It's hard to say, but Kafka provides a number of important predicates for idempotent processing out of the box for consumers and producers. Given that stream-style-apps are just a series of consumers and producers linked together, there is good reason to believe this would offer idempotent processing.

Hi, thanks for the comments. My name is Vineet and I am a co-creator of this project along with Ask.

We do plan on adding support for Kafka exactly once semantics to Faust. As @dleclere pointed out, streaming apps are simply a topology of consumers and producers. We will be adding this as soon as the underlying kafka clients add support for exactly once semantics.