Chapter 11, worked solutions, page 109
Warm-up: count the calls behind one screen
The point of the count is not the number, it is the gap. A typical “home” or “order detail” screen makes somewhere between three and seven calls, each to a different service, and uses only a slice of each response. An order detail screen might call the order service (and use status, total, line items), the shipment service (and use carrier, tracking number, expected date), and the profile service (and use name and saved address), while every response carries a dozen fields the screen never paints. The BFF version is one call returning exactly those fields, composed server-side. If your sketch shows the phone making three round trips and discarding most of three payloads, you have just drawn the chattiness and over-fetching failures the BFF exists to kill, and the tailored payload is the cure. Own the contract, not the call: the screen’s contract is that short field list, not the union of three services’ responses.
