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Bully algorithm

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In distributed computing, the bully algorithm is a method for dynamically electing a coordinator or leader from a group of distributed computer processes. The process with the highest process ID number from amongst the non-failed processes is selected as the coordinator.

Assumptions

The algorithm assumes that:[1]

  • the system is synchronous.
  • processes may fail at any time, including during execution of the algorithm.
  • a process fails by stopping and returns from failure by restarting.
  • there is a failure detector which detects failed processes.
  • message delivery between processes is reliable.
  • each process knows its own process id and address, and that of every other process.

Algorithm

The algorithm uses the following message types:

  • Election Message: Sent to announce election
  • Answer (Alive) Message: Responds to the election message
  • Coordinator (Victory) Message: Sent by winner of the election to announce victory.

When a process P recovers from failure, or the failure detector indicates that the current coordinator has failed, it performs the following sequence of actions:

  1. If P has the highest process id, it sends a Victory message to all other processes and becomes the new Coordinator. Otherwise, P broadcasts an Election message to all other processes with higher process IDs than itself.
  2. If P receives no Answer after sending an Election message, then it broadcasts a Victory message to all other processes and becomes the Coordinator.
  3. If P receives an Answer from a process with a higher ID, it sends no further messages for this election and waits for a Victory message. (If there is no Victory message after a period of time, it starts a new election.)
  4. If P receives an Election message from another process with a lower ID it sends an Answer message back and starts a new election (that is, P sends Election messages to the processes with higher ids than itself).
  5. If P receives a Coordinator message, it treats the sender as the coordinator.

See also

References

  1. ^ Jean Dollimore, Tim Kindberg, George F. Coulouris, "Distributed systems : concepts and design (Third Edition)," in Distributed systems : concepts and design (Third Edition). Addison–Wesley, 2003.
  • Witchel, Emmett (2005). "Distributed Coordination". Retrieved May 4, 2005.
  • Hector Garcia-Molina, Elections in a Distributed Computing System, IEEE Transactions on Computers, Vol. C-31, No. 1, January (1982) 48–59
  • L. Lamport, R. Shostak, and M. Pease, "The Byzantine Generals Problem" ACM Transactions on Programming Languages and Systems, Vol. 4, No. 3, July 1982.
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