/*
 * Licensed to the Apache Software Foundation (ASF) under one or more contributor license
 * agreements. See the NOTICE file distributed with this work for additional information regarding
 * copyright ownership. The ASF licenses this file to You 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 org.apache.geode.cache.util;



/**
 * GatewayConflictResolver is a Cache-level plugin that is called upon to decide what to do with
 * events that originate in other systems and arrive through the WAN Gateway. A
 * GatewayConflictResolver is invoked if the current value in a cache entry was established by a
 * different distributed system (with a different distributed-system-id) than an event that is
 * attempting to modify the entry. It is not invoked if the event has the same distributed system ID
 * as the event that last changed the entry.
 *
 * @since GemFire 7.0
 */
public interface GatewayConflictResolver {
  /**
   * This method is invoked when a change is received from another distributed system and the last
   * modification to the affected cache entry did not also come from the same system.
   * <p>
   * The given GatewayConflictHelper can be used to allow the change to be made to the cache,
   * disallow the modification or make a change to the value to be stored in the cache.
   * <p>
   * This method is invoked under synchronization on the cache entry in order to prevent it from
   * concurrent modification
   * <p>
   * For any two events, all GatewayConflictResolvers must make the same decision on the resolution
   * of the conflict in order to maintain consistency. They must do so regardless of the order of
   * the events.
   * <p>
   *
   * @param event the event that is in conflict with the current cache state
   * @param helper an object to be used in modifying the course of action for this event
   */
  void onEvent(TimestampedEntryEvent event, GatewayConflictHelper helper);

}
