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Short story of time
- 1. A SHORT STORY ABOUT TIME
- 2. INTRODUCTION Bartosz Sypytkowski ▪ @Horusiath ▪ b.sypytkowski@gmail.com ▪ bartoszsypytkowski.com
- 3. Timekeeping in distributed systems UTC How clocks work System time NTP Logical time AGENDA
- 4. Timekeeping in distributed systems Node 1 Bob Joe SET X = 2SET X = 1
- 5. Timekeeping in distributed systems Node 1 Node 2 Bob Joe SET X = 2SET X = 1
- 6. “Time is not a thing, it’s a subjective experience.”
- 7. CLOCKS
- 8. PENDULUM CLOCK • Frequency: < 1Hz • Precision: ~1 sec.
- 9. DIGITAL CLOCK • Frequency: 50Hz – 60Hz • Precision: 16-20ms
- 10. QUARTZ CLOCK • Frequency: 32kHz – 2MHz • Precision: 0.5µs - 31µs
- 11. ATOMIC CLOCK • Frequency (Cessium-133): 9,192,631,770Hz • Precision: ~0.1ns
- 12. GPS CLOCK SYNC
- 13. UNIX TIME Number of seconds since epoch (1970/01/01) It has exactly 86 400s per day…
- 14. • Day does not take 24h… • … it’s not even a constant. A TRUTH ABOUT DAY
- 15. UTC ATOMIC TIME BASED ON ATOMIC CLOCK SOLAR/ASTRONOMICAL TIME BASED ON EARTH ROTATION UTC A COMPROMISE BASED ON ATOMIC TIME IRREGULARLY ADJUSTED TO SOLAR TIME
- 17. CONSEQUENCES OF NON-MONOTONIC TIME Node 1 Node 2 Bob Joe ts = 60 SET X = 2SET X = 1 ts = 20
- 18. LEAP SECONDS IN PRACTICE
- 20. NETWORK TIME PROTOCOL 1. Skew – speedup/slowdown clock tick rate 2. Step – set a new value when difference is too big (128ms)
- 21. NTP PROBLEMS ▪ Stepping interferes with happen-before relationships. ▪ It’s slow. ▪ It assumes, that network is symmetric. PRECISION TIME PROTOCOL ▪ Available in Windows Server 2019 (opt-in) ▪ Takes into account a latency per each network device
- 22. OS TIME API WINDOWS wall-clock time LINUX wall-clock time WINDOWS monotonic time LINUX monotonic time clock_gettime(CLOCK_REALTIME) clock_gettime(CLOCK_MONOTONIC)QueryPerformanceCounter(&time) GetSystemTimePreciseAsFileTime(&date)
- 23. “In transactions, timekeeping is more about order of operations than actual time.”
- 24. SEQUENCERS Set X = 1 ts = 1 Set Y = 2 ts = 2 Set X = 3 ts = 3 Delete Y ts = 4 Set Y = 1 ts = 5 Delete X ts = 6 Monotonically incrementing counter timeline Node A
- 25. DOTS Set X = 1 A:1 Set X = 3 A:2 Set Y = 1 A:3 Delete X A:4 timeline Node A Set Y = 2 B:1 Delete Y B:2 timeline Node B
- 26. VECTOR CLOCKS Set X = 1 Set X = 3 Set Y = 1 Delete X Node A Set Y = 2 Delete Y Node B A 1 A 1 B 1 A 2 B 1 A 2 B 2 A 3 B 2 A 4 B 2
- 27. COMPAR ING VECTOR CLOCKS PARTIAL ORDERING Equals Greater than Less than Concurrent A 2 B 3 C 1 A 2 B 3 C 1 A 2 B 3 C 2 A 2 B 3 C 1 A 2 B 2 C 1 A 2 B 3 C 1 A 2 B 3 C 2 A 2 B 2 C 1
- 28. COMPAR ING VECTOR CLOCKS RESOLVING CONCURRENT UPDATES Node 1 Node 2 Bob Joe t1 SET X = 2SET X = 1 t2
- 29. COMPAR ING VECTOR CLOCKS RESOLVING CONCURRENT UPDATES Node 1 Node 2 Bob Joe t1 SET X = 2SET X = 1 t2 max (t1, t2) max (t1, t2)
- 31. VECTOR CLOCK MERGE MAX ( , ) = A 1 B 3 A 2 B 1 C 1 A 2 B 3 C 1
- 32. Overhead: Nr. of nodes * (Key size + seq. nr.) Examples: 3 × (4 + 8) = 36 bytes 10 × (16 + 8) = 240 bytes DATETIME / SEQ. NR. Overhead: 8 bytes VECTOR CLOCKS
- 34. DOTTED VERSION VECTORS REPRESENTATION D 4 C 5 B 7 A 5 D 8 C 10 C 11 A 9 version vector dot cloud +
- 35. HOW TO DETERMINE “SEEN BY ALL”? DOT VECTOR CLOCK ???
- 36. MATRIX CLOCKS 2 2 1 2 3 1 2 2 2 A B C A B C Observers Observed
- 37. Keeping time in real systems: https://www.youtube.com/watch?v=BRvj8PykSc4 Windows plans for leap seconds: https://blogs.technet.microsoft.com/networking/2018/07/18/top10-ws2019-hatime/ Dotted version vectors: http://basho.com/posts/technical/vector-clocks-revisited-part-2-dotted-version-vectors/ REFERENCES
- 38. THANK YOU
Editor's Notes
- Fun fact: this issue didn’t happened in Google, because they have used so called “leap second smearing” – when they knew that leap second is getting closer, they started to drift their atomic clocks on purpose to slowly compensate for that leap second. The problem is that this is not official way of doing things, so Google’s internal reference clocks were not inline will any other reference clocks for some time. Sometimes providers (like Amazon) officially announce leap seconds incoming. NTP also can send a packet informing about incoming leap second.