23,000
cart writes a second when a sale opens
Against just 1,200 orders a second. The forgiving cart takes about 15× checkout’s traffic, so each half gets the storage and consistency it can afford.
You removed the phone on your laptop. Why is it back?
A leaderless store kept both edits as versions; a union merge never loses an add, so deletes resurface. Today’s carts store one item per line.
Which total do you charge?
Bad: Re-price, then charge
FEST500 expires mid-OTP. They’re charged a total they never saw.
Good: Charge the quote
Priced once, valid 15 minutes. “Place order” names it and the total shown.
16M
units tied up by holding at add-to-cart
Most sit in carts that will be abandoned. Hold at “Place order” instead, about 1.1M, with conditional updates and holds that expire.
1,500 “Place order” calls a second. If 0.1% are retries, that’s 5,400 duplicate orders an hour.
The trick
The checkout ID is the key: minted when checkout opens, reused on refresh, scoped down every layer.
Your bank confirms at 20:23. Pay-by was 20:17. Now what?
Nest the clocks, attempt ≤ pay-by < hold, so a payment in time always finds its stock. A later one re-holds the stock or is refunded.
Midnight: traffic jumps 10× in a minute
- Pre-scale an hour ahead, forecast + 50%
- Admit “Place order” at the narrowest pipe’s rate
- Shed recommendations first, never cart writes or orders
- Bucket the hot stock row and coupon
Remember
The cart never refuses a write. Checkout charges a binding quote exactly once: expiring holds, one checkout ID as the key.