KV: hinted handoff

Amazon’s shopping cart, which must take an item even while the machines behind it are failing. This build makes our store take that write, and then shows the one promise it breaks to do it.

Before you start. KV: quorum stored every key on three machines, its replicas, and made a write wait for W of them to confirm and a read ask R of them, keeping the value with the biggest version number. When R + W was more than 3, every read shared a machine with the last write and so saw it; the price was that with two of the three replicas down, a W = 2 write was refused. The cut phone line (CAP, done properly) named that price: a machine that cannot reach the others must either answer with what it has or refuse. This lesson chooses to answer, and builds what that takes.