While writing about the Idempontet Filter Patter in my recent Camel Design Patterns book, I wanted to visualise the Idempontet Consumer EIP on the Camel routes but couldn't find an icon for it. Inspired by existing Normalizer and Resequencer icons, I've created an Idempotent Consumer Icon.
Similarly, for the Idempotent Consumer I've used a circle and square to represent different incoming messages (that is not different data formats, but different business data). And to emphasise that it is a stateful pattern and can remember past messages, I've put the duplicate square messages apart from one another. Then from the icon we can see that, the pattern removes the duplicate square messages and allows only unique message to pass on.
Couple of days after that, I've read A Decade of Enterprise Integration Patterns: A Conversation with the Authors where Gregor said this:
Olaf: And what would you do differently now?
Gregor: I would make an icon for the Idempotent Receiver pattern, which describes a receiver that can process the same message multiple times without any harm. Somehow we seem to have missed that one.
Not sure whether one day the icon will become part of the existing EIP icon set or not, but I've found it visually helpful to depict the idempotent behaviour. You can download a .png, visio and OmniGraffle stencil from here.