[2020] COMPSCI 711 Parallel and Distributed Computing - Logical clocks and applications
Question 2 [10 marks] - Logical clocks and applications
Discuss logical clocks, compare Lamport clocks with vector clocks, and - as an application - illustrate the Money count distributed snapshot.
(a) Define logical time in terms of the happens-before relation between events.
(b) Consider the following events sequences at processes P1, P2, P3, where si and ri are corresponding send and receive events.
P1 time-line: s1, r2, r6, r5
P2 time-line: s2, r1, s3, r4, s5
P3 time-line: s4, r3, s6
(c) Annotate all events with Lamport clock values – one line for each process, e.g.
P1 time-line: s1:1, …
(d) Consider that each process starts with an initial value of 100 $$ and each message represents a transfer of 10 $$.
(e) According to the Money Count algorithm, what are the processes’ balances at Lamport time 4.5?
(f) Annotate all events with vector clock values, e.g.
P1 time-line: s1:(1,0,0), …
(g) Critically compare Lamport clocks with vector clocks (essential differences, why would one use vector clocks instead of Lamport clocks).
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