Leader election

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.

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. An algorithm solves the leader election problem if:

A valid leader election algorithm must meet the following conditions:

An algorithm for leader election may vary in following aspects:

A ring network is a connected-graph topology in which each node is exactly connected to two other nodes, i.e., for a graph with n nodes, there are exactly n edges connecting the nodes. A ring can be unidirectional, which means processors only communicate in one direction (a node could only send messages to the left or only send messages to the right), or bidirectional, meaning processors may transmit and receive messages in both directions (a node could send messages to the left and right).

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