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In the order processing example introduced in the previous patterns , each order item that is not currently in stock could be supplied by one of multiple external suppliers. However, the suppliers may or may not have the respective item in stock themselves, they may charge a different price and may be able to supply the part by a different date. To fill the order in the best way possible, we should request quotes from all suppliers and decide which one provides us with the best term for the requested item.

How do you maintain the overall message flow when a message needs to be sent to multiple recipients, each of which may send a reply?

Use a Scatter-Gather that broadcasts a message to multiple recipients and re-aggregates the responses back into a single message.

The Scatter-Gather routes a request message to the a number of recipients. It then uses an Aggregator to collect the responses and distill them into a single response message.

Related patterns: Aggregator, Introduction to Composed Messaging Examples, Asynchronous Implementation with MSMQ, Asynchronous Implementation with TIBCO ActiveEnterprise, Composed Message Processor, Publish-Subscribe Channel, Recipient List, Return Address, Splitter

For a full description of the pattern see:

Enterprise Integration Patterns Gregor Hohpe and Bobby Woolf ISBN 0321200683 650 pages Addison-Wesley

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