trunk/activemq-core/src/main/java/org/apache/activemq/broker/region/policy/PendingMessageLimitStrategy.java

/**
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.activemq.broker.region.policy;

import org.apache.activemq.broker.region.TopicSubscription;

/**
 * A pluggable strategy to calculate the maximum number of messages that are allowed to be pending on 
 * consumers (in addition to their prefetch sizes).
 * 
 * Once the limit is reached, non-durable topics can then start discarding old messages.
 * This allows us to keep dispatching messages to slow consumers while not blocking fast consumers
 * and discarding the messages oldest first.
 *  
 * 
 */
public interface PendingMessageLimitStrategy {

    /**
     * Calculate the maximum number of pending messages (in excess of the prefetch size)
     * for the given subscription
     * 
     * @return the maximum or -1 if there is no maximum
     */
    int getMaximumPendingMessageLimit(TopicSubscription subscription);

}