ExchangeRateUpdater: A Lightweight .NET API for Retrieving CNB Exchange Rates - Vladimir Vikhanov

[Woptope]

Exchange Rate Provider - Backend Task

This pull request introduces a .NET 8 API application that fetches and exposes exchange rates from the Czech National Bank (CNB).
It provides a single endpoint that accepts a date and one or more currency codes, returning the exchange rates relative to CZK.

The solution follows clean architecture principles, ensuring separation of concerns across different layers:

• Core: Business logic, domain models, CNB client, and provider logic.
• API: Controllers, DTOs, Swagger integration, and logging configuration.
• Tests: Unit tests with mocked HTTP responses to validate different scenarios.

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Solution Highlights

Separation of Concerns

• Core: Contains domain models (Currency, ExchangeRate), CNB client, and provider logic.
• API: Handles controllers, DTOs, Swagger integration, and logging setup.
• Tests: Includes unit tests that mock CNB API responses.

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Logging & Configuration

• Uses Serilog for structured logging.
• Logs to console, rolling file, and Visual Studio Debug window.
• Reads base URL from appsettings.json via the strongly-typed CnbOptions class.

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Swagger / OpenAPI Integration

• Endpoints and DTOs are annotated with [SwaggerOperation] and [SwaggerSchema] for clear documentation.
• Supports testing via Swagger UI without external tools.

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Unit Testing & Error Handling

• Dedicated test project with:
  • Mocked CNB responses for valid JSON, invalid JSON, and 500 errors.
  • Coverage of both happy path and edge cases.

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Extensibility & Future Improvements

• Security: Add authentication & authorization for sensitive operations.
• Caching: Implement Redis or another caching layer to improve performance.
• New Features: Support for exporting results (CSV/Excel) and multi-currency transformations.
• Extended Endpoints: Expand API functionality, e.g., adding historical exchange rate retrieval.
• Increased Test Coverage: Extend unit tests to cover more edge cases, including extreme date ranges.
• Logging Enhancements: Integrate external log aggregators for better observability.

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Usage

1. Run the API and navigate to /swagger.
2. Provide:
   • Date in YYYY-MM-DD format.
   • Comma-separated currency codes (e.g., "USD,EUR,CZK").
3. The API returns exchange rates in JSON format.

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Why This Approach?

• Simplicity & Focus: Retrieves real exchange rates from CNB, structures them into domain models, and exposes them via a clean, well-documented API.
• Scalability & Maintainability: SOLID design principles ensure clarity and long-term maintainability.