What

Implementation of ExchangeRateProvider for the Czech National Bank (CNB) using their public daily exchange rate endpoint.

Data source: https://www.cnb.cz/en/financial_markets/foreign_exchange_market/exchange_rate_fixing/daily.txt

Design decisions

The original skeleton was a single flat file. I reorganised it into three layers to keep concerns separate:

• Domain — Currency, ExchangeRate, CnbDailyTxtParser. No external dependencies.
• Application — IExchangeRateProvider interface. Decouples consumers from the implementation.
• Infrastructure — ExchangeRateProvider, HTTP client, configuration. All I/O lives here.

Runtime — Upgraded the project from .NET 6 (original skeleton) to .NET 10. .NET 6 reached end of life in November 2024 and no longer receives security patches. .NET 10 is the current LTS release and also enables modern C# features used throughout the implementation (<Nullable>enable</Nullable>, collection expressions, ArgumentNullException.ThrowIfNull, TimeProvider).

Program — The original skeleton instantiated ExchangeRateProvider directly and printed rates via Console.WriteLine with a hardcoded currency list. Replaced with a proper DI container, structured logging, command-line currency arguments, and exit codes.

Implementation highlights

Parser — handles the pipe-delimited format, normalises amounts greater than 1 (e.g. JPY is quoted per 100 units), and supports both comma and dot as decimal separators since CNB uses comma on the Czech endpoint and dot on the English one.

Caching — Results are cached in memory with a configurable TTL (default 1 hour). Concurrent requests are coalesced via SemaphoreSlim so only one HTTP call is made at a time. If-Modified-Since / HTTP 304 support avoids re-parsing unchanged data. Cache is preserved on errors so a transient failure does not evict valid data.

Resilience — Polly retry pipeline with exponential backoff and jitter on 408, 429, 5xx and network exceptions. All parameters (URL, timeout, retries, TTL) are externalised to appsettings.json and validated at startup.

Contract — Respects all three rules from the original skeleton: only rates published by CNB are returned, no inverse rates are calculated, and unknown currencies are silently ignored.

How to run

cd jobs/Backend/Task
dotnet run                        # defaults to EUR and USD
dotnet run -- EUR USD JPY GBP    # specific currencies

Tests

Unit tests covering: parser edge cases, TTL and cache behaviour, HTTP 304 handling, concurrency (single HTTP request under concurrent callers), error propagation, snapshot preservation on failure, and domain invariants.

cd jobs/Backend
dotnet test Task.Tests/

Open questions for the team

• Parser strictness — currently a malformed line throws and aborts the entire fetch. An alternative would be to skip invalid lines and log a warning, continuing with the valid ones. Depends on whether partial data is acceptable or not.

• Cache TTL — defaulting to 1 hour, but CNB publishes once per day (~14:30 CET). The right value depends on how time-sensitive the rates are for the business.

• Stale cache on CNB outage — currently if the CNB is down and the cache has expired, the error propagates to the caller. An alternative would be to serve the last known snapshot regardless of TTL, accepting stale data over no data. Depends on whether partial availability is preferable to a hard failure for the business.

• CZK as fixed target currency — all rates are expressed as X/CZK. If the business needs CZK/X, cross rates between non-CZK currencies, or rates against a different base, the current design would need to change.

• Single daily fixing — CNB publishes one rate per currency per day. If the business requires bid/ask spreads, intraday rates, or historical lookups, a different data source would be needed.