One replica is down. The cart still takes your write.

transcript

Your cart lives on three machines. One of them just died. You tap add to cart. Does the store say no?

The brief was a shopping cart. One rule: never reject a write. Insist on all three replicas, and one dead machine stops the cart. So the store does something sloppy. On purpose.

Your key belongs on A, B and C. C isn't answering. So the coordinator writes to A, B, and the next healthy node, D. It needs two acknowledgements. A and B give them. Write accepted.

But here's the catch. D was never meant to hold your cart. So it stores the copy with a hint. This belongs to C. No master here. Nodes gossip, and D hears that C is back. D sends the copy over, and deletes its own. Hinted handoff.

Handoff covers a replica that comes back. Copies can still disagree. Read repair. A read sees two versions? Return the newest. Fix the stale one. In the background, replicas compare Merkle trees. Hash trees over key ranges. Matching roots? Identical. A mismatch narrows level by level. Only differing keys move.

A dead replica didn't stop the cart. A neighbour held its copy. Sloppy quorum to stay writable. Handoff and repair to heal. Take the write now. Heal the copies later.

Full breakdown in the lecture.