> Is this Magical Pixie Dust I can sprinkle on my application?
> No, not quite. Exactly-once processing is an end-to-end guarantee and the application has to be designed to not violate the property as well. If you are using the consumer API, this means ensuring that you commit changes to your application state concordant with your offsets as described here.
I think that is a pretty clear statement that end-to-end exactly once semantics doesn't come for free. It states that there needs to be additional application logic to achieve this and also specifies what must be done.
Right. But that's at the end of a separate article, while the post which this HN discussion is about throws around the words "exactly once" a lot more casually. The argument is over the use of the words "exactly once". They should just refer to the feature as "transactions" or "idempotent producer".
> Is this Magical Pixie Dust I can sprinkle on my application?
> No, not quite. Exactly-once processing is an end-to-end guarantee and the application has to be designed to not violate the property as well. If you are using the consumer API, this means ensuring that you commit changes to your application state concordant with your offsets as described here.
I think that is a pretty clear statement that end-to-end exactly once semantics doesn't come for free. It states that there needs to be additional application logic to achieve this and also specifies what must be done.