Requirements

Design an Automated Teller Machine (ATM) system. The system should allow users to perform basic banking operations such as depositing cash, withdrawing cash, checking balance, and transferring funds. The system should also be designed with security and concurrency in mind. Imagine you're designing this for a bank that wants a robust, scalable, and maintainable solution. Focus on low-level design principles, object-oriented design, and design patterns to create a flexible and extensible system. # Requirements ## Functional Requirements **Authentication:** Users should be able to authenticate using a card and PIN. The system should handle incorrect PIN attempts and temporarily lock the account after a certain number of failed attempts. **Balance Inquiry:** Users should be able to check their account balance. **Cash Withdrawal:** Users should be able to withdraw cash from their account. The system should verify sufficient funds and available cash in the ATM. It should dispense the correct denominations. **Cash Deposit:** Users should be able to deposit cash into their account. The system should validate the deposited bills. **Fund Transfer:** Users should be able to transfer funds between their own accounts (e.g., checking to savings). **Transaction History:** Users should be able to view a history of their transactions. This can be a simple in-memory representation for this exercise. **Admin Override:** An administrative function to override limits (for example, to load cash into the ATM). This is not a core requirement, but think about how your design would accommodate such functionality. ## Non-Functional Requirements **Security:** The system must be secure and protect user data. This includes PIN handling, transaction integrity, and preventing unauthorized access. **Concurrency:** The system must handle concurrent transactions from multiple users without data corruption or deadlocks. **Extensibility:** The system should be designed to easily support new transaction types (e.g., bill payment) or new hardware (e.g., different types of card readers). **Maintainability:** The system should be well-structured and easy to understand and maintain. **Testability:** The system should be designed to be easily testable with unit tests. **Fault Tolerance:** The ATM should continue to function even if certain components fail (e.g., a particular denomination dispenser is empty). This aspect should be touched upon during the design discussion, showing how you might accommodate the possibility. **Auditability:** All transactions should be logged for auditing purposes. **Currency Support:** The system should support multiple currencies. ## Core Entities **ATM:** Represents the ATM machine itself. This is the main entry point for user interactions. It should manage the overall workflow, including authentication, transaction processing, and dispensing cash. It needs to be thread-safe. **CardReader:** Handles reading the user's card. Abstract this to allow for different card reader implementations. **Authenticator:** Handles user authentication (PIN verification, account locking). Consider different authentication mechanisms in the future (biometrics). **Account:** Represents a user's bank account. Holds account balance and transaction history. Account information should be protected from concurrent access. **Transaction:** An abstract class or interface for different transaction types (Deposit, Withdrawal, Transfer). Use a pattern to create and process transactions. **TransactionProcessor:** Processes different types of transactions. Consider how to extend this to support new transaction types. **CashDispenser:** Manages the ATM's cash inventory and dispenses cash in the correct denominations. Design this for fault tolerance if a particular denomination is empty. **DepositSlot:** Handles cash deposits, validating the bills. **Currency:** Represents a currency and its denominations. **AuditLogger:** Logs all transactions for auditing purposes. Use a suitable pattern to ensure transactions are reliably logged.

Design an Automated Teller Machine (ATM) System

Requirements

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Functional Requirements

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Non-Functional Requirements

Core Entity Identification

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