There's lots of words in this article (and design doc: http://goo.gl/EsdTXo ) but it's missing a very simple key points?

1. State diagram: What are the messages exchanged between Producer and Consumer (and how many round trips to confirm a message)

2. At what point does each side consider the message to have been delivered?

3. Has this been tested empirically? (i.e: Setup producer, consumer, and partition/kill each side randomly to see if messages get lost)

The one thing I don't understand is the following. The two parties communicate by message passing. At some point the message will transition to a new state (i.e: delivered). That transition cannot happen on both sides at the same time. So how do you handle the failure of sending of the last message? Do you stage messages until after the timeout period has passed?