/*
 * 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.partition;

import com.hazelcast.core.Partition;
import com.hazelcast.core.PartitionService;

import java.util.EventListener;

/**
 * PartitionLostListener provides the ability to be notified upon a possible data loss when a partition has no owner and backups.
 * <p>
 * <b>IMPORTANT:</b> Please note that it may dispatch false-positive partition lost events when partial network split errors
 * occur.
 * <p>
 * Partition loss detection algorithm works as follows:
 * When the cluster is initialized, each node becomes owner of some of the partitions, and backup of the some other partitions.
 * On a node failure, the first backup node becomes owner for a partition and all other backup nodes are shifted up in the
 * partition backup order. Therefore, some nodes sync themselves from the new partition owner node since they are shifted up in
 * the partition table and do not contain the backup data yet. Whey they issue a partition sync request, they set themselves in
 * a custom state that indicates that a node is waiting for a partition sync for a particular backup. When the sync operation is
 * completed, that sync flag is reset.
 * <p>
 * Lets say the owner of the partition crashes before it sends backup data to sync-waiting partitions.
 * The first backup which will become the owner of the partition checks its replica-sync flags. If they are set, it detects that
 * the owner failed before the backup node completes the sync process and issues a partition lost event.
 *
 * @see Partition
 * @see PartitionService
 * @since 3.5
 */
public interface PartitionLostListener extends EventListener {

    /**
     * Invoked when a partition loses its owner and all backups.
     *
     * @param event the event that contains the partition ID and the backup count that has been lost
     */
    void partitionLost(PartitionLostEvent event);
}

/*
 * 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.partition;

import com.hazelcast.core.Partition;
import com.hazelcast.core.PartitionService;

import java.util.EventListener;

/**
 * PartitionLostListener provides the ability to be notified upon a possible data loss when a partition loses a replica.
 * <p>
 * When the cluster is initialized, each node becomes owner of some of the partitions, and backup of the some other partitions.
 * We call owner of partition "primary replica" and backup nodes "backup replicas". Partition replicas are ordered.
 * A primary replica node keeps all data that is mapped to the partition. If a Hazelcast data structure is configured with
 * 1 backup, its data is put into the primary replica and the first backup replica. Similarly, data of a Hazelcast data structure
 * that is configured with 2 backups is put into the primary replica, the first backup replica, and the second backup replica.
 * The idea is same for higher backup counts.
 * <p>
 * When a node fails, primary replicas of its partitions are lost. In this case, ownership of each partition owned by
 * the unreachable node is transferred to the first available backup node. After this point, other backup nodes sync themselves
 * from the new partition owner node in order to populate the missing backup data.
 * <p>
 * In this context, the partition loss detection algorithm works as follows:
 * {@link PartitionLostEvent#lostBackupCount} denotes the replica index up to the which partition replicas are lost.
 * - 0 means that only the primary replica is lost. In other words, the node which owns the partition is unreachable,
 * hence removed from the cluster.
 * If there is a data structure configured with no backups, its data is lost for this partition.
 * - 1 means that both the primary replica and the first backup replica are lost. In other words, the partition owner node and
 * the first backup node have became unreachable. If a data structure is configured with less than 2 backups, its data is lost
 * for this partition.
 * - The idea works same for higher backup counts.
 *
 * Please note that node failures that do not involve a primary replica does not lead to partition lost events.
 * For instance, if a backup node crashes when owner of the partition is still alive, a partition lost event is not fired.
 * In this case, Hazelcast tries to assign a new backup replica to populate the missing backup.
 *
 * @see Partition
 * @see PartitionService
 * @since 3.5
 */
public interface PartitionLostListener extends EventListener {

    /**
     * Invoked when a partition loses its owner and all backups.
     *
     * @param event the event that contains the partition ID and the backup count that has been lost
     */
    void partitionLost(PartitionLostEvent event);
}

Improve partition lost listener javadoc