Concurrency & IsolationdebuggingAdvanced

Two users booked the same seat

Symptoms

Occasionally two customers are assigned the same seat, despite a "check if taken, then book" flow.
Only happens under concurrent load.

-- both requests, interleaved:
T1: SELECT count(*) FROM bookings WHERE seat = '12A';  -- 0, so free
T2: SELECT count(*) FROM bookings WHERE seat = '12A';  -- 0, so free
T1: INSERT INTO bookings (seat, user_id) VALUES ('12A', 1);
T2: INSERT INTO bookings (seat, user_id) VALUES ('12A', 2);  -- both succeed

Investigate

Inspect areas in any order (0/4 inspected). When you think you know the root cause, commit to it.

The check-then-insert logic

Isolation level

Constraints on the bookings table

Application locking