com.hazelcast.quorum.QuorumFunction Maven / Gradle / Ivy
/*
* Copyright (c) 2008-2018, Hazelcast, Inc. All Rights Reserved.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.hazelcast.quorum;
import com.hazelcast.core.Member;
import java.util.Collection;
/**
* A function that can be used to conclude absence/presence of quorum.
* The quorum function is consulted:
*
* - When a cluster membership change occurs (member added or removed)
* - Whenever a hearbeat is received from a member, in case the {@code QuorumFunction} also
* implements {@link HeartbeatAware}
*
*
* A {@code QuorumFunction} that implements {@link HeartbeatAware} will be also notified of heartbeats.
* Additionally, a {@code QuorumFunction} that implements {@link com.hazelcast.core.MembershipListener}
* will be notified of membership events. All listener invocations are executed before the
* {@code QuorumFunction}'s {@link #apply(Collection)} method is invoked.
*
* IMPORTANT: The term "quorum" simply refers to the count of members in the cluster required for an operation to succeed.
* It does NOT refer to an implementation of Paxos or Raft protocols as used in many NoSQL and distributed systems.
* The mechanism it provides in Hazelcast protects the user in case the number of nodes in a cluster drops below the
* specified one.
*/
public interface QuorumFunction {
/**
* Determines if quorum is present based on the current list of members in the cluster.
* Lite members are excluded and only data members are provided to this method.
*
* This method should not block nor execute time-consuming operations, otherwise it may stall other quorum function
* invocations.
*
* @param members snapshot of current member list
* @return boolean presence/absence of quorum
*/
boolean apply(Collection members);
}