/*
 * 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.query;

import org.apache.geode.cache.CacheCallback;

/**
 * Application plug-in interface for handling continuous query events after they occur. The listener
 * has two methods, one that is called when there is an event satisfied by the CQ and the other
 * called when there is an error during CQ processing.
 *
 * @since GemFire 5.5
 */

public interface CqListener extends CacheCallback {

  /**
   * This method is invoked when an event is occurred on the region that satisfied the query
   * condition of this CQ. This event does not contain an error. If CQ is executed using
   * ExecuteWithInitialResults the returned result may already include the changes with respect to
   * this event. This could arise when updates are happening on the region while CQ registration is
   * in progress. The CQ does not block any region operation as it could affect the performance of
   * region operation. Its up to the application to synchronize between the region operation and CQ
   * registration to avoid duplicate event being delivered.
   *
   * @param aCqEvent the {@link CqEvent} that triggered the listener
   *
   * @see org.apache.geode.cache.query.CqQuery#executeWithInitialResults
   */
  void onEvent(CqEvent aCqEvent);

  /**
   * This method is invoked when there is an error during CQ processing. The error can appear while
   * applying query condition on the event. e.g if the event doesn't has attributes as specified in
   * the CQ query. This event does contain an error. The newValue may or may not be available, and
   * will be null if not available.
   *
   * @param aCqEvent the {@link CqEvent} which encountered the error
   */
  void onError(CqEvent aCqEvent);
}
