feat: Add Frankfurter API integration using WebClient, Strategy Pattern, and In-Memory Store

[luthfiyyah-a]

Overview

This PR implements a financial data aggregator that integrates with the Frankfurter Exchange Rate API. The system is designed to fetch, transform, and serve Indonesian Rupiah (IDR) focused financial data with custom business logic.

Key Technical Implementations

• Strategy Design Pattern: Centralized controller logic by delegating resource-specific transformations to dedicated strategy classes (IDRDataFetcher).
• WebClient Integration: Implemented a reactive WebClient via a custom FactoryBean for efficient and configurable external API communication.
• Immutable Data Store: Created a thread-safe, in-memory storage using UnmodifiableMap to ensure data integrity after preloading.
• ApplicationRunner: Automated data preloading during application startup to ensure immediate availability of financial data.
• ASCII-Based Spread Calculation: Implemented a custom personalization logic for USD buy spread based on specific string ASCII values.

Testing Coverage

• Unit Tests: Verified controller logic using standalone MockMvc and validated individual strategy transformation rules.
• Integration Tests: Performed end-to-end verification using MockWebServer to simulate the Frankfurter API and confirm successful data preloading.
• Global Exception Handling: Implemented @RestControllerAdvice to ensure standardized JSON error responses.

Preview

Endpoint	Description
GET /api/finance/data/latest_idr_rates	Latest rates with custom spread
GET /api/finance/data/historical_idr_usd	Historical IDR to USD data
GET /api/finance/data/supported_currencies	List of available currencies

More Information

See the readme for more information