Backend tasks - Ashleigh Adams

[ash-leigh]

Overview

This PR implements a production-ready Exchange Rate Provider for the Czech National Bank (CNB), following clean architecture principles and industry best practices for maintainability, reliability, and scalability. To test and run the solution please see instructions in README at route of projects repository

Key Design Decisions

1. CNB Data Source
   While there is a couple of official data sources I used CNB's official REST API with Swagger documentation https://api.cnb.cz/cnbapi/swagger-ui.html#//exrates so that we could have confidence in the formal API contract
2. Clean Architecture with CQRS Pattern
   Structured the solution into four distinct layers (Domain, Application, Infrastructure, API) with clear dependency flow and implemented CQRS for query handling.
3. Factory Pattern for Multi-Currency Support
   Implemented ExchangeRateServiceFactory with service key routing for currency-specific providers.
4. Polly for Resilience & Retry Logic
   Integrated Polly with exponential backoff, and timeout policies for HTTP calls to CNB API.
5. Comprehensive Validation Strategy
   Implemented multi-layer validation
6. Integration & Unit Test Coverage
7. Docker & OpenAPI Documentation
   Dockerfile and Scalar UI for interactive API documentation.

Engineering Best Practices

1. Centralised build and package management using Directory.Build.props and Directory.Packages.props
2. Applied [ExcludeFromCodeCoverage] attribute to test project Program.cs files and other infrastructure code that doesn't require testing.
3. Utilised copilot for boiler plate code and initial unit tests. Running Claude Sonnet 4.3, example prompt:

/tests #SomeClass provide lean unit tests using xUnit, FakeItEasy, Shouldly and FakeLogger

Backlog

Features that would be good to implement next

1. Implement more intelligent caching strategy
   At the moment data from CNB is cached every hour but given the banks documentation provides the time its exchange rates as usually declared (14:30 weekdays, excluding holidays) there is opportunity to ensure the cache does not go stale.

2. Return ValidFor for rates
   Returning the date at which the exchange rate is valid would be beneficial to consumers.

[wiz-trial-29da161873]

Wiz Scan Summary

Scanner	Findings
Vulnerabilities	-
Sensitive Data	-
Secrets	-
IaC Misconfigurations	1  3
SAST Findings	-
Total	1  3

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