ATM System

Design the software for an ATM: card insertion, PIN authentication, balance enquiry, cash withdrawal with note dispensing, and deposits. It combines a State machine for the session with Chain of Responsibility for dispensing notes, plus careful thinking about failures between the ATM and the bank.

Requirements

Functional

• Insert card, enter PIN (3 attempts, then retain the card / block).
• Operations: balance enquiry, withdraw cash, deposit cash, change PIN, mini statement.
• Withdrawal: amount must be a multiple of Rs 100 and within daily limits; the ATM must be able to dispense it with available notes.
• Dispense using Rs 2000, 500, 200, 100 notes, minimising note count.
• Print or display receipts; eject card at session end.

Non-functional

• Money must never be lost or double-debited, even if the ATM crashes mid-transaction.
• The bank is a remote system; network calls can time out.
• Session timeouts for inactivity.
• Hardware components (card reader, keypad, dispenser, printer) are behind interfaces.