Correctness under load.
WHAT THE RISKS ARE
Double spending from concurrent requests Balances read before another transaction commits Duplicate processing of the same event
WHAT PREVENTS DOUBLE SPENDING
Locking the balance row during the transaction A constraint preventing negative balances Serialised processing per account
WHY A CHECK-THEN-ACT PATTERN FAILS
Two requests both read a sufficient balance before either writes.
WHAT ISOLATION LEVELS AFFECT
Which anomalies are possible.
WHAT TO USE FOR MONEY
An isolation level preventing the anomalies that matter, understood rather than assumed.
WHAT OPTIMISTIC CONCURRENCY PROVIDES
Detecting conflict at write time, and retrying.
WHERE IT SUITS
Low-contention accounts.
WHERE IT DOES NOT
Accounts with heavy concurrent activity.
WHAT EVENTUAL CONSISTENCY MEANS
Reads may be stale briefly.
WHERE THAT IS ACCEPTABLE
Reporting and analytics.
WHERE IT IS NOT
Balance checks before a debit.
WHAT TO TEST
Concurrent operations on the same account, deliberately, at volume.
WHY
These defects do not appear in sequential testing and are catastrophic in production.
WHAT TO MONITOR
Negative balances, which should be impossible.
