Design Ticketmaster

Question 1

Who is trying to buy?

Accepted Answer

A million of them, in the same minute. Start there.

Question 2

What absorbs the first wave?

Accepted Answer

The gateway is the only thing that sees the full million and is where admission is decided before anything expensive runs.

Question 3

A million people arrive for fifty thousand seats. What do the other 950,000 hit?

Accepted Answer

Letting everyone through means every request contends for the same rows and the whole thing falls over serving errors. A waiting room admits people at a rate the booking path can actually serve — it turns a stampede into a queue, which is both survivable and fairer.

Question 4

Where is the "this seat is being bought right now" state?

Accepted Answer

Holds are short-lived, enormously contended, and must expire on their own. That is an in-memory store with atomic operations and a TTL — not a row you lock for five minutes while somebody finds their card.

Question 5

Where does a confirmed booking live?

Accepted Answer

The confirmation is the one thing in this system that must never be lost or duplicated. It is a transactional write to a durable store, and it is the source of truth the inventory is only a working copy of.

Question 6

Your client writes bookings directly. What enforces one seat, one buyer?

Accepted Answer

The uniqueness rule lives in a service that can check the hold, take the payment and write the booking as one decision. A client cannot be trusted with any of that.

Question 7

What is the peak request rate you are designing for?

Accepted Answer

1,000,000 people arriving within a minute is roughly 17,000 requests a second, against a few hundred at rest. Designing for the average here is how the on-sale becomes an outage.

Question 8

Someone reserves a seat and closes the tab. What happens to it?

Accepted Answer

A hold with a TTL releases itself; a database lock held across a human payment step does not, and pins a connection while it waits. The interesting part is what happens when the hold expires between payment and confirmation — which is why the durable write, not the hold, is the source of truth.

Blueprint — system design