The Contract at the Counter (API design)

The System Design board about how two services meet. They never see inside each other. They meet at a counter, across a written list of what you may ask for and what comes back, and this lesson is about writing that list so strangers can still use it in three years. There is a video of this board: the contract at the counter.

Before you start. You need two facts: that programs talk to each other over a network, and that a network sometimes loses the message, so the sender is left not knowing whether it arrived (How a parcel finds any house on earth has the packets underneath, if you want them). The rest of the story is an auction site, a court order and a menu.