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Leader Election

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  1. Leader Election Bernard Chen

  2. Outlines • Introduction to leader election • Complete Topology • Logical Ring Topology • Tree Topology • Secure Extrema Finding Algorithm (SEFA) • References

  3. What is leader selection? • The existence of a centralized controller greatly simplifies process synchronization • However, if the central controller breaks down, the service availability can be limited • The problem can be alleviated if a new controller (leader) can be chosen

  4. When do we need leader election? • 1. The initiation of a system of processes • 2. When an existing leader fails (The detection of failures is normally based on a time-out mechanism)

  5. Two Types of Election Criteria • 1. a leader election can be based on a global priority. This type of election is called extrema finding. (Every node has a fixed evaluation value) • 2. processes in the group can vote for a leader based on a node’s preference, it’s called a preference-based leader election algorithm.


  6. Leader Election vs. Mutual Exclusion • In many aspects, leader election and mutual exclusion are the same; they both try to reach an agreement for identifying a unique process • However, there are some differences

  7. Leader Election vs. Mutual Exclusion • The two major differences are: • A mutual exclusion must ensure that no process is starved, while a leader election is more concerned with the fast and successful termination of the election process • Another distinction is that the result for leader election need to be announced to all processes

  8. Outlines • Introduction to leader election • Complete Topology • Logical Ring Topology • Tree Topology • Secure Extrema Finding Algorithm (SEFA) • References

  9. Complete Topology • In a complete topology, each process in the group can reach any other process in the same group in onemessage hop. • Each process as a global priority, and the highest-priority is elected leader

  10. The Bully Algorithm • It’s a extrema-finding algorithm • A process starts a leader election if it suspects that the coordinator has failed. • If a process Pheavy receives an election message from a lighter process Plight, it sends a take-over message to Plight. Plight is out of the race. • If a process doesn’t get a take-over message back, it wins, and sends a victory message to all other processes.

  11. The Bully Algorithm • The bully election algorithm • Process 4 holds an election • Process 5 and 6 respond, telling 4 to stop • Now 5 and 6 each hold an election

  12. The Bully Algorithm • Process 6 tells 5 to stop • Process 6 wins and tells everyone

  13. Outlines • Introduction to leader election • Complete Topology • Logical Ring Topology • Tree Topology • Secure Extrema Finding Algorithm (SEFA) • References

  14. Logical Ring Topology • A logical ring is easy to construct and offers the unique property that a message initiated by any node will return to the node, indicating completion of a round of broadcast

  15. Leader Election • Initiation: • A process sends an ELECTION message to its successor (or next alive process) with its ID • Each process adds its own ID and forwards the ELECTION message • Leader Election: • Message comes back to initiator • Initiator announces the winner by sending another message around the ring

  16. Ring Algorithm: Initiation

  17. Ring Algorithm - Election

  18. Outlines • Introduction to leader election • Complete Topology • Logical Ring Topology • Tree Topology • References

  19. Tree Topology • To construct a logical ring structure is easy if the underlying network supports broadcast facilities • However, in Ad Hoc networks, user mobility may results in frequent leader election, making the process a critical component of system operation

  20. Leader election in Ad Hoc networks • Most of the leader election algorithms for mobile ad hoc networks elect a “random” leader and hence are not extrema-finding • SEFA (Vasudevan S., 2003) is a round-based hierarchy-building approach which also considered secure problem • Good reviews can be found in [3](G. Tel. 2000) and [4](N.Lynch 1996)

  21. Secure Extrema Finding Algorithm (SEFA) • SEFA is an extrema-finding algorithm in which all nodes use a Common Election Algorithm (CEA) to determine their rank • Decision factors might include node identity, battery lifes, or level of trust…etc • Larger the CEA value, the more “desirable” the node is a leader

  22. Algorithm Example

  23. Algorithm Example

  24. Outlines • Introduction to leader election • Complete Topology • Logical Ring Topology • Tree Topology • Secure Extrema Finding Algorithm (SEFA) • References

  25. References • Chow, Randy, et. al.,Distributed Operating Systems & Algorithms, Addison Wesley, March 18, 1997 • Vasudevan, S.; DeCleene, B.; Immerman, N.; Kurose, J.; Towsley, D.; “Leader election algorithms for wireless ad hoc networks”, DARPA Information Survivability Conference and Exposition, 2003. Proceedings • G. Tel. Introduction to Distributed Algorithms. Second Edition, 2000. Cambridge University Press • N.Lynch Distributed Algorithms. 1996, Morgan Kaufmann Publications • V. Park and M. Corson. A Highly adaptive Distributed Routing Algorithm for mobile Wireless Networks. In proceediing of IEEE INFOCOM, April 1997

  26. Thank You • Questions??