Commerce & Aggregation

Aggregating catalogs and offers across many sellers you don’t control: price and stock ingest via feeds and polite crawling, landed-cost ranking over item + shipping + tax, staleness-bounded caches, and checkout-time revalidation so the price you show survives to the charge.

Explainers

• Buy the Cheapest Book — It reads like a database query — sort by price, show the top row — but the two words the query hides are the whole design. “Cheapest” is never the sticker: it is a freshly-computed landed cost (item + shipping + tax, in stock, in the right condition) over prices you don’t control. And “buy” is a promise the price has to survive one click later. Built from zero: the scope dialogue that pulls both words apart, the commit-first envelope whose scary number is 11.6k external fetches a second to keep a billion offers a day fresh (which is why you don’t), the naive live-fan-out that waits on the slowest of 33 sellers and its break, the two ingest lanes (marketplace writes you own vs external feeds you crawl), tiered freshness and a crawl budget, the signature landed-cost ranking computed live where the sticker-cheapest almost never wins, a cached aggregate with a staleness clock, search over the catalog, and the load-bearing correctness beat — checkout-time price revalidation with a tolerance policy, reservation against the last-copy race, and an idempotent order that never double-charges. Grounded in Amazon’s Featured Offer and Google Merchant Center docs.