ATM machine

The problem

An ATM accepts a card, checks the PIN, then lets the customer check balance, withdraw cash or deposit, and returns the card. It talks to a bank to authorise operations and holds a limited amount of cash.

States

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Figure SDA-LLD-06-1 · ATM states · © forcsguy.com

In Idle, "enter PIN" makes no sense; in Authenticated, "insert card" makes no sense. The State pattern puts those rules inside state objects instead of if (state === ...) chains.

interface AtmState {
  insertCard(atm: Atm, card: Card): void;
  enterPin(atm: Atm, pin: string): void;
  withdraw(atm: Atm, cents: number): void;
}

class Idle implements AtmState {
  insertCard(atm, card) { atm.card = card; atm.setState(new HasCard()); }
  enterPin() { throw new Error("insert a card first"); }
  withdraw() { throw new Error("insert a card first"); }
}
class HasCard implements AtmState {
  tries = 0;
  insertCard() { throw new Error("card already inserted"); }
  enterPin(atm, pin) {
    if (atm.bank.checkPin(atm.card!, pin)) atm.setState(new Authenticated());
    else if (++this.tries >= 3) { atm.retainOrEject(); atm.setState(new Idle()); }
  }
  withdraw() { throw new Error("enter the PIN first"); }
}

Who is responsible for what

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Figure SDA-LLD-06-2 · Withdrawal sequence · © forcsguy.com

• Bank owns the balance and account rules. ATM owns hardware, cash on hand and the session.
• Cash dispenser holds counts per denomination and decides which notes to give (greedy from the largest, with a fallback).
• Every step is logged so a failure between "dispensed" and "confirmed" can be reconciled later (a mismatch report, or an automatic reversal).

Design notes

• Treat PINs as secrets: never store or log them; the bank verifies, the ATM only forwards an encrypted PIN block.
• Idempotency: a retried authorise call must not debit twice; use a transaction id.
• Timeouts: an idle session returns to Idle and ejects the card.
• Extensions: deposits (count and hold until verified), fund transfers, receipts, and multiple card types.

Real-world reading