Kafka actually does call these transactions! However (and this is a loooong discussion I can't really dig into right now) there's sort of two ways to look at "e

Kafka actually does call these transactions! However (and this is a loooong discussion I can't really dig into right now) there's sort of two ways to look at "exactly once". One is in the sense that DB transactions are "exactly once"; a transaction's effects shouldn't be duplicated or lost. But in another sense "exactly once" is a sort of dataflow graph property that relates messages across topic-partitions. That's a little more akin to ACID "consistency". You can use transactions to get to that dataflow property, in the same sort of way that Serializable transaction systems guarantee certain kinds of domain-level consistency. For example, Serializability guarantees that any invariant preserved by a set of transactions, considered purely in isolation, is also preserved by concurrent histories of those transactions. I think you can argue Kafka intends to reach "exactly-once semantics" through transactions in that way.