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 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     */
031    public 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    }