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Even when a sender application only sends a message once, the receiver application may receive the message more than once.

How can a message receiver deal with duplicate messages?

Design a receiver to be an Idempotent Receiver--one that can safely receive the same message multiple times.

The term idempotent is used in mathematics to describe a function that produces the same result if it is applied to itself, i.e. f(x) = f(f(x)). In Messaging this concepts translates into the a message that has the same effect whether it is received once or multiple times. This means that a message can safely be resent without causing any problems even if the receiver receives duplicates of the same message.

Related patterns: Guaranteed Delivery

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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