Synchronization

1. Synchronization Chapter 5

2. Clock Synchronization • When each machine has its own clock, an event that occurred after another event may nevertheless be assigned an earlier time.

3. Physical Clocks (1) • Computation of the mean solar day.

4. Physical Clocks (2) • TAI (International Atomic Time) seconds are of constant length, unlike solar seconds. • Leap seconds are introduced when necessary to keep in phase with the sun.

5. Clock Synchronization Algorithms • The relation between clock time and UTC (Universal Coordinated Time) when clocks tick at different rates.

6. Cristian's Algorithm • Getting the current time from a time server.

7. Cristian's Algorithm • Two problems: • Time must never run backwards, so backwards changes must be introduced gradually • update clock more slowly, for example • The delay for the time to get back may be large, and may be unpredictable (have to subtract propagation time) • Can measure several times and take average • Can omit values that exceed some threshold • Can take only the smallest, since this may be most representative of pur propagation time

8. The Berkeley Algorithm • The time daemon asks all the other machines for their clock values • The machines answer • The time daemon tells everyone how to adjust their clock

9. Lamport Timestamps • Three processes, each with its own clock. The clocks run at different rates. • Lamport's algorithm corrects the clocks.

10. Lamport Timestamps • In 1978, Leslie Lamport noticed that for most applications synchronization is not important, but only that events be ordered in time. • Lamport defined some concepts: • 1) if a and b are events in the same process, and a occurs before b, then ab "a happened before b" • 2) if a is the event of sending a message and b is the event of • receipt of that message, then ab

11. Lamport Timestamps • 3) if ab and bc then ac • Sometimes we draw a space-time diagram • time -----> • ---------- a ------------- b ------------------------- Process p1 • ------------- c ---- d -------------- g -------------- Process p2 • ------------------- e --------------------- f -------- Process p3

12. Lamport Timestamps • ---------- a ------------- b ------------------------- Process p1 • ------------- c ---- d -------------- g -------------- Process p2 • ------------------- e --------------------- f -------- Process p3 • Suppose there are messages from c to b, d to f, and e to g • cb • df • eg • dg • cd ==> cf

13. Lamport Timestamps • ---------- a ------------- b ------------------------- Process p1 • ------------- c ---- d -------------- g -------------- Process p2 • ------------------- e --------------------- f -------- Process p3 • two events a and b are said to be concurrent if it is not true that ab and it is not true that ba • d and e are concurrent

14. Example: Totally-Ordered Multicasting • An application of Lamport timestamps: • Updating a replicated database and leaving it in an inconsistent state.

15. Example: Synchronization with Event Counts and Sequencers • Paper by D. P. Reed and R. K. Kanodia, ``Synchronization with Eventcounts and Sequencers,'' Communications of the ACM, 22, 2 (February 1979), pp.115-123