[dclusin]

Sequence numbers on messages sent by producers coupled with some sort of persistence by the sender. This way when a gap is detected some sort of resend protocol can be implemented to achieve the delivery guarantee.

[jlgreco]

So the sender just keeps trying until it gets an ACK. Fair enough, though I imagine you'd want to run multiple nodes even with this.

[dclusin]

Correct. Reliability and Availability are two separate but related goals.