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 * @version $Revision: 564271 $ 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 }