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Understanding Lamport Clocks and Event Order in Distributed Systems

A detailed overview of Lamport clocks and event ordering in distributed systems, exploring concepts like light cones, logical clocks, and causality. Learn about processes, event propagation, and the challenges of determining event order.

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Understanding Lamport Clocks and Event Order in Distributed Systems

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  1. Dave Eckhardt de0u@andrew.cmu.edu Lamport clocks

  2. Synchronization • No class Friday • Spring Carnival (“Mobot” races @ noon)

  3. Outline (not) • Chapter 15 (“Distributed System Structures”) • Zooming past distributed systems • Process migration! • Network protocol stacks • “The Internet in one easy lesson” • You can read it yourselves... • ...and you probably should.

  4. Outline • Lamport clocks • Covered in 17.1, 17.2 (different focus from today) • Time, Clocks, and the Ordering of Events in a Distributed System • CACM 21:7 (1978)

  5. Overview • Light cones • Meeting for beer • “Happened before” partial order • Logical clocks • Advanced techniques

  6. Light cones • Concept • Effects propagate at or below speed of light • Objects, light/radio/X-rays, gravity • Knowledge of events limited the same way • Event propagation modeled by expanding sphere • Four-dimensional “cone”

  7. Light cones

  8. Light cones

  9. Light cones

  10. Light cones

  11. Light cones

  12. Light cones

  13. Light cones

  14. Light cones

  15. Light cones • Future light cone • The part of spacetime influenced by an event • Past light cone • The part of spacetime that could have influenced an event

  16. Meeting for Beer • P1 transmits “Panther Hollow Inn” to blackboard

  17. Meeting for Beer • P1 transmits “Panther Hollow Inn” to blackboard • P1 transmits to P2 • Hey, P2, let's go have a beer. • I have transmitted the bar's name to the blackboard. • See you there!

  18. Meeting for Beer • P1 transmits “Panther Hollow Inn” to blackboard • P1 transmits to P2 • Hey, P2, let's go have a beer. • I have transmitted the bar's name to the blackboard. • See you there! • P2 receives P1's message

  19. Meeting for Beer • P1 transmits “Panther Hollow Inn” to blackboard • P1 transmits to P2 • Hey, P2, let's go have a beer. • I have transmitted the bar's name to the blackboard. • See you there! • P2 receives P1's message • P2 queries blackboard

  20. Meeting for Beer • P1 transmits “Panther Hollow Inn” to blackboard • P1 transmits to P2 • Hey, P2, let's go have a beer. • I have transmitted the bar's name to the blackboard. • See you there! • P2 receives P1's message • P2 queries blackboard • It says “Squirrel Cage” - how???

  21. Meeting for Beer

  22. Meeting for Beer

  23. Meeting for Beer

  24. Meeting for Beer

  25. Meeting for Beer

  26. Meeting for Beer

  27. What went wrong? • P1 thought • Blackboard update happened before invitation • P2 thought • Invitation happened before blackboard update • When does an event “happen”? • When its effects propagate “everywhere relevant” • What does “happen before” mean? • Could that green node really be so slow?

  28. Universe Model • System = set of processes • Process = sequence of events • Event • Internal: ++x; • Message transmission • Message reception

  29. “Happened before” partial order • A happens before B (A  B) • If A and B happen inside a process, in that order • A = transmission, B = reception, of same message • If A  B and B  C, then A  C • A and B are concurrent when • A ! B and B ! A • Observe A ! A

  30. Space-time Diagram •  • inside a process, or • follow a message • p0  r2 • concurrent • p0, q0, r0 • p1, q1 • q1, r0 • p1, r0

  31.  means “possibly causes” • p0 possibly causes p1 • ...by storing something in P's memory • p0 possibly causes q1 • Message could trigger q1 • Concurrent events • ...cannot cause each other

  32. Logical clocks • Can we assign timestamps to events? • Want • If A  B then C(A) < C(B) • Events inside Pi • a  b  Ci(a) < Ci(b) • Message from Pi to Pj • a=Pi's send, b=Pj's receive  Ci(a) < Cj(b)

  33. Logical clocks • Events inside Pi • Increment Ci() between successive events • Message from Pi to Pj • Sender: place timestamp T in message: Ci(send) • Receiver: ensure Cj(receive) > T

  34. Meeting for Beer

  35. Meeting for Beer

  36. Meeting for Beer

  37. Meeting for Beer

  38. Meeting for Beer

  39. Meeting for Beer

  40. What this means • P1 wants • <“PHI” on board> happened before <P2 read board> • Equivalent to “59 < 58” (oops) • The events were concurrent • “PHI” could not cause P2's bar trip

  41. Fixing the problem • P1 should wait for board to acknowledge • “PHI” causes ACK • ACK causes “Meet me at...” • “Meet me at...” causes bar trip • Then: “PHI” causes bar trip

  42. Extensions • Define total ordering of system events • Typical (timestamp, process #) tuple comparison • Process # used to break timestamp ties • Distributed agreement algorithms • Such as “fair distributed mutual exclusion” • Requests must be granted “in order” • See text: 17.2 • Adding physical (real-time) clocks

  43. Summary • Light cones • “Happened before" partial order • Potential causality • Another definition of concurrency • Timestamps track message causality

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