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 org.apache.activemq.broker.region.TopicSubscription;
020
021/**
022 * A pluggable strategy to calculate the maximum number of messages that are allowed to be pending on 
023 * consumers (in addition to their prefetch sizes).
024 * 
025 * Once the limit is reached, non-durable topics can then start discarding old messages.
026 * This allows us to keep dispatching messages to slow consumers while not blocking fast consumers
027 * and discarding the messages oldest first.
028 *  
029 * 
030 */
031public interface PendingMessageLimitStrategy {
032
033    /**
034     * Calculate the maximum number of pending messages (in excess of the prefetch size)
035     * for the given subscription
036     * 
037     * @return the maximum or -1 if there is no maximum
038     */
039    int getMaximumPendingMessageLimit(TopicSubscription subscription);
040
041}