Codemia | Master System Design Interviews Through Active Practice

My Solution for Design an ATM with Score: 9/10

by nectar4678

Requirements

The ATM system needs to cater to the following key functionalities:

User Authentication: Validate users securely using PIN or biometric authentication.
Withdrawals: Allow users to withdraw money from their accounts, ensuring sufficient balance.
Deposits: Enable users to deposit cash or checks.
Balance Inquiry: Display the current balance of the selected account.
Transfers: Facilitate transferring funds between accounts (e.g., savings to checking) or to another user’s account.
Account Management: Support multiple account types like savings and checking.
Security Measures: Include mechanisms to prevent fraud and unauthorized access.
Failure Handling: Handle scenarios such as insufficient cash in the ATM, card retention for failed authentication attempts, or network failures.

Define Core Objects

The primary objects in this system are:

ATM: Represents the ATM machine, handling transactions and user interactions.
Card: Represents a user’s ATM card, associated with their accounts.
Account: A bank account (e.g., savings, checking) linked to the user.
Transaction: Represents various transactions like withdrawal, deposit, balance inquiry, and transfer.
User: Represents the person using the ATM and their associated details.
Bank: Represents the backend system that validates transactions and holds account details.

Analyze Relationships

ATM ↔ User: The ATM interacts with the user for authentication and transaction input.

Card ↔ User: The user provides their card to the ATM for identification.

Card ↔ Account: The card is linked to one or more accounts.

Transaction ↔ Account: Transactions modify the balance of the associated account.

ATM ↔ Bank: The ATM communicates with the bank to validate transactions and update account details.

Establish Hierarchy

Account Types:

SavingsAccount (inherits from Account)
CheckingAccount (inherits from Account)

Transaction Types:

Withdrawal (inherits from Transaction)
Deposit (inherits from Transaction)
BalanceInquiry (inherits from Transaction)
Transfer (inherits from Transaction)

ATM Components:

CashDispenser
Screen
Keypad
CardReader

Design Patterns

Singleton Pattern: Ensure only one instance of the ATM interface or bank service exists.

Factory Pattern: Use for creating transaction objects based on user input.

Observer Pattern: Notify the bank of any changes in account state or transactions.

Strategy Pattern: Use for different account types (savings, checking) and transaction handling.

Define Class Members (write code)

class ATM:
    def __init__(self, bank):
        self.bank = bank
        self.cash_dispenser = CashDispenser()
        self.card_reader = CardReader()
        self.screen = Screen()
        self.keypad = Keypad()
    
    def authenticate_user(self, card, pin):
        return self.bank.validate_pin(card, pin)
    
    def execute_transaction(self, transaction):
        return transaction.perform()

class Card:
    def __init__(self, card_number, user):
        self.card_number = card_number
        self.user = user

class Account:
    def __init__(self, account_number, balance):
        self.account_number = account_number
        self.balance = balance
    
    def deposit(self, amount):
        self.balance += amount
    
    def withdraw(self, amount):
        if self.balance >= amount:
            self.balance -= amount
            return True
        return False

class Transaction:
    def perform(self):
        pass

class Withdrawal(Transaction):
    def __init__(self, account, amount):
        self.account = account
        self.amount = amount
    
    def perform(self):
        if self.account.withdraw(self.amount):
            return "Withdrawal Successful"
        return "Insufficient Funds"

class Bank:
    def validate_pin(self, card, pin):
        # Logic to validate card and PIN
        pass

Adhere to SOLID Guidelines

Single Responsibility: Each class has a clear responsibility, such as handling transactions or managing accounts.
Open/Closed Principle: The system can be extended with new transaction types without modifying existing classes.
Liskov Substitution: Derived classes like SavingsAccount can replace the base Account class without altering behavior.
Interface Segregation: Components like Screen and Keypad focus on specific responsibilities.
Dependency Inversion: The ATM depends on abstractions (e.g., Transaction) rather than concrete classes.

Consider Scalability and Flexibility

The design can handle additional features like:

Adding new transaction types, such as bill payments or mobile recharges.
Supporting different authentication mechanisms (e.g., biometric, OTP).
Allowing multiple languages on the ATM screen.

Create/Explain your diagram(s)

Future improvements

Add support for digital wallets or QR code-based payments.
Implement real-time fraud detection mechanisms.
Enable NFC-based contactless transactions.
Provide users with a mobile app for remote control of ATM functionalities (e.g., pre-booked cash withdrawal).