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 java.util.List;
020
021import org.apache.activemq.broker.region.MessageReference;
022import org.apache.activemq.broker.region.Subscription;
023import org.apache.activemq.filter.MessageEvaluationContext;
024
025/**
026 * Abstraction to allow different dispatching policies to be plugged
027 * into the region implementations.  This is used by a queue to deliver
028 * messages to the matching subscriptions.
029 * 
030 * 
031 */
032public interface DispatchPolicy {
033    
034    /**
035     * Decides how to dispatch a selected message to a collection of consumers.  A safe
036     * approach is to dispatch to every subscription that matches.  Queue Subscriptions that 
037     * have not exceeded their pre-fetch limit will attempt to lock the message before 
038     * dispatching to the client.  First subscription to lock the message wins.  
039     * 
040     * Order of dispatching to the subscriptions matters since a subscription with a 
041     * large pre-fetch may take all the messages if he is always dispatched to first.  
042     * Once a message has been locked, it does not need to be dispatched to any 
043     * further subscriptions.
044     * 
045     * The list will be safe to iterate over when this method is called
046     * 
047     * @return true if at least one consumer was dispatched or false if there are no active subscriptions that could be dispatched
048     */
049    boolean dispatch(MessageReference node, MessageEvaluationContext msgContext, List<Subscription> consumers) throws Exception;
050
051}