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    1   /**
    2    * Licensed to the Apache Software Foundation (ASF) under one or more
    3    * contributor license agreements.  See the NOTICE file distributed with
    4    * this work for additional information regarding copyright ownership.
    5    * The ASF licenses this file to You under the Apache License, Version 2.0
    6    * (the "License"); you may not use this file except in compliance with
    7    * the License.  You may obtain a copy of the License at
    8    *
    9    *      http://www.apache.org/licenses/LICENSE-2.0
   10    *
   11    * Unless required by applicable law or agreed to in writing, software
   12    * distributed under the License is distributed on an "AS IS" BASIS,
   13    * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   14    * See the License for the specific language governing permissions and
   15    * limitations under the License.
   16    */
   17   package org.apache.activemq.broker.region.policy;
   18   
   19   import org.apache.activemq.broker.region.TopicSubscription;
   20   
   21   /**
   22    * A pluggable strategy to calculate the maximum number of messages that are allowed to be pending on 
   23    * consumers (in addition to their prefetch sizes).
   24    * 
   25    * Once the limit is reached, non-durable topics can then start discarding old messages.
   26    * This allows us to keep dispatching messages to slow consumers while not blocking fast consumers
   27    * and discarding the messages oldest first.
   28    *  
   29    * @version $Revision: 563921 $
   30    */
   31   public interface PendingMessageLimitStrategy {
   32   
   33       /**
   34        * Calculate the maximum number of pending messages (in excess of the prefetch size)
   35        * for the given subscription
   36        * 
   37        * @return the maximum or -1 if there is no maximum
   38        */
   39       int getMaximumPendingMessageLimit(TopicSubscription subscription);
   40   
   41   }

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