001    /**
002     * Licensed to the Apache Software Foundation (ASF) under one or more
003     * contributor license agreements.  See the NOTICE file distributed with
004     * this work for additional information regarding copyright ownership.
005     * The ASF licenses this file to You under the Apache License, Version 2.0
006     * (the "License"); you may not use this file except in compliance with
007     * the License.  You may obtain a copy of the License at
008     *
009     *      http://www.apache.org/licenses/LICENSE-2.0
010     *
011     * Unless required by applicable law or agreed to in writing, software
012     * distributed under the License is distributed on an "AS IS" BASIS,
013     * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014     * See the License for the specific language governing permissions and
015     * limitations under the License.
016     */
017    package org.apache.activemq.broker.region.policy;
018    
019    import javax.jms.MessageListener;
020    
021    import org.apache.activemq.command.ActiveMQDestination;
022    import org.apache.activemq.command.Message;
023    
024    /**
025     * Represents some kind of query which will load initial messages from some source for a new topic subscriber.
026     * 
027     * 
028     */
029    public interface MessageQuery {
030        
031        /**
032         * Executes the query for messages; each message is passed into the listener
033         * 
034         * @param destination the destination on which the query is to be performed
035         * @param listener is the listener to notify as each message is created or loaded
036         */
037        void execute(ActiveMQDestination destination, MessageListener listener) throws Exception;
038    
039        /**
040         * Returns true if the given update is valid and does not overlap with the initial message query.
041         * When performing an initial load from some source, there is a chance that an update may occur which is logically before
042         * the message sent on the initial load - so this method provides a hook where the query instance can keep track of the version IDs
043         * of the messages sent so that if an older version is sent as an update it can be excluded to avoid going backwards in time.
044         * 
045         * e.g. if the execute() method creates version 2 of an object and then an update message is sent for version 1, this method should return false to 
046         * hide the old update message.
047         * 
048         * @param message the update message which may have been sent before the query actually completed
049         * @return true if the update message is valid otherwise false in which case the update message will be discarded.
050         */
051        boolean validateUpdate(Message message);
052    
053    }