# slides.06 - Distributed Systems Principles and Paradigms...

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Distributed Systems Principles and Paradigms Chapter 06 (version April 7, 2008 ) Maarten van Steen Vrije Universiteit Amsterdam, Faculty of Science Dept. Mathematics and Computer Science Room R4.20. Tel: (020) 598 7784 E-mail:[email protected], URL: www.cs.vu.nl/ steen/ 01 Introduction 02 Architectures 03 Processes 04 Communication 05 Naming 06 Synchronization 07 Consistency and Replication 08 Fault Tolerance 09 Security 10 Distributed Object-Based Systems 11 Distributed File Systems 12 Distributed Web-Based Systems 13 Distributed Coordination-Based Systems 00 – 1 /

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Clock Synchronization Physical clocks Logical clocks Vector clocks 06 – 1 Distributed Algorithms/6.1 Clock Synchronization
Physical Clocks (1/3) Problem: Sometimes we simply need the exact time, not just an ordering. Solution: Universal Coordinated Time (UTC): Based on the number of transitions per second of the cesium 133 atom (pretty accurate). At present, the real time is taken as the average of some 50 cesium-clocks around the world. Introduces a leap second from time to time to compensate that days are getting longer. UTC is broadcast through short wave radio and satel- lite. Satellites can give an accuracy of about ± 0.5 ms. 06 – 2 Distributed Algorithms/6.1 Clock Synchronization

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Physical Clocks (2/3) Problem: Suppose we have a distributed system with a UTC-receiver somewhere in it we still have to distribute its time to each machine. Basic principle: Every machine has a timer that generates an in- terrupt H times per second. There is a clock in machine p that ticks on each timer interrupt. Denote the value of that clock by C p ( t ) , where t is UTC time. Ideally, we have that for each machine p , C p ( t ) = t , or, in other words, dC / dt = 1 . 06 – 3 Distributed Algorithms/6.1 Clock Synchronization
Physical Clocks (3/3) Fast clock Perfect clock Slow clock Clock time, C dC dt > 1 dC dt = 1 dC dt < 1 UTC, t In practice: 1 ρ dC dt 1 + ρ . Goal: Never let two clocks in any system differ by more than δ time units synchronize at least every δ / ( 2 ρ ) seconds. 06 – 4 Distributed Algorithms/6.1 Clock Synchronization

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Global Positioning System (1/2) Basic idea: You can get an accurate account of the time as a side-effect of GPS. Principle: Height x (-6,6) r = 10 (14,14) r = 16 Point to be ignored Problem: Assuming that the clocks of the satellites are accurate and synchronized: It takes a while before a signal reaches the re- ceiver The receiver’s clock is definitely out of synch with the satellite 06 – 5 Distributed Algorithms/6.1 Clock Synchronization
Global Positioning System (2/2) Δ r is unknown deviation of the receiver’s clock. x

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slides.06 - Distributed Systems Principles and Paradigms...

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