The two lane sync idea is the part I will remember from this drill. Sending price and stock through a high priority queue while bulk imports wait in their own low priority lane explains the stale price example clearly. A search result at NPR 1,200 that is really NPR 1,350 at checkout is exactly the kind of bug that erodes trust. I also liked the typeahead math. Doing the heavy counting hourly and leaving each keystroke as one Redis GET makes the 50 to 100 ms budget feel achievable instead of aspirational.