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.camel.processor.aggregate; 018 019import org.apache.camel.Exchange; 020 021/** 022 * A strategy for aggregating two exchanges together into a single exchange. 023 * <p/> 024 * On the first invocation of the {@link #aggregate(org.apache.camel.Exchange, org.apache.camel.Exchange) aggregate} 025 * method the <tt>oldExchange</tt> parameter is <tt>null</tt>. The reason is that we have not aggregated anything yet. 026 * So its only the <tt>newExchange</tt> that has a value. Usually you just return the <tt>newExchange</tt> in this 027 * situation. But you still have the power to decide what to do, for example you can do some alternation on the exchange 028 * or remove some headers. And a more common use case is for instance to count some values from the body payload. That 029 * could be to sum up a total amount etc. 030 * <p/> 031 * It is possible that <tt>newExchange</tt> is <tt>null</tt> which could happen if there was no data possible 032 * to acquire. Such as when using a {@link org.apache.camel.processor.PollEnricher} to poll from a JMS queue which 033 * is empty and a timeout was set. 034 * <p/> 035 * Possible implementations include performing some kind of combining or delta processing, such as adding line items 036 * together into an invoice or just using the newest exchange and removing old exchanges such as for state tracking or 037 * market data prices; where old values are of little use. 038 * <p/> 039 * If an implementation also implements {@link org.apache.camel.Service} then any <a href="http://camel.apache.org/eip">EIP</a> 040 * that allowing configuring a {@link AggregationStrategy} will invoke the {@link org.apache.camel.Service#start()} 041 * and {@link org.apache.camel.Service#stop()} to control the lifecycle aligned with the EIP itself. 042 * 043 * @version 044 */ 045public interface AggregationStrategy { 046 047 // TODO: In Camel 3.0 we should move this to org.apache.camel package 048 049 /** 050 * Aggregates an old and new exchange together to create a single combined exchange 051 * 052 * @param oldExchange the oldest exchange (is <tt>null</tt> on first aggregation as we only have the new exchange) 053 * @param newExchange the newest exchange (can be <tt>null</tt> if there was no data possible to acquire) 054 * @return a combined composite of the two exchanges 055 */ 056 Exchange aggregate(Exchange oldExchange, Exchange newExchange); 057}