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.camel.processor.aggregate;
018    
019    import 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     * 
039     * @version $Revision: 885364 $
040     */
041    public interface AggregationStrategy {
042    
043        /**
044         * Aggregates an old and new exchange together to create a single combined exchange
045         *
046         * @param oldExchange the oldest exchange (is <tt>null</tt> on first aggregation as we only have the new exchange)
047         * @param newExchange the newest exchange (can be <tt>null</tt> if there was no data possible to acquire)
048         * @return a combined composite of the two exchanges
049         */
050        Exchange aggregate(Exchange oldExchange, Exchange newExchange);
051    }