Case Study of Architecture & Design Patterns for a Credit Card Banking System

 

Architecture & Design Patterns

This documental post is a sample case study over the architectural considerations, module design, implementation approaches, and database model for a secure and scalable Credit Card Banking Online Website. The focus is on a layered, domain-driven approach, leveraging established design patterns for robustness and maintainability.

1. Overall System Architecture

The proposed architecture adopts a Layered (N-Tier) Architecture as its foundation, which provides clear separation of concerns, simplifies development, and enhances scalability. This structure can evolve towards a microservices approach for specific complex domains (e.g., fraud detection) as the system grows.

Key Architectural Layers:

  1. Presentation Layer (UI/Frontend):

    • Technology: ASP.NET Core (Razor Pages or MVC) for Server-Side Rendering (SSR) and/or a modern JavaScript framework (React, Angular, Vue.js) for a Single Page Application (SPA) communicating with the API. For this document, we'll assume ASP.NET Core Razor Pages for simplicity and full-stack .NET.

    • Functionality: User interface, user interaction, data display, input validation, session management.

  2. API/Application Layer:

    • Technology: ASP.NET Core Web API.

    • Functionality: Exposes endpoints for the UI and potential third-party integrations, handles request validation, orchestrates business logic, and manages data flow between the UI and the domain layer. This layer acts as the entry point for all application functionalities.

  3. Domain/Business Logic Layer:

    • Technology: Plain C# classes, interfaces, and domain models. Independent of any specific framework.

    • Functionality: Contains the core business rules, entities, value objects, and aggregates. This is the heart of the application, ensuring that business rules are enforced consistently regardless of how operations are initiated.

  4. Infrastructure/Data Access Layer:

    • Technology: Entity Framework Core with SQL Server.

    • Functionality: Handles persistence concerns, communicates with the database, implements repositories, and manages Unit of Work. It also includes external service integrations (e.g., SMS gateways, email services).

Supporting Components:

  • Database: SQL Server (relational database for transactional data).

  • Caching: Distributed cache (e.g., Redis) for frequently accessed, less volatile data.

  • Message Broker: (e.g., RabbitMQ, Azure Service Bus) for asynchronous communication, event-driven processes (e.g., fraud alerts, statement generation), and decoupling services.

  • Logging & Monitoring: Centralized logging (e.g., Serilog, ELK Stack), APM tools (e.g., Application Insights) for observability.

  • Security: Identity Management (ASP.NET Core Identity, IdentityServer for OAuth/OpenID Connect).

2. Module Design and Implementation

We will break down the Credit Card Banking website into key functional modules, detailing their design and implementation using appropriate patterns.

2.1. User Authentication & Authorization Module

  • Purpose: Manages user registration, login, password recovery, session management, and role/policy-based access control.

  • Key Components:

    • ApplicationUser: Extends IdentityUser for custom user properties (e.g., FirstName, LastName).

    • ApplicationRole: Extends IdentityRole for custom roles (e.g., CardHolder, Administrator, CustomerService).

    • ApplicationDbContext: Inherits from IdentityDbContext for user/role persistence.

    • AccountController (API) / LoginPage (Razor Page): Handles user registration, login, logout, password resets.

    • JwtService / TokenService: Generates and validates JSON Web Tokens (JWT) for API authentication.

    • CustomAuthorizationHandler: Implements complex authorization policies.

  • Design Patterns:

    • Identity Pattern: Leveraging ASP.NET Core Identity for robust user management.

    • Dependency Injection: Injecting UserManager, SignInManager, RoleManager, and TokenService.

    • Strategy Pattern (for Authentication Schemes): ASP.NET Core handles this internally with schemes (Cookie, JWT Bearer).

  • Implementation Notes:

    • Registration process: User provides details, password hashed via Identity, confirmation email sent (via email service).

    • Login: Validates credentials, issues JWT (for API) or sets a cookie (for web app).

    • Authorization: [Authorize] attribute on controllers/actions for role or policy checks. Policies defined in Startup.cs (AddAuthorization(options => { ... })).

2.2. Account Management Module

  • Purpose: Allows users to view their credit card accounts, current balance, credit limit, available credit, statement date, and minimum payment due. Also handles new card applications.

  • Key Components:

    • Account Entity: Represents a credit card account (e.g., AccountId, UserId, CardNumber, CreditLimit, CurrentBalance, Status).

    • Card Entity: Represents the physical/virtual card details (e.g., CardId, AccountId, ExpiryDate, CVV, PIN, IsActive).

    • AccountService: Business logic for fetching account details, creating new accounts, updating account status.

    • IAccountRepository / AccountRepository: Data access for Account and Card entities.

    • AccountController / AccountDetailsPage: UI/API endpoints for account operations.

  • Design Patterns:

    • Repository Pattern: IAccountRepository abstracts data access from the AccountService.

    • Unit of Work Pattern: DbContext acts as the Unit of Work, ensuring SaveChanges commits all changes atomically.

    • Domain Model: Account and Card are rich domain objects with encapsulated behavior.

  • Implementation Notes:

    • Credit card numbers should be masked/encrypted at rest and in transit.

    • Logic for calculating available credit (CreditLimit - CurrentBalance) resides in the Account domain entity or AccountService.

    • New card applications might involve a workflow and status changes (e.g., "Pending Approval", "Approved", "Declined").

2.3. Transaction Processing Module

  • Purpose: Records all credit card transactions (purchases, refunds, cash advances), applies interest, and manages transaction history.

  • Key Components:

    • Transaction Entity: Represents a single transaction (e.g., TransactionId, AccountId, Amount, Type (Debit/Credit), Description, MerchantName, TransactionDate, Status).

    • TransactionService: Business logic for processing transactions, applying fees, interest calculation, and validation.

    • ITransactionRepository / TransactionRepository: Data access for Transaction entities.

    • TransactionController / TransactionHistoryPage: UI/API endpoints for viewing and managing transactions.

  • Design Patterns:

    • Repository Pattern: ITransactionRepository for data access.

    • Strategy Pattern (for Transaction Types/Fees): Could be used if different transaction types have vastly different processing rules or fee structures.

    • Observer Pattern / Event-Driven: A TransactionProcessedEvent could be published after a transaction is recorded, triggering updates for balance, fraud checks, or statement generation.

  • Implementation Notes:

    • Debit transactions increase CurrentBalance, Credit transactions decrease it.

    • Real-time balance updates are critical.

    • Transaction status (e.g., "Pending", "Posted", "Failed") should be managed.

    • Fraud detection hooks can be integrated here (e.g., a message published to a fraud service).

2.4. Billing & Payments Module

  • Purpose: Generates monthly statements, calculates minimum payments and due dates, and processes user-initiated payments to their credit card.

  • Key Components:

    • Statement Entity: Represents a monthly statement (e.g., StatementId, AccountId, StatementDate, OpeningBalance, ClosingBalance, MinimumPaymentDue, DueDate).

    • Payment Entity: Records payments made by the user (e.g., PaymentId, AccountId, Amount, PaymentDate, PaymentMethod, Status).

    • BillingService: Logic for statement generation (often a scheduled background task), minimum payment calculation, interest accrual.

    • PaymentService: Processes incoming payments, updates account balance, communicates with payment gateways.

    • IStatementRepository / StatementRepository: Data access for Statement entities.

    • IPaymentRepository / PaymentRepository: Data access for Payment entities.

    • BillingController / StatementPage / PaymentPage: UI/API for viewing statements and making payments.

  • Design Patterns:

    • Repository Pattern: For Statement and Payment.

    • Strategy Pattern (for Payment Methods): E.g., ProcessBankTransfer, ProcessDebitCardPayment.

    • Command Pattern: A MakePaymentCommand could encapsulate payment initiation.

    • Scheduler Pattern (via IHostedService): For automated statement generation.

  • Implementation Notes:

    • Payment processing would integrate with a secure payment gateway. Payment details are typically not stored directly in the banking system database (PCI DSS compliance).

    • Statements are generated periodically (e.g., end of month) as a batch process.

    • Payment status needs to be robustly handled (e.g., "Pending", "Success", "Failed", "Refunded").

2.5. Fraud Detection (High-Level)

  • Purpose: Identify and flag suspicious activities.

  • Key Components:

    • FraudDetectionService: Analyzes transaction patterns.

    • FraudAlert Entity: Records detected fraud.

    • Message Queue: Transactions are published to a queue, consumed by a separate fraud detection microservice for asynchronous processing.

  • Design Patterns:

    • Event-Driven Architecture: Transactions raise events that the fraud module consumes.

    • Microservice: Could be a separate, highly scalable service.

2.6. Reporting & Analytics

  • Purpose: Generate various reports for users (e.g., yearly summaries, spending categories) and for internal banking operations.

  • Key Components:

    • ReportService: Business logic for compiling reports.

    • Views/Stored Procedures: Optimized database views or stored procedures for complex reporting queries.

  • Design Patterns:

    • CQRS (Query side): Could be a strong candidate here if reporting requirements become very complex or demand different data models/stores than the transactional system.

    • Data Transfer Objects (DTOs): Used to shape data specifically for reports.

3. SQL Server Database Model

The database design focuses on a relational model, optimized for transactional consistency and integrity.

Core Entities and Tables:

  1. Users Table (for Authentication)

    • UserId (PK, GUID/NVARCHAR) - Maps to AspNetUsers.Id

    • FirstName (NVARCHAR(50))

    • LastName (NVARCHAR(50))

    • DateOfBirth (DATE)

    • PhoneNumber (NVARCHAR(20))

    • Email (NVARCHAR(256), UNIQUE)

    • (Inherits many from AspNetUsers like UserName, PasswordHash, EmailConfirmed, etc.)

  2. Accounts Table

    • AccountId (PK, INT/GUID) - Unique identifier for the credit card account.

    • UserId (FK, GUID/NVARCHAR) - Links to Users.UserId.

    • CardNumber (NVARCHAR(16), UNIQUE) - Masked/Encrypted credit card number (e.g., last 4 digits stored, full encrypted value in vault).

    • CreditLimit (DECIMAL(18, 2)) - Maximum credit allowed.

    • CurrentBalance (DECIMAL(18, 2)) - Current outstanding balance.

    • AvailableCredit (DECIMAL(18, 2)) - Calculated: CreditLimit - CurrentBalance.

    • OpeningDate (DATETIME)

    • Status (NVARCHAR(50)) - e.g., 'Active', 'Closed', 'Frozen', 'Pending'.

    • AnnualInterestRate (DECIMAL(5, 4))

    • MinimumPaymentPercentage (DECIMAL(5, 4))

  3. Cards Table

    • CardId (PK, INT/GUID) - Unique identifier for the physical/virtual card.

    • AccountId (FK, INT/GUID) - Links to Accounts.AccountId.

    • CardHolderName (NVARCHAR(100)) - Name on the card.

    • ExpiryDate (DATE)

    • CVV (NVARCHAR(4)) - Encrypted or only last digits stored.

    • PIN (NVARCHAR(4)) - Encrypted or not stored directly.

    • IssueDate (DATETIME)

    • IsActive (BIT) - Whether the card is currently active.

    • CardType (NVARCHAR(50)) - e.g., 'Visa', 'MasterCard', 'Amex'.

  4. Transactions Table

    • TransactionId (PK, BIGINT)

    • AccountId (FK, INT/GUID) - Links to Accounts.AccountId.

    • Amount (DECIMAL(18, 2))

    • TransactionType (NVARCHAR(20)) - e.g., 'Purchase', 'Refund', 'CashAdvance', 'Payment', 'Interest', 'Fee'.

    • Description (NVARCHAR(255))

    • MerchantName (NVARCHAR(100)) - For purchases.

    • TransactionDate (DATETIME)

    • PostingDate (DATETIME) - When it's officially recorded on the account.

    • Status (NVARCHAR(50)) - e.g., 'Pending', 'Posted', 'Declined', 'Reversed'.

    • ReferenceNumber (NVARCHAR(100), UNIQUE) - External transaction ID.

  5. Statements Table

    • StatementId (PK, INT/GUID)

    • AccountId (FK, INT/GUID) - Links to Accounts.AccountId.

    • StatementDate (DATE)

    • OpeningBalance (DECIMAL(18, 2))

    • ClosingBalance (DECIMAL(18, 2))

    • TotalCredits (DECIMAL(18, 2)) - Sum of all credits in the statement period.

    • TotalDebits (DECIMAL(18, 2)) - Sum of all debits in the statement period.

    • MinimumPaymentDue (DECIMAL(18, 2))

    • DueDate (DATE)

    • IsPaid (BIT) - Whether the minimum payment is met.

  6. Payments Table

    • PaymentId (PK, BIGINT)

    • AccountId (FK, INT/GUID) - Links to Accounts.AccountId.

    • Amount (DECIMAL(18, 2))

    • PaymentDate (DATETIME)

    • PaymentMethod (NVARCHAR(50)) - e.g., 'BankTransfer', 'DebitCard'.

    • Status (NVARCHAR(50)) - e.g., 'Pending', 'Processed', 'Failed', 'Refunded'.

    • ConfirmationCode (NVARCHAR(100), UNIQUE) - From payment gateway.

  7. UserContactInfo (Optional - for multiple contacts per user)

    • ContactId (PK, INT/GUID)

    • UserId (FK, GUID/NVARCHAR)

    • ContactType (NVARCHAR(50)) - e.g., 'Email', 'Phone', 'Address'.

    • ContactValue (NVARCHAR(255))

    • IsPrimary (BIT)

  8. FraudAlerts Table (for Fraud Detection)

    • AlertId (PK, BIGINT)

    • TransactionId (FK, BIGINT, NULLABLE) - Links to a suspicious transaction.

    • AccountId (FK, INT/GUID) - Links to affected account.

    • AlertType (NVARCHAR(50)) - e.g., 'UnusualSpending', 'ForeignTransaction'.

    • AlertDetails (NVARCHAR(MAX))

    • AlertDate (DATETIME)

    • Status (NVARCHAR(50)) - e.g., 'New', 'Investigating', 'Resolved'.

ER Diagram Description:

Given the inability to render a visual ER diagram, I'll describe the relationships:

  • Users (One) to Accounts (Many): One user can have multiple credit card accounts. Accounts.UserId is a Foreign Key referencing Users.UserId.

  • Accounts (One) to Cards (Many): One credit card account can have multiple physical/virtual cards associated with it (e.g., primary and supplementary cards). Cards.AccountId is a Foreign Key referencing Accounts.AccountId.

  • Accounts (One) to Transactions (Many): One account can have many transactions. Transactions.AccountId is a Foreign Key referencing Accounts.AccountId.

  • Accounts (One) to Statements (Many): One account generates multiple statements over time. Statements.AccountId is a Foreign Key referencing Accounts.AccountId.

  • Accounts (One) to Payments (Many): One account can receive many payments. Payments.AccountId is a Foreign Key referencing Accounts.AccountId.

  • Users (One) to UserContactInfo (Many): One user can have multiple contact details. UserContactInfo.UserId is a Foreign Key referencing Users.UserId.

  • Accounts (One) to FraudAlerts (Many): One account can trigger multiple fraud alerts. FraudAlerts.AccountId is a Foreign Key referencing Accounts.AccountId.

  • Transactions (One) to FraudAlerts (Optional One): A fraud alert may be associated with a specific transaction. FraudAlerts.TransactionId is a Foreign Key referencing Transactions.TransactionId, and is nullable as an alert might be account-level.

4. Key Design Patterns Employed

This architecture leverages several fundamental design patterns to achieve maintainability, testability, and scalability.

  1. Model-View-Controller (MVC) / Razor Pages Pattern (Presentation Layer):

    • MVC: Separates application into M (data/logic), V (UI), and C (handling input/orchestrating).

    • Razor Pages: A page-centric approach simplifying UI development with code-behind.

    • Benefit: Clear separation of concerns for the frontend, making development and testing of UI components more manageable.

  2. Repository Pattern (Infrastructure/Data Access Layer):

    • Purpose: Abstracts the data access logic from the business logic. Instead of direct DbContext calls, the business services interact with interfaces like IAccountRepository or ITransactionRepository.

    • Benefit: Decouples the domain layer from the persistence technology (e.g., allows switching from SQL Server to NoSQL with minimal impact on business logic), improves testability (can easily mock repositories).

  3. Unit of Work Pattern (Infrastructure/Data Access Layer):

    • Purpose: Ensures that a series of operations are treated as a single, atomic transaction. In EF Core, the DbContext naturally embodies this pattern by tracking changes and allowing a single SaveChanges() call to commit all changes.

    • Benefit: Maintains data consistency and integrity, simplifies transaction management across multiple repository operations.

  4. Dependency Injection (DI) (Across all Layers):

    • Purpose: A core principle in .NET Core, where components receive their dependencies from an external source (the DI container) rather than creating them.

    • Benefit: Promotes loose coupling, making components independent and easily swappable. Enhances testability by allowing mocking of dependencies.

  5. Strategy Pattern (Domain/Business Logic Layer - e.g., Payments, Interest Calculation):

    • Purpose: Defines a family of algorithms, encapsulates each one, and makes them interchangeable. The strategy lets the algorithm vary independently from clients that use it.

    • Benefit: Useful for scenarios like different payment processing methods (bank transfer, debit card, credit card), different interest calculation methodologies, or varying fraud detection rules. Improves extensibility without modifying core logic.

  6. Observer Pattern (Domain/Application Layer - Event-Driven Communication):

    • Purpose: Defines a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.

    • Benefit: Enables loose coupling between components that need to react to state changes or events. For instance, when a Transaction is Posted, it could publish a TransactionProcessedEvent that FraudDetectionService or StatementGenerationService observes and reacts to asynchronously via a message queue.

  7. Command Query Responsibility Segregation (CQRS) (Advanced Consideration):

    • Purpose: Separates the read (query) and write (command) models of an application. Commands perform actions, and queries retrieve data. Often implemented with different data stores or optimized paths for each.

    • Benefit: Can provide significant performance and scalability benefits for systems with high read-to-write ratios or complex read models. The query side can be highly optimized (e.g., denormalized views, different database technology) without impacting the transactional write side. Might be considered for reporting and analytics initially, potentially expanding if justified.

This architecture provides a solid, extensible, and maintainable foundation for a Credit Card Banking Online Website, addressing critical concerns like security, scalability, and modularity.

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