Logical clocks

Transcript

1. Logical Clocks Dani Kim (dani.kim@geekple.com) 5VFTEBZ 4FQUFNCFS 
2. Why use it? • In a distributed system, used to determine the order of events 5VFTEBZ 4FQUFNCFS 
3. Lamport timestamps 1/2 • will not be perfectly synchronized • but, simple algorithm • minimal overhead • each process P has its own clock C 5VFTEBZ 4FQUFNCFS 
4. Lamport timestamps 2/2 • Increments counter before each event • When a process sends a message, it includes its counter with the message • On receiving a message, receiver’s counter = max(received counter, own counter) + 1 5VFTEBZ 4FQUFNCFS 
5. Happend-Before (->) • If ‘a’ and ‘b’ occur on same process, a->b if event ‘a’ occur before ‘b’ • if ‘a’ is the sending of a message and ‘b’ is the reception of the message sent in event ‘a’, a->b 5VFTEBZ 4FQUFNCFS 
6. Concurrent (||) • “not (a->b)” and “not (b->a)” • these events are concurrent • a||b 5VFTEBZ 4FQUFNCFS 
7. Example 1 P1 P2 P3 P4 1 1 5VFTEBZ 4FQUFNCFS 
8. Example 1 P1 P2 P3 P4 1 1 2 2 3 4 5VFTEBZ 4FQUFNCFS 
9. Example 1 P1 P2 P3 P4 1 1 2 2 3 4 5 5VFTEBZ 4FQUFNCFS 
10. Example 1 P1 P2 P3 P4 1 1 2 2 3 4 5 5VFTEBZ 4FQUFNCFS 
11. Features • If a->b, then C(a) < C(b) • But C(a) < C(b) does not imply a->b • Just C(a) <= C(b) implies “not (b->a)” • “not (b->a)” is equivalent a->b or a||b • (P1, 1) <= (P3, 3), (P1, 1) || (P3, 3) 5VFTEBZ 4FQUFNCFS 
12. Vector Clocks 1/2 • more powerful of Lamport timestamp • use several clocks instead of a single clock 5VFTEBZ 4FQUFNCFS 
13. Vector Clocks 2/2 • Each time a process experiences an internal event, it increments its own logical clock in the vector by one • Each time a process prepares to send a message, it increments its own logical clock in the vector by one and then send its entire vector along with the message being sent • Each time a process receives a message, it increments its own logical clock in the vector by one and updates each element in its vector by taking the maximum of the value in its own vector clock and the value in the vector in the received message • http://en.wikipedia.org/wiki/Vector_clocks 5VFTEBZ 4FQUFNCFS 
14. Example [1,0,0,0] P1 P2 P3 P4 [0,1,0,0] [0,0,0,1] 5VFTEBZ 4FQUFNCFS 
15. Example [1,0,0,0] P1 P2 P3 P4 [0,1,0,0] [0,0,0,1] [1,2,0,0] [0,1,1,0] [0,1,2,1] [0,1,3,1] 5VFTEBZ 4FQUFNCFS 
16. Example [1,0,0,0] P1 P2 P3 P4 [0,1,0,0] [0,0,0,1] [1,2,0,0] [0,1,1,0] [0,1,2,1] [0,1,3,1] [2,1,3,1] 5VFTEBZ 4FQUFNCFS 
17. Example [1,0,0,0] P1 P2 P3 P4 [0,1,0,0] [0,0,0,1] [1,2,0,0] [0,1,1,0] [0,1,2,1] [0,1,3,1] [2,1,3,1] 5VFTEBZ 4FQUFNCFS 
18. Pseudo code function is_concurrent(own[], other[]) greater=false, less=false FOR i = 0 to numberOfElements by 1 IF own[i] > other[i] THEN greater = true ELSE own[i] < other[i] less = true IF greater AND less THEN RETURN true ELSE RETURN false 5VFTEBZ 4FQUFNCFS 
19. Features •if x->y, then VC(x) < VC(y) •if VC(x) < VC(y), then x->y •if VC(x) < VC(y), then “not (y->x)” 5VFTEBZ 4FQUFNCFS 
20. Conclusion • They do not resolve conflicts by themselves 5VFTEBZ 4FQUFNCFS 
21. References • http://en.wikipedia.org/wiki/Vector_clock • http://en.wikipedia.org/wiki/Logical_clock 5VFTEBZ 4FQUFNCFS