Design ATM (LLD)

1. Requirements

• Functional

  • Authenticate cardholder via PIN; session scoped to one card until logout or timeout.
  • Balance inquiry, cash withdrawal with denomination limits, deposit envelope stub (optional).
  • Ledger integration to authorize and commit funds; abort on insufficient balance or daily limit.

• Non-Functional

  • PIN never stored in plain text; secure keypad interface assumed as boundary.
  • Idempotent transactions with reference ids to survive retries.

• Assumptions / Out of Scope

  • Single ATM terminal process; host banking core is another service.

2. Core Entities

3. Class Diagram

4. State / Sequence Diagram (where relevant)

5. Design Patterns Applied

• Facade — ATMController hides dispenser, gateway, session. Facade pattern.
• Proxy — BankingGateway as remote proxy to core. Proxy pattern.
• Command — Transaction objects for logging and undo audit. Command pattern.

6. Implementation

Go

package atm

type Card struct {
    AccountID string
}

type Session struct {
    Card          Card
    Authenticated bool
}

type ATMController struct {
    Session   *Session
    Bank      BankingGateway
    Dispenser CashDispenser
    PinCheck  PinValidator
}

func (controller *ATMController) Withdraw(amount int64, idempotencyKey string) error { /* ... */ }

JavaScript

class ATMController {
  constructor({ bankingGateway, cashDispenser, pinValidator }) {
    this.bankingGateway = bankingGateway;
    this.cashDispenser = cashDispenser;
    this.pinValidator = pinValidator;
    this.session = null;
  }
  insertCard(card) { /* ... */ }
  withdraw({ amountCents, idempotencyKey }) { /* ... */ }
}

7. Concurrency / Thread Safety

• Collisions: Two concurrent withdrawal requests on same session from malfunctioning hardware unlikely; network retries duplicate commits.
• Granularity: Idempotency keys on bank calls; single active session flag on terminal.
• Go: Mutex on ATMController during open session.

8. Extensibility & Followups

• Multi-currency and forex rates from host.
• Cassette refill scheduling and jam detection events.
• Edge cases: partial dispense, communication failure after cash out, forced reconciliation.