algorithms - Happened-before and Causal order - Computer Science Stack Exchange
Time Synchronization. (Part 2: Lamport and vector clocks) The happened before relation (→) is necessary for causal ordering. 6 p. 1 p. 2 p. 3. Problem: We first need to introduce a notion of ordering before we can order anything. The happened-before relation on the set of events in a distributed system. knowing the physical time at which they occurred. 1. Happens-Before (HB) Relation. • Lamport defined the happens-before relation (denoted by.
This is my most often cited paper.
But I have rarely encountered anyone who was aware that the paper said anything about state machines. People seem to think that it is about either the causality relation on events in a distributed system, or the distributed mutual exclusion problem.
People have insisted that there is nothing about state machines in the paper. I've even had to go back and reread it to convince myself that I really did remember what I had written.
Define Happened-Before Relationship. Explain implementation of logical clocks with an example
Lamport is implicitly using a short-hand notation which in digital design we call register transfers and each process is implicitly an algorithmic state machine. What we really care about is how each event affects the entire state of each process.
Each register transfer statement is a succinct way of talking about a set of state transitions that depend on the prior state. The register transfer statement Y: So what happened in statement a very much impacts the final state produced by statement b.
An algorithm requires that some process acts as a coordinator.
The question is how to select this special process dynamically. In many systems the coordinator is chosen by hand e.
If a coordinator is chosen dynamically, to what extent can we speak about a centralized or distributed solution?
Is a fully distributed solution, i.
Each process has an associated priority weight. The process with the highest priority should always be elected as the coordinator.4.9 Time and Ordering: Lamport Timestamps 14:35
How do we find the heaviest process? If a process Pheavy receives an election message from a lighter process Plight, it takes over the election and sends a message to Plight.
Plight is out of the race. Process priority is obtained by organizing processes into a logical ring.
Lecture #3: Theoretical Foundations -- Clocks in a Distributed Environment
Process with the highest priority should be elected as coordinator. Any process can start an election by sending an election message containing its id to its successor.
If a successor is down, the message is passed on to the next successor.