Implement Czech National Bank Exchange Rate Provider

jobs/Backend/Task/ExchangeRateUpdater.slnx

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+<Solution>
+  <Folder Name="/src/">
+    <Project Path="src/Application/ExchangeRateUpdater.Application.csproj" />
+    <Project Path="src/Domain/ExchangeRateUpdater.Domain.csproj" />
+    <Project Path="src/Infrastructure/Providers/CzechNationalBank/ExchangeRateUpdater.Providers.CzechNationalBank.csproj" />
+  </Folder>
+
+  <Folder Name="/tests/">
+    <Project Path="tests/UnitTests/ExchangeRateUpdater.UnitTests.csproj" />
+  </Folder>
+</Solution>

jobs/Backend/Task/docs/readme.md

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+# Exchange Rate Updater – Notes & Considerations
+
+> **TL;DR**  
+> A clean, immutable, and test-focused solution that fetches and parses daily CNB exchange rates, emphasizing correctness, validation, and separation of concerns
+
+<br>
+
+
+### _How to Run The Application_
+
+```bash
+# navigate to src/Application folder
+cd src/Application
+
+# restore dependencies
+dotnet restore
+
+# build the solution
+dotnet build
+
+# run unit tests
+dotnet test
+
+# run the application
+dotnet run --project ExchangeRateUpdater.Application.csproj
+```
+
+<br>
+<br>
+
+
+## 1. Problem Statement
+
+_See Challenge Instructions here: [Mews Backend .NET Challenge Instructions](https://github.com/MewsSystems/developers/blob/master/jobs/Backend/DotNet.md)_
+
+
+The goal of this challenge is to fetch daily foreign exchange rates published by the Czech National Bank (CNB), parse the returned data, and expose the rates in a clean, domain-driven model.
+
+The CNB endpoint returns exchange rates as a plain-text, pipe-separated file containing:
+- A header with the publication date and working day number
+- A column definition row
+- One row per currency exchange rate
+
+The solution is expected to:
+- Retrieve the data over HTTP
+- Parse and validate the response format
+- Convert the raw data into strongly typed domain objects
+- Handle invalid or unexpected input gracefully
+
+<br>
+
+## 2. Assumptions
+The following assumptions were made to keep the solution focused and explicit:
+
+- The CNB response format is stable and follows the structure found in the [sample_data.txt](sample_data.txt)
+- All exchange rate values are positive
+- The application aims to display the convertion of 1 unit of given currency. Example: 1 EUR -> CZK
+- The target currency is implicitly CZK, as implied by the [CNB source](https://www.cnb.cz/en/financial-markets/foreign-exchange-market/central-bank-exchange-rate-fixing/central-bank-exchange-rate-fixing/daily.txt)
+- The full response fits comfortably in memory
+- This is a read-only, batch-style operation (no persistence required)
+
+<br>
+
+## 3. Approach
+The solution is structured around a clear separation of concerns:
+
+_It uses IoC and DI to allow new implementations to be added without breaking existing ones_
+
+1. **HTTP Client Layer**
+   - Responsible for retrieving raw data from the CNB endpoint
+   - Configured with timeouts, retries, and resilience policies
+
+2. **Parsing Layer**
+   - Converts the pipe-separated text response into structured models
+   - Validates headers, column definitions, and data rows and fails early in case the contract ever changes
+   - Produces deterministic parsing results or meaningful exceptions
+
+3. **Domain Layer**
+   - Models currencies and exchange rates as immutable value objects
+   - Enforces basic domain invariants (e.g. positive exchange rates)
+
+4. **Application Layer**
+   - Orchestrates the flow between client, parser, and domain
+   - Exposes a clean API for consuming exchange rate data
+
+The design favors clarity and correctness over premature optimization.
+
+<br>
+
+## 4. Design Decisions
+
+### Immutability
+- Domain entities and DTOs are implemented as immutable types
+- Value-based equality is used where identity is defined purely by data
+- This reduces side effects and simplifies reasoning and testing
+
+### Separation of Concerns
+- Parsing logic is isolated from HTTP and orchestration logic
+- Domain models are independent of infrastructure concerns
+- This makes individual components easier to test and evolve
+
+### Explicit Validation
+- The parser validates headers, column definitions, and row structure
+- Failures are detected early and reported with clear exception messages
+
+### Resilience
+- HTTP calls are protected using retry and circuit breaker policies
+- Transient failures are handled without leaking infrastructure concerns into the domain
+
+<br>
+
+## 5. Immutability & Modeling
+The following modeling strategy was applied:
+
+- **Records**
+  - Domain value objects such as `Currency` and `ExchangeRate`
+  - DTOs and parsing result models
+  - Configuration objects loaded from application settings
+
+- **Classes**
+  - Services, clients, and parsers
+  - Components that encapsulate behavior or orchestration
+
+This distinction reflects the difference between *data* and *behavior* in the system.
+
+<br>
+
+## 6. Edge Cases & Error Handling
+The solution explicitly handles:
+
+- Empty or whitespace-only responses
+- Missing or malformed headers
+- Unexpected column definitions
+- Invalid numeric values
+- Incomplete or malformed data rows
+
+When errors occur, the system:
+- Fails fast during parsing
+- Throws domain-specific exceptions with descriptive messages
+- Avoids silently ignoring invalid data
+
+<br>
+
+## 7. Testing Strategy
+The testing approach focuses on correctness and determinism:
+
+- Unit tests for domain entities and invariants
+- Comprehensive parser tests covering:
+  - Valid inputs
+  - Invalid formats
+  - Edge cases and error scenarios
+- Tests are isolated, fast, and do not rely on external systems
+
+Unit tests for higher-level application behavior and Integration tests were intentionally omitted due to scope.
+
+<br>
+
+## 8. Limitations
+Known limitations include:
+
+- No caching of results between runs
+- No dynamically parsing program args as currencies input
+- No persistence layer
+- No localization or formatting concerns addressed
+- No retry backoff customization exposed via configuration
+
+These were accepted trade-offs given the scope of the challenge.
+
+<br>
+
+## 9. Possible Improvements
+With additional time, the following enhancements could be made:
+
+- Add integration tests with a mocked HTTP server
+- Implement better Domain Exceptions for proper retries / logging
+- Implement greceful failures for malformed Exchange Rates 
+- Consider adding caching
+- Support additional rate providers via a common abstraction
+- Expose richer observability (metrics, structured logs)

jobs/Backend/Task/docs/sample_data.txt

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+23 Dec 2025 #248
+Country|Currency|Amount|Code|Rate
+Australia|dollar|1|AUD|13.818
+Brazil|real|1|BRL|3.694
+Bulgaria|lev|1|BGN|12.434
+Canada|dollar|1|CAD|15.064
+China|renminbi|1|CNY|2.936
+Denmark|krone|1|DKK|3.256
+EMU|euro|1|EUR|24.320
+Hongkong|dollar|1|HKD|2.652
+Hungary|forint|100|HUF|6.218
+Iceland|krona|100|ISK|16.432
+IMF|SDR|1|XDR|28.206
+India|rupee|100|INR|23.043
+Indonesia|rupiah|1000|IDR|1.230
+Israel|new shekel|1|ILS|6.452
+Japan|yen|100|JPY|13.226
+Malaysia|ringgit|1|MYR|5.077
+Mexico|peso|1|MXN|1.150
+New Zealand|dollar|1|NZD|12.049
+Norway|krone|1|NOK|2.051
+Philippines|peso|100|PHP|35.072
+Poland|zloty|1|PLN|5.747
+Romania|leu|1|RON|4.778
+Singapore|dollar|1|SGD|16.051
+South Africa|rand|1|ZAR|1.237
+South Korea|won|100|KRW|1.393
+Sweden|krona|1|SEK|2.247
+Switzerland|franc|1|CHF|26.193
+Thailand|baht|100|THB|66.364
+Turkey|lira|100|TRY|48.183
+United Kingdom|pound|1|GBP|27.861
+USA|dollar|1|USD|20.631