Happened-Before Relation

Most of the time it doesn't matter if all the processes share the same time, but they need to agree on the order in which events have occurred. The concept of order is necessary.

Happened-Before Relation

• If a and b are two events of the same process, and a is before b, then a -> b.

• If a is the sender of a message, and b is the receiver of the same message, then a -> b.

• If a -> b and b -> c the a -> c.

This partial order of events is also the concept used in systems with concurrent processes.