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 }