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 */
017package org.apache.activemq.broker.region.policy;
018
019import javax.jms.MessageListener;
020
021import org.apache.activemq.command.ActiveMQDestination;
022import 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 */
029public 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}