Mumbai University > Information Technology > Sem 6 > Distributed System

Marks: 10Marks

In synchronize logical clocks lamport defined a relation called happens-before.

the happens-before relation can be observed directly in two situations.

If a and b are events in same process and a occurs before b, the a b is true.

If a is the event of a message being sent by another process then a b is also true.

A message cannot be received before it is sent, even at the same time it is sent , since it takes a finite(non-zero) amount of time to arrive.

The happen-before is a transitive relation so if a b & b c then a c.

If two events, x & y happen in different processes that do not exchange message, then x y is not true but neither is y x.

the events are said to be concurrent which simply means that nothing can be said about when the event happened which event happened first.

Using these method , there is a way to assign time to all events in a distributed system subject to the following condition.

If a and b represent to sending and receiving of a message, respectively c(a)>c(b),

If a happens before b in the same process, c(a)<c(b)< p="">

for all distinctive event a and b,

e.g. totally ordered multicasting.