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;
019import org.apache.camel.Exchange;
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 {
047    // TODO: In Camel 3.0 we should move this to org.apache.camel package
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);