1. Snap-Stabilizing Committee Coordination Borzoo Bonakdarpour, Stéphane Devismes, and Frank Petit

2. Committee Coordination Cap Estérel, mardi 24 mai 2011, ALGOTEL'2011 2

3. Committee Coordination Cap Estérel, mardi 24 mai 2011, ALGOTEL'2011 3 Synchronization Wait

4. Committee Coordination Cap Estérel, mardi 24 mai 2011, ALGOTEL'2011 4 Exclusion

5. Committee Coordination Cap Estérel, mardi 24 mai 2011, ALGOTEL'2011 5 Progress Wait

6. Snap-Stabilization Cap Estérel, mardi 24 mai 2011, ALGOTEL'2011 6

7. Related property: Self-Stabilization Cap Estérel, mardi 24 mai 2011, ALGOTEL'2011 7

8. Other properties Maximal Concurrency Fairness 2-phase discussion time Cap Estérel, mardi 24 mai 2011, ALGOTEL'2011 8

9. Maximal Concurrency Cap Estérel, mardi 24 mai 2011, ALGOTEL'2011 9 Wait

10. (Professor) Fairness May 17, 2011 IEEE International Parallel and Distributed Processing Symposium (IPDPS'11) Every professor eventually participates in a committee meeting that it is a member of. Assumption [2]) (otherwise, impossible [2]) Fairness [2] Y.-K. Tsay and R. Bagrodia. Some impossibility results in interprocess synchronization. Distributed Computing, 6(4):221–231, 1993. Every professor wants to participate in a committee meeting infinitely often.

11. Impossibility of Fairness and Maximal Concurrency Cap Estérel, mardi 24 mai 2011, ALGOTEL'2011 11 Wait We design an algorithm for each property

12. 2-Phase Discussion Time Before: atomic event assumption – All members start and finished the meeting simultaneously 2-Phase discussion: – Phase 1 (essential discussion) : all participants must discuss – Phase 2 (voluntary discussion) : participants discuss freely The meeting ends when any participant decide to leave the meeting Cap Estérel, mardi 24 mai 2011, ALGOTEL'2011 12

13. Algorithms Snap-stabilizing 2-Phase Committee Coordination with Maximal Concurrency Snap-stabilizing 2-Phase Fair Committee Coordination Cap Estérel, mardi 24 mai 2011, ALGOTEL'2011 13

14. Snap-stabilizing 2-Phase Committee Coordination with Maximal Concurrency Cap Estérel, mardi 24 mai 2011, ALGOTEL'2011 14 Step 1: Looking for a committee to participate L L L L L L L

15. Snap-stabilizing 2-Phase Committee Coordination with Maximal Concurrency Cap Estérel, mardi 24 mai 2011, ALGOTEL'2011 15 Step 2: Choosing a committee where all members are also looking L L L L L L L

16. Snap-stabilizing 2-Phase Committee Coordination with Maximal Concurrency Step 3: Getting ready to participate in a meeting Cap Estérel, mardi 24 mai 2011, ALGOTEL'2011 L L L L L L L Ok

17. Snap-stabilizing 2-Phase Committee Coordination with Maximal Concurrency Cap Estérel, mardi 24 mai 2011, ALGOTEL'2011 17 Step 4: Committee meeting convenes Ok L L L

18. Snap-stabilizing 2-Phase Committee Coordination with Maximal Concurrency 18 Step 5: After essential discussion, professors leave the meeting Cap Estérel, mardi 24 mai 2011, ALGOTEL'2011 18 Ok L L L D D D D

19. Snap-stabilizing 2-Phase Fair CC Selection of the committee: ID ? Cap Estérel, mardi 24 mai 2011, ALGOTEL'2011 19 L L L L L L OkD D D D 1 3 2 5 4 6 7

20. Snap-stabilizing 2-Phase Fair CC Selection of the committee: Token-Based Priority (1/2) Let P, Q be processes If P hold a token, but not Q, then – P has a higher than Q Otherwise – Priorities are given by the IDs Cap Estérel, mardi 24 mai 2011, ALGOTEL'2011 20

21. Snap-stabilizing 2-Phase Fair CC Selection of the committee: Token-Based Priority (1/2) Cap Estérel, mardi 24 mai 2011, ALGOTEL'2011 21 L L L L L OkD D D D 1 3 2 5 4 6 7 T T L

22. When release the token ? Cap Estérel, mardi 24 mai 2011, ALGOTEL'2011 22 L L L L L 1 3 5 4 6 7 2 T If the token holder has no guarantee to eventually convene a meeting, it releases the token

23. Snap-stabilizing 2-Phase Fair Committee Coordination Recall: Every professor wants to participate in a committee meeting infinitely often. (required assumption) Mainly differ by the way the token is handled: – The token holder only releases the token at the end a meeting Cap Estérel, mardi 24 mai 2011, ALGOTEL'2011 23

24. Locked Professors Cap Estérel, mardi 24 mai 2011, ALGOTEL'2011 24 L L L Ok L 1 3 5 6 4 2 T Locked

25. Concurrency: Algo 1 vs. Algo 2 Algo 1: Maximal matching Algo 2: almost… Cap Estérel, mardi 24 mai 2011, ALGOTEL'2011 25 L L Ok L 1 3 5 4 2 T Locked

26. Conclusion Summary – We considered the distributed committee coordination problem – We showed that satisfying fairness and maximal concurrency is impossible even if professors desire to participate in meetings infinitely often – We proposed a snap-stabilizing algorithms for each conflicting property. Future work – Dynamic join / leave of professor – Priorities on committee Cap Estérel, mardi 24 mai 2011, ALGOTEL'2011 26