2022-04-26
  • Topic: Time
  • Clocks: every device has a clock
    • Voltage -> electrical oscillation -> 0-1 pulse
    • Clock skew (always exists): difference b/w two clocks
      • cannot have two clocks fully synchronized
    • Clock drift: difference b/w a clock and a nominal perfect reference clock
    • There is no way to synchronize two clocks
  • Sources of time: International Atomic Time, UTC time, GPS (broadcast UTC)
  • Clock synchronization: based on source of time (with error D); sync with each other
  • Clock compensation:
    • Two clocks drift at rate R msecs/sec
    • Max difference 2R msecs/sec
    • Mush resync in D/2R to agree within D
  • Synchronization methods
    • Async:
      • Cristian's algorithm:
        • Con's: single point of failure/bottleneck, security problem
        • Solutions: multiple server, N > 3f rules (f = faulty server)
      • Berkeley algorithm:
        • characteristic: no central reference clock; consistency matters (not correctness)
        • Con's: master election problem
      • Network Time Protocol
  • Logical Time
    • Logical time is essentially some kind of causal order
    • Recap: state change is an event. This event is "pushed" by some other event (cause)
    • Cause implies Order; Order does not necessarily implies Cause
    • Event ordering
    • Logical clocks (Lamport): monotonically increasing software counter
      • one for each process
      • message carries timestamp
      • If received timestamp later than current, process the message
      • If received timestamp earlier than current, there is a PROBLEM
        • Solution one: do not process messages that advances time too far
        • Solution two: constantly send timestamp even though there is no event
        • Solution three: rollback to the state right before the received timestamp and re-do all rollbacked operations, inform others that need to rollback (preemption)
          • A memory that keeps every historical operation
          • Keep a record for minimum timestamp (reduce memory consumption)