Logical Time and Clocks1 | Internal use only
  2. 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. 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. 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. 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. 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. 7. Condition 6 • Transitivity : –e→f –f→g – Then e → g7 | Internal use only
  8. 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. 9 | Internal use only
  10. 10. Lamport Logical Clocks10 | Internal use only
  11. 11. Vector time11 | Internal use only
  12. 12. May be Continued ... So any12 | Internal use only
  13. 13. Thank You !!13 | Internal use only
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