💱 Currency Exchange API

A secure, scalable, and well-architected .NET Web API for retrieving and converting currency exchange rates. Built with Clean Architecture, this project demonstrates solid design principles and enterprise-grade patterns including JWT authentication, Serilog logging, Circuit Breaker resilience, and external API integration with pagination support.

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✅ Features

• 🔐 JWT Authentication
  Secure access using JSON Web Tokens. Users can register and log in via the AuthController.

• 🔄 Currency Conversion
  Convert amounts between currencies using live data from external providers. Factory Pattern is implemented to switch between providers dynamically.

• 📅 Latest and Historical Exchange Rates
  Retrieve the latest rates or a time series of exchange rates within a specified date range, with pagination support for large datasets.

• ❌ Invalid Currency Validation
  Currencies like TRY, PLN, THB, and MXN are considered exceptional and excluded from conversions. Custom validators reject requests involving these currencies.

• 🧱 Clean Architecture
  Proper separation of concerns with layers:

  • Domain (Core business logic)
  • Application (CQRS, MediatR handlers, DTOs)
  • Infrastructure (External API implementations like Frankfurter)
  • API (Controllers, filters, middleware)

• 🧪 CQRS Pattern with MediatR
  Queries and commands are handled in a clean and testable way using the MediatR library.

• 🔁 Factory Pattern
  Implemented for currency provider resolution to allow easy extension (e.g., switching from Frankfurter to another provider).

• 📊 Pagination Support
  For historical data retrieval endpoints, with customizable page size and index.

• 🛡 Resilience with Circuit Breaker
  Integrated with Polly to gracefully handle external API failures using Circuit Breaker policies.

• 📂 Serilog Logging
  Structured, persistent, and searchable logging for monitoring and debugging.

• 🌐 Swagger Integration
  Full support for testing all endpoints via Swagger UI. JWT bearer token authentication supported through Swagger interface.

• 🔁 Unit Tests
  Implemented unit test with using xUnit

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🗂 Project Structure

/src
├── CurrencyExchange.API --> ASP.NET Core Web API (Presentation Layer)
├── CurrencyExchange.Application --> Application Layer (CQRS, Validators, DTOs)
├── CurrencyExchange.Domain --> Domain Entities and Interfaces
├── CurrencyExchange.Infrastructure --> External APIs, Provider Implementations

📌 Assumptions Made

• You have SQL Server installed and accessible.

• You need to manually update the appsettings.json file by modifying the accessible SQL user under the DbConnectionOptions section before running the project.

• This project is assumed to run in a development environment initially.

• Redis is not integrated in this version but planned for future releases.

• The caching layer uses in-memory cache (IMemoryCache) for development/testing purposes.

⚙️ Setup Instructions

• Clone the repository to your local machine.
• Update the DbConnectionOptions section in appsettings.json to use a valid and accessible SQL Server user.
• Run the project — the database will be created automatically using the Code First approach.
• Upon first run, initial seed data (such as user roles Admin, User) will be inserted into the database.
• Create a new user by sending a request to the api/users/createUser endpoint via Swagger.
• After creating the user, log in through api/auths/login, then copy the returned token. Use Authorize in Swagger with Bearer {token} to access secured endpoints.

📦 Dependencies

• .NET 8+
• MediatR
• FluentValidation
• Polly
• Serilog
• AspNetCoreRateLimit
• Swashbuckle (Swagger)
• Microsoft.Extensions.Caching.Memory

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🔮 Possible Future Enhancements

• Redis Integration: Replace the in-memory cache with a distributed Redis cache to improve performance and scalability across multiple instances.
• Cache Invalidation: Add a cache eviction strategy that ties into authentication or role updates, so sensitive currency data can be revalidated securely.
• Docker Support: A docker-compose.yaml file is planned for deployment simplification. Due to time constraints, it is not included in this version.
• Rate Limiting per User: Enhance the current IP-based rate limiting with user-based rate limiting to protect sensitive endpoints more securely.
• Refresh Tokens: Implement long-term authentication flow with refresh tokens for improved JWT security.