From Coulouris, Dollimore, Kindberg and Blair Distributed Systems: Concepts and Design Edition 5, © Addison-Wesley 2012 Slides for Chapter 14: Time and Global States

Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 Figure 14.1 Skew between computer clocks in a distributed system

Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 Figure 14.2 Clock synchronization using a time server m r m t p Time server,S

Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 Figure 14.3 An example synchronization subnet in an NTP implementation Note: Arrows denote synchronization control, numbers denote strata.

Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 Figure 14.4 Messages exchanged between a pair of NTP peers T i T i-1 T i-2 T i-3 Server B Server A Time mm'm'

Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 Figure 14.5 Events occurring at three processes

Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 Figure 14.6 Lamport timestamps for the events shown in Figure 14.5

Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 Figure 14.7 Vector timestamps for the events shown in Figure 14.5

Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 Figure 14.8 Detecting global properties

Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 Figure 14.9 Cuts m 1 m 2 p 1 p 2 Physical time e 1 0 Consistent cut Inconsistent cut e 1 1 e 1 2 e 1 3 e 2 0 e 2 1 e 2 2

Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 Figure Chandy and Lamport’s ‘snapshot’ algorithm Marker receiving rule for process p i On p i ’s receipt of a marker message over channel c: if (p i has not yet recorded its state) it records its process state now; records the state of c as the empty set; turns on recording of messages arriving over other incoming channels; else p i records the state of c as the set of messages it has received over c since it saved its state. end if Marker sending rule for process p i After p i has recorded its state, for each outgoing channel c: p i sends one marker message over c (before it sends any other message over c).

Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 Figure Two processes and their initial states

Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 Figure The execution of the processes in Figure 14.11

Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 Figure Reachability between states in the snapshot algorithm S init S final S snap actual execution e 0,e 1,... recording begins ends pre-snap: e ' 0,e ' 1,...e ' R-1 post-snap: e ' R,e ' R+1,... '

Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 Figure Vector timestamps and variable values for the execution of Figure 14.9 m 1 m 2 p 1 p 2 Physical time Cut C 1 (1,0)(2,0)(4,3) (2,1)(2,2)(2,3) (3,0) x 1 = 1x 1 = 100x 1 = 105 x 2 = 100x 2 = 95x 2 = 90 x 1 Cut C 2

Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 Figure The lattice of global states for the execution of Figure Sij = global state after i events at process 1 and j events at process 2 S 00 S 10 S 20 S 21 S 30 S 31 S 32 S 22 S 23 S 33 S 43 Level

Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 Figure Algorithms to evaluate possibly  and definitely 

Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 Figure Evaluating definitely 