System design interview question

Design a ticket booking system

Ticketmaster, BookMyShow, any sale where demand dwarfs supply: “design a ticket booking system” tests two things at once — surviving a surge of a million people in a minute, and never selling one seat twice while they fight over it. It is the classic question about contention. Here is how to walk into it, and then how to practise it until the answer is yours.

Practise Ticket Booking

Multiple choicePick the design from four
Fill the gapsFill the gaps in one drawn for you
Build from scratchBuild it on a blank canvas

Practise it on a canvas

Free with an account.

The question, as an interviewer would set it

A ticketing site sells seats for concerts. For a popular show, a million people arrive in the same minute for fifty thousand seats. Each buyer picks specific seats, and a chosen seat must be held for them for ten minutes while they pay, so nobody else can take it in that time; if they do not pay, the seat returns to sale. No seat may ever be sold twice. The seat map is viewed far more often than seats are bought, and showing a seat as free for a few seconds after it was taken is acceptable, because the purchase itself is checked. The site must not fall over under the surge, even if that means buyers wait their turn.

What it is really testing

Every phrase that matters in the brief is a requirement in disguise. Spot them before you draw a single box.

“a million people arrive in the same minute”
A line that absorbs the surge and admits buyers at a steady rate.
“held for them for ten minutes while they pay”
Holds that expire on their own.
“No seat may ever be sold twice”
Each sale decided in one atomic transaction.
“viewed far more often than seats are bought”
Serve the seat map from memory, where slightly stale is fine.
“even if that means buyers wait their turn”
Queue arrivals rather than serving or refusing everyone at once.

Back of the envelope

Arrivals: 1 million in the first minute → ~17,000 a secondAn assumption to say out loud. If the interviewer has a number in mind, they will give it to you.
Seats: 50,000 → 20 buyers for every seatMost people will not get one; the design decides who does, and how fairly.
Seat map views: ~10 refreshes each → ~10 million in the rushReads dwarf purchases, and a few seconds out of date is acceptable for them.
Sales: 50,000 in ~15 minutes → ~55 a secondTiny next to the arrivals. The hard part is the crowd and the contention, not the sales rate.

Questions to ask first

Do buyers pick exact seats, or just a number of tickets?
How long is a seat held while the buyer pays?
How big is the biggest on-sale, and how quickly does it sell out?
Is it acceptable for buyers to wait in line before they can choose?
How much of a limit should one buyer have, and what about bots?

The trade-offs you will be pushed on

1.A million people arrive in one minute. What keeps the site up?
2.A buyer holds seats and walks away. How do the seats come back without anyone cleaning up?
3.A hold expires a second before payment completes. Who gets the seat?
4.What guarantees a seat is never sold twice, even when two sales finish at once?

Now answer it

The same design always gets the same score, with the reason behind every requirement — then defend it against the trade-offs above.