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The order-processing example presented in the Content-Based Router and Splitter patterns processes an incoming order consisting of individual line items. Each line item requires an inventory check with the respective inventory system. After all items have been verified we want to pass the validated order message to the next processing step.

How can you maintain the overall message flow when processing a message consisting of multiple elements, each of which may require different processing?

Use Composed Message Processor to process a composite message. The Composed Message Processor splits the message up, routes the sub-messages to the appropriate destinations and re-aggregates the responses back into a single message.

The Composed Message Processor uses an Aggregator to reconcile the requests that were dispatched to the multiple inventory systems. Each processing unit sends a response message to the aggregator stating the inventory on hand for the specified item. The Aggregator collects the individual responses and processes them based on a predefined algorithm as described under Aggregator.

Related patterns: Aggregator, Content-Based Router, Pipes and Filters, Splitter

Find the full description of this pattern in: Enterprise Integration Patterns Gregor Hohpe and Bobby Woolf ISBN 0321200683 650 pages Addison-Wesley

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