|
In distributed computing, leader election is the process of designating a single process as the organizer of some task distributed among several computers (nodes). Before the task is begun, all network nodes are either unaware which node will serve as the "leader" (or coordinator) of the task, or unable to communicate with the current coordinator. After a leader election algorithm has been run, however, each node throughout the network recognizes a particular, unique node as the task leader. The network nodes communicate among themselves in order to decide which of them will get into the "leader" state. For that, they need some method in order to break the symmetry among them. For example, if each node has unique and comparable identities, then the nodes can compare their identities, and decide that the node with the highest identity is the leader. The definition of this problem is often attributed to LeLann, who formalized it as a method to create a new token in a token ring network in which the token has been lost. Leader election algorithms are designed to be economical in terms of total bytes transmitted, and time. The algorithm suggested by Gallager, Humblet, and Spira for general undirected graphs has had a strong impact on the design of distributed algorithms in general, and won the Dijkstra Prize for an influential paper in distributed computing. Many other algorithms were suggested for different kind of network graphs, such as undirected rings, unidirectional rings, complete graphs, grids, directed Euler graphs, and others. A general method that decouples the issue of the graph family from the design of the leader election algorithm was suggested by Korach, Kutten, and Moran. ==Definition== The problem of leader election is for each processor eventually to decide that whether it is a leader or not subject to only one processor decides that it is the leader.〔H. Attiya and J. Welch, ''Distributed Computing: Fundamentals, Simulations and Advance Topics'', John Wiley & Sons inc., 2004, chap. 3〕 An algorithm solves the leader election problem if: # States of processors are divided into elected and not elected states. Once elected, it remains as elected (similarly if not elected). # In every execution, exactly one processor becomes elected and the rest determine that they are not elected. A valid leader election algorithm must meet the following conditions:〔I. Gupta, R. van Renesse, and K. P. Birman,2000, A Probabilistically Correct Leader Election Protocol for Large Groups, ''Technical Report'' , Cornell University〕 # Termination: the algorithm should finish eventually within a finite time one leader is selected. In randomized approaches this condition is sometimes weakened (for example, requiring termination with probability 1). # Uniqueness: there is exactly one processor that considers itself as leader. # Agreement: all other processors know who the leader is. An algorithm for leader election may vary in following aspects:〔R. Bakhshi, W. Fokkink, J. pang, and J. Van de Pol,c2008 "Leader Election in Anonymous Rings:Franklin Goes Probabilistic", ''TCS'', Vol. 273, pp. 57-72.〕 *Communication mechanism: the processors are either synchronous in which processes are synchronized by a clock signal or asynchronous where processes run at arbitrary speeds. *Process names: whether processes have a unique identity or are indistinguishable (anonymous). *Network topology: for instance, ring, acyclic graph or complete graph. *Size of the network: the algorithm may or may not use knowledge of the number of processes in the system. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「leader election」の詳細全文を読む スポンサード リンク
|