Kafka has exactly-once semantics that mostly work. They also have "idempotent producer" which ostensibly provides atomic transactions as well. Sounds awesome,

Kafka has exactly-once semantics that mostly work. They also have "idempotent producer" which ostensibly provides atomic transactions as well. Sounds awesome, but the implementation pretty much breaks kafka in that one can no longer scale. Topic and partition are all constrained to a single broker. Hot spots abound!