hazelcast-master/hazelcast/src/main/java/com/hazelcast/spi/merge/SplitBrainMergePolicy.java
/*
 * Copyright (c) 2008-2023, 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.spi.merge;

import com.hazelcast.core.HazelcastInstance;
import com.hazelcast.core.HazelcastInstanceAware;
import com.hazelcast.instance.impl.Node;
import com.hazelcast.internal.services.NodeAware;
import com.hazelcast.nio.serialization.DataSerializable;

/**
 * Policy for merging data structure values
 * after a split-brain has been healed.
 * <p>
 * The values of merging and existing {@link MergingValue}s are
 * always in the in-memory format of the backing data structure.
 * This can be a serialized format, so the content cannot be
 * processed without deserialization. For most merge policies
 * this will be fine, since the key or value are not used.
 * <p>
 * The deserialization is not done eagerly for two main reasons:
 * <ul>
 * <li>The deserialization is quite expensive and
 * should be avoided, if the result is not needed.</li>
 * <li>There is no need to locate classes of stored entries
 * on the server side, when the entries are not deserialized.
 * So you can put entries from a client by using {@link
 * com.hazelcast.config.InMemoryFormat#BINARY} with a different
 * classpath on client and server. In this case a deserialization
 * could throw a {@link java.lang.ClassNotFoundException}.</li>
 * </ul>
 * If you need the deserialized data you can call
 * {@link MergingValue#getValue()}
 * or {@link MergingEntry#getKey()},
 * which will deserialize the data lazily.
 * <p>
 * A merge policy can implement {@link HazelcastInstanceAware} to get the
 * {@link HazelcastInstance} injected. This can be used to retrieve the
 * user context via {@link HazelcastInstance#getUserContext()}, which is
 * an easy way to get user dependencies that are otherwise hard to obtain.
 * <p>
 * A merge policy can also implement {@link NodeAware}
 * to get an instance of {@link Node} injected.
 *
 * @param <V> the (deserialized) type of the merging value
 * @param <T> the type of the required merging value, e.g. a simple
 *            {@code MergingValue<V>} or a composition like {@code
 *            MergingEntry<String, V> & MergingHits & MergingLastAccessTime}
 * @param <R> the type of the merged value as returned by {@link #merge(MergingValue, MergingValue)}
 * @since 3.10
 */
public interface SplitBrainMergePolicy<V, T extends MergingValue<V>, R>
        extends DataSerializable {

    /**
     * Selects the value of either the merging or the
     * existing {@link MergingValue} which should be merged.
     * <p>
     * Note that the existing {@link MergingValue} instance may be {@code null}
     * if no matching data could be found to the merging {@link MergingValue}.
     *
     * @param mergingValue  {@link MergingValue} instance that has the
     *                      merging data of the smaller sub-cluster
     * @param existingValue {@link MergingValue} instance that has the existing
     *                      data or {@code null} if no matching data exists
     * @return the selected value for merging
     */
    R merge(T mergingValue, T existingValue);
}
