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My book uses it; see Idempotent Receiver. While we were working on the book, Gregor noticed the importance of this term floating around, idempotent, and felt it was a pattern in messaging. We came up with two examples:

  • A message with content structured such that it could be received repeatedly and would only change the receiver the first time. For example, rather than saying to add $10 to an account, an idempotent version of the message would say to set the account balance to $110.
  • A receiver implemented to remember the messages it had received, detect duplicates, and ignore all but one of them (usually the first), ensuring that a message with duplicates would nevertheless be processed only once.
It's the latter application of the concept that made it into the book.

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