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.
The algorithm assumes that:
- 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.
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, P performs the following actions:
- 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.
- If P receives no Answer after sending an Election message, then it broadcasts a Victory message to all other processes and becomes the Coordinator.
- 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 restarts the process at the beginning.)
- If P receives an Election message from another process with a lower ID it sends an Answer message back and starts the election process at the beginning, by sending an Election message to higher-numbered processes.
- If P receives a Coordinator message, it treats the sender as the coordinator.
- 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.