1. Slides for Chapter 11: Coordination and Agreement From Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edition 3, © Addison-Wesley 2001

2. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Figure 11.1 A network partition

3. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Figure 11.2 Server managing a mutual exclusion token for a set of processes

4. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Figure 11.3 A ring of processes transferring a mutual exclusion token

5. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Figure 11.4 Ricart and Agrawala’s algorithm On initialization state := RELEASED; To enter the section state := WANTED; Multicast request to all processes;request processing deferred here T := request’s timestamp; Wait until (number of replies received = (N – 1)); state := HELD; On receipt of a request at p j (i ≠ j) if (state = HELD or (state = WANTED and (T, p j ) < (T i, p i ))) then queue request from p i without replying; else reply immediately to p i ; end if To exit the critical section state := RELEASED; reply to any queued requests;

6. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Figure 11.5 Multicast synchronization p 3 34 Reply 34 41 34 p 1 p 2 Reply

7. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Figure 11.6 Maekawa’s algorithm – part 1 On initialization state := RELEASED; voted := FALSE; For p i to enter the critical section state := WANTED; Multicast request to all processes in V i – {p i }; Wait until (number of replies received = (K – 1)); state := HELD; On receipt of a request from p i at p j (i ≠ j) if (state = HELD or voted = TRUE) then queue request from p i without replying; else send reply to p i ; voted := TRUE; end if Continues on next slide

8. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Figure 11.6 Maekawa’s algorithm – part 2 For p i to exit the critical section state := RELEASED; Multicast release to all processes in V i – {p i }; On receipt of a release from p i at p j (i ≠ j) if (queue of requests is non-empty) then remove head of queue – from p k, say; send reply to p k ; voted := TRUE; else voted := FALSE; end if

9. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Figure 11.7 A ring-based election in progress Note: The election was started by process 17. The highest process identifier encountered so far is 24. Participant processes are shown darkened

10. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Figure 11.8 The bully algorithm The election of coordinator p 2, after the failure of p 4 and then p 3

11. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Figure 11.9 Open and closed groups

12. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Figure 11.10 Reliable multicast algorithm

13. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Figure 11.11 The hold-back queue for arriving multicast messages

14. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Figure 11.12 Total, FIFO and causal ordering of multicast messages Notice the consistent ordering of totally ordered messages T 1 and T 2, the FIFO-related messages F 1 and F 2 and the causally related messages C 1 and C 3 – and the otherwise arbitrary delivery ordering of messages.

15. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Figure 11.13 Display from bulletin board program Bulletin board: os.interesting Item FromSubject 23A.HanlonMach 24G.JosephMicrokernels 25A.HanlonRe: Microkernels 26T.L’HeureuxRPC performance 27M.WalkerRe: Mach end

16. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Figure 11.14 Total ordering using a sequencer

17. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Figure 11.15 The ISIS algorithm for total ordering 2 1 1 2 2 1 Message 2 Proposed Seq P 2 P 3 P 1 P 4 3 Agreed Seq 3 3

18. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Figure 11.16 Causal ordering using vector timestamps

19. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Figure 11.17 Consensus for three processes

20. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Figure 11.18 Consensus in a synchronous system

21. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Figure 11.19 Three byzantine generals p 1 (Commander) p 2 p 3 1:v 2:1:v 3:1:u p 1 (Commander) p 2 p 3 1:x1:w 2:1:w 3:1:x Faulty processes are shown shaded

22. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 3 © Addison-Wesley Publishers 2000 Figure 11.20 Four byzantine generals p 1 (Commander) p 2 p 3 1:v 2:1:v 3:1:u Faulty processes are shown shaded p 4 1:v 4:1:v 2:1:v3:1:w 4:1:v p 1 (Commander) p 2 p 3 1:w1:u 2:1:u 3:1:w p 4 1:v 4:1:v 2:1:u3:1:w 4:1:v