Design a flash sale system

A store sells a limited release at a set time: in the brief, 10,000 pairs of a sneaker in 8 sizes, one per customer, on sale at 10:00:00 to about 2 million people. Almost everyone will be told no. The job is to tell them quickly and fairly, sell every pair exactly once, and keep the site up for the few thousand who can buy.

It looks like ticket booking, and the difference matters. Seats are all different, so booking holds specific seats on a map. Pairs of one size are identical, so the state is one number per size, and the hard parts move: one count everyone hits, a line sized to the stock, bots, and showing it was fair.

The practice brief states the requirements up front; confirm them in the first few minutes. The core is a count that can’t go below zero, a launch that survives the crowd, and pairs that reach real customers.

Work out only the numbers the design depends on, with rough figures from the brief. The one that shapes everything is how many people there are for each pair.

A drop has an opening time and a state; each size has a stock count, the count of record. An entry is a place in line that gains a pass once let through. A reservation holds one pair for one buyer, with a status and an expiry. An order is paid against a reservation with a key the app chose, and a purchase row keyed on drop and customer enforces one pair each.

Each step gets its own call, because each has its own answer: not open yet, your place, sold out, held until a time, paid. Joining returns a place; the pass arrives only when the shopper is let through, and reserving requires it.

Build the design in two paths: the page everyone loads, and the purchase path the few thousand use. The deep dives then harden the parts an interviewer pushes on.