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1 | Internal use only
Logical Time and ClocksLogical Time and Clocks
2 | Internal use only
The need for logical time
• Unlike conventional sequential programs. the computations performed
by distributed systems
– do not yield a linear sequence of eventsdo not yield a linear sequence of events
– system inherently define a partial orderingsystem inherently define a partial ordering
– genuinely concurrent events have no influence on one genuinely concurrent events have no influence on one
anotheranother.
3 | Internal use only
The relation “happened before,”
• Condition 1: 'Sequential behaviour'. If events ee and f f occur
in the same process instance pp, and ff occurs after ee,
– 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 → f
4 | Internal use only
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 → e
5 | Internal use only
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 → f
6 | Internal use only
Condition 5
• Asynchronous (buffered) message-passing.
– If event e is an asynchronous send and event f is the
corresponding receive,
• then e → f
7 | Internal use only
Condition 6
• Transitivity :
– e → f
– f → g
– Then e → g
8 | Internal use only
Concurrent Events
• a -/-> e , e -/-> a since they occur at different processes
• Such events are called concurrent
• Therefore a || e
9 | Internal use only
10 | Internal use only
Lamport Logical Clocks
11 | Internal use only
Vector time
12 | Internal use only
May be Continued ...
So any
13 | Internal use only
Thank You !!