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Composed Message Processor

The Composed Message Processor from the EIP patterns allows you to process a composite message by splitting it up, routing the sub-messages to appropriate destinations and the re-aggregating the responses back into a single message.

In Camel we provide two solutions

The difference is when using only a Splitter it aggregates back all the splitted messages into the same aggregation group, eg like a fork/join pattern.
Whereas using the Aggregator allows you group into multiple groups, a pattern which provides more options.

Using the splitter alone is often easier and possibly a better solution. So take a look at this first, before involving the aggregator.

Example using both Splitter and Aggregator

In this example we want to check that a multipart order can be filled. Each part of the order requires a check at a different inventory.

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// split up the order so individual OrderItems can be validated by the appropriate bean
        .when().method("orderItemHelper", "isWidget")

// collect and re-assemble the validated OrderItems into an order again
    .aggregate(new MyOrderAggregationStrategy()).header("orderId").completionTimeout(100).completionTimeoutCheckerInterval(10)

Using the Spring XML Extensions

  <from uri="direct:start"/>
        <method bean="orderItemHelper" method="isWidget"/>
	<to uri="bean:widgetInventory"/>
	<to uri="bean:gadgetInventory"/>
    <to uri="seda:aggregate"/>

  <from uri="seda:aggregate"/>
  <aggregate strategyRef="myOrderAggregatorStrategy" completionTimeout="1000">
    <to uri="mock:result"/>

To do this we split up the order using a Splitter. The Splitter then sends individual OrderItems to a Content Based Router which checks the item type. Widget items get sent for checking in the widgetInventory bean and gadgets get sent to the gadgetInventory bean. Once these OrderItems have been validated by the appropriate bean, they are sent on to the Aggregator which collects and re-assembles the validated OrderItems into an order again.

When an order is sent it contains a header with the order id. We use this fact when we aggregate, as we configure this .header("orderId") on the aggregate DSL to instruct Camel to use the header with the key orderId as correlation expression.

For full details, check the example source here:


Example using only Splitter

In this example we want to split an incoming order using the Splitter eip, transform each order line, and then combine the order lines into a new order message.

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// this routes starts from the direct:start endpoint
// the body is then splitted based on @ separator
// the splitter in Camel supports InOut as well and for that we need
// to be able to aggregate what response we need to send back, so we provide our
// own strategy with the class MyOrderStrategy.
    .split(body().tokenize("@"), new MyOrderStrategy())
        // each splitted message is then send to this bean where we can process it
        // this is important to end the splitter route as we do not want to do more routing
        // on each splitted message
    // after we have splitted and handled each message we want to send a single combined
    // response back to the original caller, so we let this bean build it for us
    // this bean will receive the result of the aggregate strategy: MyOrderStrategy

Using XML

If you use XML, then the <split> tag offers the strategyRef attribute to refer to your custom AggregationStrategy

The bean with the methods to transform the order line and process the order as well:

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public static class MyOrderService {

    private static int counter;

     * We just handle the order by returning a id line for the order
    public String handleOrder(String line) {
        LOG.debug("HandleOrder: " + line);
        return "(id=" + ++counter + ",item=" + line + ")";

     * We use the same bean for building the combined response to send
     * back to the original caller
    public String buildCombinedResponse(String line) {
        LOG.debug("BuildCombinedResponse: " + line);
        return "Response[" + line + "]";

And the AggregationStrategy we use with the Splitter eip to combine the orders back again (eg fork/join):

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 * This is our own order aggregation strategy where we can control
 * how each splitted message should be combined. As we do not want to
 * loos any message we copy from the new to the old to preserve the
 * order lines as long we process them
public static class MyOrderStrategy implements AggregationStrategy {

    public Exchange aggregate(Exchange oldExchange, Exchange newExchange) {
        // put order together in old exchange by adding the order from new exchange

        if (oldExchange == null) {
            // the first time we aggregate we only have the new exchange,
            // so we just return it
            return newExchange;

        String orders = oldExchange.getIn().getBody(String.class);
        String newLine = newExchange.getIn().getBody(String.class);

        LOG.debug("Aggregate old orders: " + orders);
        LOG.debug("Aggregate new order: " + newLine);

        // put orders together separating by semi colon
        orders = orders + ";" + newLine;
        // put combined order back on old to preserve it

        // return old as this is the one that has all the orders gathered until now
        return oldExchange;

Using This Pattern

If you would like to use this EIP Pattern then please read the Getting Started, you may also find the Architecture useful particularly the description of Endpoint and URIs. Then you could try out some of the Examples first before trying this pattern out.

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