If you write: int balance = 0; int get {...} void set(int balance) {...} void withdraw(int amt) { if amnt <= balance { balance -= amnt; }

If you write: int balance = 0; int get {...} void set(int balance) {...} void withdraw(int amt) { if amnt <= balance { balance -= amnt; } } it will be flagged immediately in code review as a race condition and/or something that doesn't guarantee its implied balance>=0 invariant. Because threading. But lift the logic up into a REST controller like pretty much all web app backends: GET /balance/get POST /balance/set POST /balance/withdraw And it will sail straight through review. (Because we pretend each caller isn't its own Thread.)