Exactly-once delivery really depends where and how you actually "deliver". Even if your writes are idempontent you need to know if they have been properly commi

Exactly-once delivery really depends where and how you actually "deliver". Even if your writes are idempontent you need to know if they have been properly committed on the other side, not that simple. If the system you are committing your output offers version control and/or proper transactional support then delivery can be eventually re-conciliated. Apache Flink's snapshotting algorithm solely guarantees exactly-once application state access, plain and simple.