Time in Distributed Systems

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Time in Distributed Systems

• Distributed Systems, Lecture 7

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Why Time is Important?

• If you work in the industry, you never have to
  worry about this
• Youll rarely have to architect a
  time-coordination framework

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Why is Time important?

• There is no such thing as absolute time
• Read A Brief History of Time (Stephan Hawking)
• Lets assume there was a big bang, t0
• UNIX epoch (big bang) Jan 1, 1971
• Mostly for causal ordering

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Ugliness of Time

• Humans measure it in astronomical terms
• Computers measure it either as
• Quartz crystal oscillations
• Atomic Caesium - 133 transitions between
  hyperfine levels of the ground state
• These things get out of synch
• We add leap seconds

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Clocks and Clock Drifts

• Quartz Clocks
• Accuracy 106 (1 second in 11.6 days)
• Atomic Clocks
• Accuracy 1013 (1 second in 300,000 years)

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UTC

• Universal Time Coordinated
• Managed by an atomic clock
• Propagated by radio and satellite signals
• Delay?
• Speed of light

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Cristians algorithm

• Client asks the Server
• Sets its time to
• Server transmission delay
• Assumptions
• Consistent Delay
• Symmetric Delay
• Problems
• Server can crash
• Security

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Berkeleys Algorithm

• Server Polls clients
• Keeps an estimate of client delay
• Server takes a fault-tolerant average
• Drop out-liers
• Take an average (including its own)
• Propagate the delta (not time)

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NTP

• Hierarchical time dissemination
• Time Correction cant role back time
• Root talks to strata 1, strata 1 talks to strata
  2 and so on
• Hosts ranked on strata number
• Use a dispersion filter to rank hosts
• Dispersion filter variation of delay

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What we really care about

• Causal ordering
• Lamport clocks
• Monotonically increasing counter per process
  (host)
• Only meaningful for one pair
• No global ordering

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Vector Clocks

• Lamport Clocks
• One clock for every process (host)
• Vector of size N, for an N node system
• Imposes total ordering
• Not very scalable