Implementation of Exchange Rate Task

[sergiomadd]

Exchange rate updater

This is a .NET 6 console app that provides exchange rates from the Czech National Bank (CNB) for the CZK currency.

Decisions made for the task

Data Source

I decided to use the official CNB API: https://api.cnb.cz/. It is a documented official data source, updated daily, that provides foreign currency exchange rates in a fixed format.

• Endpoint for latest rates https://api.cnb.cz/cnbapi/exrates/daily?lang=EN

Console app

I decided to go with a console app as it is the app type provided by the template. Also since I already have a public open source api project, I decided it would fit perfectly to display my skills in another .NET technology.

.NET 6

I chose to remain on the provided .NET 6 version rather than upgrading, since the task requirements did not specify an upgrade, and the project template builds correctly out of the box with .NET 6.
In a real-world scenario, upgrading the target framework of an existing project can introduce many breaking changes, and it would require extensive testing, and careful planning. Framework upgrades should be thoroughly though out and coordinated with other parts of the software.
Using .NET 6 does not pose any limitations for this project, and I am fully comfortable working with older .NET versions when appropriate.

Architecture

The app uses a layered architecture with dependency inversion, built on top of .NET Generic Host.

• It uses dependency injection to provide loose coupling and easy testing, as components depend on abstractions.
• It uses a HttpClient with retry policy using Polly. This adds support for transient failures.
• It uses configuration management for external settings. Makes it easy to configure different environments.
• It comes with a separate testing project with unit testing, without requiring the real API or console interaction.

Logging

For logging I used ILogger and System.Console interchangeably, with separation of responsibilities.

• ILogger for system relevant logs, process tracking and errors.
• System.Console for user relevant logs and user interaction.

MenuHandler

I have added a console menu with a simple lifecycle loop for the user interaction layer. This way I separate the business logic from the user UI. It has these available programs:

• [1] Get latest exchange rates
  • Displays the latest exchange rates that match the supported currencies
  • Uses ExchangeRateProvider.GetExchangeRates
• [2] Get exchange rates for a date
  • Asks the user to input a date until its valid
  • Displays exchange rates for the day that match the supported currencies
  • Uses ExchangeRateProvider.GetExchangeRatesFromDay
• [3] Compare exchange rates between 2 dates
  • Asks the user to input 2 dates one by one until they are valid
  • Displays the calculated difference between the rates of the 2 dates
  • Uses ExchangeRateProvider.CompareExchangeRatesBetweenDates
• [0] Exit
  • Closes the application

On each action finish, apart from [0], it prompts the user to Press Enter to continue. On Enter it clears the console and displays the menu, thus completing the loop.

ExchangeRateSource

I decided to use a IExchangeRateSource to implement a data obtainer. This design provides a swappable source, so if in the future instead of getting the data from a api, I needed to get it from a database, I would only need create a new implementation of the interface and register it.

ExchangeRateProvider

It implements the interface IExchangeRateProvider which allows for easy testing. It uses private helper methods to assist in its function, and makes the logic reusable.

• ProcessExchangeRates Processes incoming ExchangeRateDTO to match supported currencies
• CompareExchangeRates Compares 2 lists of ExchangeRateDTO, using only those who match the supported currencies, and returning a list of ExchangeRateDifference with calculated difference values.
• IsExchangeRateDTOValid Validates if the provided ExchangeRateDTO is valid.
• ExchangeRateMapDTOToModel Maps provided ExchangeRateDTO to the internal domain model.

This design implements SRP, where the GetExchangeRates method is only in charge of orchestrating the data intake and the processing of the exchange rates. Also it makes the helper methods reusable for other future functions in the provider.

Why add ExchangeRateDifference entity?

This additional entity allows for separation of concerns and keeps the domain model focused on each entities function. It also improves maintainability as it ensures updates to calculations don't affect the ExchangeRate entity.

Error Handling

I implemented a ExchangeRateSourceException to help with maintainable error handling. In addition, exceptions are caught and logged as close to their origin as possible. It allows for detailed logging and shields from throwing low level exceptions, revealing implementation specific details to above layers.

Known issues

• ILogger logging sometimes lags behind Console.WriteLine. This is caused by the ILogger buffer, as it asynchronously prints messages, compared to Console.WriteLine which writes right away. This issue can sometimes place logger messages in between console messages.
  • Fix: Introduce a dedicated ConsoleLogger with flushable wrapper, to make sure the ILogger buffer empties before proceeding with execution
• Sometimes at app startup the menu doesn't get displayed right away. This happens cause the main method is async, and the console initialized before the first Console.WriteLine in the menu handler is executed.

Improvements

• Update .NET to latest version. This would benefit app maintenance and security, but as explained above i ruled it out of the scope of this template project.
• Add external json files for storing supported currency data.
• Add caching layer for exchange rates. Exchange rates are static and they only update once a day with a new one. This kind of data would benefit from caching as the data wouldn't change for 2 request of the same rate.
• Utilize more available endpoints of the CNB API for more features.
  • Monthly averages
  • Monthly cumulative averages
  • Quarterly averages
• Add integration testing for the MenuHandler interactions.
• Replace numerical console programs with named ones. [1] -> lt for latest rates
• Add IExchangeRateDifferenceCalculator for managing different ways of calculating the rate differences, instead of doing the calculation on the CompareExchangeRates method. This would separate concerns, and be able to add different calculation methods, like LogarithmicDifferenceCalculator, without modifying the implementation.

How to run the project

Requisites

• .NET 6.0 SDK installed on your machine.

Building and Launching the App

Build the solution

dotnet build ExchangeRateUpdater.sln

Run the application

dotnet run --project ExchangeRateUpdater/ExchangeRateUpdater.csproj

Executing Unit Tests

Run all tests

dotnet test ExchangeRateUpdater.Tests/ExchangeRateUpdater.Tests.csproj

Credits

Menu title font from https://patorjk.com/software/taag

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