Simultaneous requests.
WHAT A RACE CONDITION IS
Two requests doing something at the same time, producing a result neither intended.
WHERE THEY OCCUR
Stock levels Account balances Sequential reference numbers Booking limited availability Anything read, modified and written back
WHY THEY ARE MISSED
They do not occur in development, where requests arrive one at a time.
WHAT PREVENTS THEM
Database-level locking during the operation Atomic operations, where the database performs the change Unique constraints, which reject a duplicate outright
WHAT TO USE FOR COUNTERS
An atomic increment, rather than reading and writing back.
WHAT TO USE FOR STOCK AND BALANCES
A transaction with a lock on the affected row.
WHAT TO USE FOR REFERENCES
A database sequence or a unique constraint with retry.
WHAT ELSE TO CONSIDER
Duplicate submissions, from double clicks or retries.
WHAT PREVENTS THOSE
An idempotency key, recorded and checked.
WHAT TO TEST
Two simultaneous attempts at the same operation.
