Martin Fowler

Select one server amongst the cluster as leader. The leader is responsible for taking decisions on behalf of the entire cluster and propagating the decisions to all the other servers.

Every server at startup looks for an existing leader. If no leader is found, it triggers a leader election. The servers accept requests only after a leader is selected successfully. Only the leader handles the client requests. If a request is sent to a follower server, the follower can forward it to the leader server.

Leader Election

Figure 1: Election

Figure 2: Votes

Figure 3: Leader Heartbeats

For smaller clusters of three to five nodes, like in the systems which implement consensus, leader election can be implemented within the data cluster itself without dependending on any external system. Leader election happens at server startup. Every server starts a leader election at startup and tries to elect a leader. The

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