Dealing with Contention

Consider buying concert tickets online. There's one seat left for The Weeknd, and Alice and Bob both want it. They each hit "Buy Now" in the same instant. The obvious way to handle a purchase is the one most of us would reach for first. Read the current seat count, check that it's above zero, and if it is, decrement it and sell the ticket.

-- Read the current count
SELECT available_seats FROM concerts WHERE concert_id = 'weeknd_tour';

-- The app checks available_seats > 0, then writes the new value back
UPDATE concerts
SET available_seats = available_seats - 1
WHERE concert_id = 'weeknd_tour';

For a single buyer this is exactly right. The trouble starts when Alice and Bob run it at the same moment. Alice's request reads one seat available. A fraction of a millisecond later, before Alice has written anything back, Bob's request reads the same count and also sees one seat. Both check the number they just read, both conclude there's a seat to sell, and both move on to charge a card. Alice's update commits first and the count drops to zero. Bob's update commits right after, decrements again, and the count slides to negative one.

By the time the dust settles, both cards have been charged $500 and both buyers have a confirmation email for the same seat. Alice and Bob show up to the concert each thinking they own Row 5, Seat 12. One of them is getting kicked out, and the venue is stuck issuing a refund while dealing with two very angry customers.