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     * <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     */
045    public 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    }