File:Lamport-Clock-en.svg

Summary[edit]

Example for the working of a lamport logical clock, showing the causality of events (happened-before relation).

Explanation[edit]

• A, B and C are processes (oder places)
• A1, A2 ... C5 are events or activites in that process (resp. at that place)
• The boxes at the events give the current value of the Lamport clock of that process at the given point in time.
• The arrows represent the passing of messages (resp. effects or data) between the processes (resp. places)
• The dark blue (resp. dark red) areas represent the cause (resp. the effect) of the event B4.
• The light blue (resp. light red) areas represent events, that are considered to be causes or effects of B4 by the lamport clock, but are in fact independent (or concurrent)

Events are concurrent to (independend of) B4, if no information flows to or from that event from or to B4. It's easy to see that the lamport clock does not recognize some of the events as independent, even though they are. This is because the Lamport clock only satisfies the weak clock condition (see logical clock).

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