Design an ATM: card, PIN, balance, withdrawals and deposits

Design the software for an ATM. A customer inserts a card and enters a PIN, then checks the balance, withdraws cash, or deposits. The machine has a cash dispenser holding notes of fixed denominations, talks to the bank to authorize every transaction, and must behave safely when anything fails part way.

Requirements

• Functional: read the card and verify the PIN with the bank, with a limit on wrong attempts before the card is retained; show the account balance; withdraw an amount, checked against the balance, the daily limit and whether the dispenser can make it from the notes it holds; deposit cash or cheques; print or show a receipt; end the session and return the card.
• Non-functional: money is never lost: if the dispenser fails after the bank debits the account, the debit is reversed; every transaction is logged; the machine goes out of service when cash runs out or a part fails.

Clarifying questions

• Which note denominations, and should the machine prefer large or small notes?
• Is the bank connection always available? What happens while it is down?
• Does the ATM serve several banks' cards (with fees)?
• Are deposits counted by the machine, or checked later by the bank?
• Can a customer do several transactions in one session?

What a strong answer covers

• Core classes: ATM (the controller), CardReader, Keypad, Screen, CashDispenser (notes per denomination, can_dispense, dispense), DepositSlot, ReceiptPrinter, BankService (the interface to the bank), Session (card, account, attempts), and Transaction with BalanceInquiry, Withdrawal and Deposit.
• ATM states (idle, card inserted, authenticated, transaction in progress, out of service) using the state pattern.
• Withdrawal as two phases: check locally that the notes can make the amount, authorize and hold with the bank, dispense, then confirm; on dispenser failure, reverse the hold.
• Note selection with limited notes per denomination, refusing an amount the machine cannot make.
• Hardware behind interfaces so the logic is testable with fakes, and a transaction log for reconciliation.
• Tests: wrong PIN three times, an amount that cannot be made, a dispenser jam after authorization.

Follow-ups

• Add cardless withdrawal with a code from a phone app.
• The bank link drops mid-withdrawal. What does the ATM do, and how is it reconciled?
• Track cash levels and alert the operator before the machine runs out.