ExchangeRateProvider For CNB - Mews Backend Task - Omar Farooq Khan

jobs/Backend/Notes.md

@@ -0,0 +1,129 @@
+### Thoughts
+
+## Building a requirements List
+
+The task is to implement an ExchangeRateProvider for Czech National Bank. 
+Find data source on their web - part of the task is to find the source of the exchange rate data and a way how to extract it from there.
+It is up to you to decide which technology (from .NET family) or package to use. 
+Any code design changes/decisions to the provided skeleton are also completely up to you.
+The solution has to be buildable, runnable and the test program should output the obtained exchange rates.
+Goal is to implement a fully functional provider based on real world public data source of the assigned bank.
+To submit your solution, create a Pull Request from a fork.
+
+Please write the code like you would if you needed this to run on production environment and had to take care of it long-term.
+
+Should return exchange rates among the specified currencies that are defined by the source. But only those defined
+by the source, do not return calculated exchange rates. E.g. if the source contains "CZK/USD" but not "USD/CZK",
+do not return exchange rate "USD/CZK" with value calculated as 1 / "CZK/USD". If the source does not provide
+some of the currencies, ignore them.
+
+Translates to:
+- Finding a datasource
+- Able to state what Exchange rates we want to return based on a list of desired currencies
+- A "source" currency to calculate against.
+- A Desired response to return
+
+### Discovery
+I navigated the CNB website to retrieve information around any API/data that relates to Exchange rates and found the following link describing an API.
+https://www.cnb.cz/cs/casto-kladene-dotazy/Kurzy-devizoveho-trhu-na-www-strankach-CNB/
+
+For this project I wanted to be able to easily extract the data regarding exchange rates, there were two approaches to this:
+- Option 1: Take advantage of the Swagger docs and directly integrate with the API: https://api.cnb.cz/cnbapi/swagger-ui.html#/%2Fexrates 
+- Option 2: Utilising the TXT file directly from the page https://www.cnb.cz/en/financial-markets/foreign-exchange-market/central-bank-exchange-rate-fixing/central-bank-exchange-rate-fixing/
+
+For a production grade system, I decided to take advantage of the Swagger Docs as they are a typical gold standard for developers on information regarding what can be returned/errors,
+and it is safe to assume the route won't change for the API whereas the website can change.
+
+### Programming Philosophies That I followed for this project 
+- Adherence to SOLID principals
+- Testability
+- Readability
+- Observability
+- KISS & DRY
+
+### Project Configuration
+With my approach I decided to upgrade the project to utilise .net 10.
+It required little effort and would provide a long term benefit in terms of active support in a production environment and ensure security updates are available.
+I utilised the option pattern so that API configuration can be done with a simple application.json modification and allow for a source currency to be defined.
+
+Additionally I added Polly and FluentValidation to assist with building a more resilient system to solving transient network errors and validating user input respectively.
+The Test project mimics the folder structure of the ExchangeRateUpdater Project so that it is easy to identify where the tests are. They were also written with BDD in mind.
+
+I decided to define a contract `IExchangeRateClient` where if we needed to integrate with another API, we could be free to do so without much hassle
+
+## Core Principles for the development in the project
+## Resiliency 
+This was achieved using Polly in the CzechNationalBankExchangeRateClient. 
+This client uses retrys and timeouts to manage the duration of a request and give it the ablilty to deal with transient errors.
+These values are configured in appSettings.json
+
+## Observability
+ - When it comes to observability, I try to follow the idea of being able to "Follow your code via Logs", and this was a practice that I tried to follow this project too.
+I not only accounted for error cases but also what should happen normally e.g, if we recieve a response from the external api. 
+Additionally i've accounted for exceptions and Logged them individually to identify specific errors.
+I've also created a `ErrorLog` to extract the important information like the status code and message should there be an error.
+
+## Maintainability
+- I've explicitly created a "EmptyResponse" object to handle the cases where there is an unexpected failure. 
+This avoids returning a "null" that is just obscure but instead returns a specific object that describes a fault.
+- I've also used the ResultsPattern to help identify successful outcomes to those that are undesired.
+- Single responsiblity has been a core component of this project as each class has one purpose in mind. It has allowed for simple and easy tests that are quick to verify.
+
+### Testing
+- FluentAssertions as they are really readable and assertions just make sense.
+- Moq for mocking out dependencies; really useful and very familiar with/its wide known.
+- XUnit as familiar framework.
+
+### Testing Strategy
+- Generally I try to stay close to the Given_When_Then format as its clear to identify the objective behind a test.
+- I prioritised mocking dependencies and Followed the Arrange Act Assert Pattern to make the tests clear to a reader.
+- Utilising SOLID patterns also helped make tests easier as the objective behind a class was straight forward.
+- Tests share an identical name with their class and follow a similar folder structure to ensure easy traceability.
+
+What this project does:
+- Exposes a Swagger Endpoint for interacting with the API 
+  - `api/v1/exchange-rates/daily`
+    - This endpoint fetches the desired currencies and returns the Exchange based on the Source Currency for the current business day.
+
+## Getting Started
+
+Follow these steps to build, run, and test the project
+
+### 1. Build the Solution
+Compile the entire solution to resolve dependencies and create binaries.
+```bash
+dotnet build ExchangeRateUpdater.sln
+```
+
+### 2. Run the Solution
+Compile the entire solution to resolve dependencies and create binaries.
+```bash
+dotnet run --project ExchangeRateUpdater/ExchangeRateUpdater.csproj
+```
+
+### 3. Call the endpoint
+Utilise the Swagger Docs (Accessible via /swagger/index.html)
+
+Alternatively you can use the following CURL or Call the endpoint `api/v1/exchange-rates/daily` via Postman/Insomnia.
+
+```markdown
+curl -X 'GET' \
+  'http://localhost:5001/api/v1/exchange-rates/daily?currencies=USD&currencies=GBP&currencies=XAK' \
+  -H 'accept: application/json'
+```
+
+### Run the Tests
+```bash
+dotnet test ExchangeRateUpdater.Tests/ExchangeRateUpdater.Tests.csproj
+```
+
+### Note
+When running locally, utlilse the LaunchSettings configuration so that its simpler to run.
+
+Things I would do if I had more time:
+- Support for the ability to choose a specific date, this could be done via utlising the existing integrated API or Expanding to using the other Endpoints.
+  - This can easily be done by taking advantage of the `IExchangeRateClient` and `CzechNationalBankExchangeRateClient` since there is a defined contract to retrieve ExchangeRate Data and the Client is open to expansion.
+- Caching the API response to reduce unnecessary requests calls to the CNB api.
+- Add robust integration tests utilising the JustEat client for Interception https://github.com/justeattakeaway/httpclient-interception 
+- A suite of automated Postman tests used as a smoke test between environments to ensure confidence that the system still works.
+- Setup Runbook Alerts within monitoring Frameworks like Datadog so that any unexpected errors are raised to the attention of a developer asap.

jobs/Backend/Task/ExchangeRateUpdater.sln

@@ -3,7 +3,9 @@ Microsoft Visual Studio Solution File, Format Version 12.00
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+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "ExchangeRateUpdater", "ExchangeRateUpdater\ExchangeRateUpdater.csproj", "{7B2695D6-D24C-4460-A58E-A10F08550CE0}"
+EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "ExchangeRateUpdaterTests", "ExchangeRateUpdaterTests\ExchangeRateUpdaterTests.csproj", "{C0F5DCC5-1599-44FB-91AF-A5A89A65C1C9}"
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@@ -15,6 +17,10 @@ Global
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 		{7B2695D6-D24C-4460-A58E-A10F08550CE0}.Release|Any CPU.Build.0 = Release|Any CPU
+		{C0F5DCC5-1599-44FB-91AF-A5A89A65C1C9}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{C0F5DCC5-1599-44FB-91AF-A5A89A65C1C9}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{C0F5DCC5-1599-44FB-91AF-A5A89A65C1C9}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{C0F5DCC5-1599-44FB-91AF-A5A89A65C1C9}.Release|Any CPU.Build.0 = Release|Any CPU
 	EndGlobalSection
 	GlobalSection(SolutionProperties) = preSolution
 		HideSolutionNode = FALSE

jobs/Backend/Task/ExchangeRateUpdater/Business/ExchangeRateProvider.cs

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+using System.Collections.Generic;
+using System.Threading.Tasks;
+using ExchangeRateUpdater.Business.Processors;
+using ExchangeRateUpdater.Models;
+
+namespace ExchangeRateUpdater.Business
+{
+    public class ExchangeRateProvider : IExchangeRateProvider
+    {
+        private readonly IExchangeRateProcessor _exchangeRateProcessor;
+
+        public ExchangeRateProvider(IExchangeRateProcessor exchangeRateProcessor)
+        {
+            _exchangeRateProcessor = exchangeRateProcessor; 
+        }
+
+        public async Task<Result<IEnumerable<ExchangeRate>>> GetExchangeRates(string[] currencies)
+        {
+            return await _exchangeRateProcessor.ProcessExchangeRates(currencies);
+        }
+    }
+}

jobs/Backend/Task/ExchangeRateUpdater/Business/IExchangeRateProvider.cs

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+using System.Collections.Generic;
+using System.Threading.Tasks;
+using ExchangeRateUpdater.Models;
+
+namespace ExchangeRateUpdater.Business;
+
+public interface IExchangeRateProvider
+{
+    /// <summary>
+    /// Should return exchange rates among the specified currencies that are defined by the source. But only those defined
+    /// by the source, do not return calculated exchange rates. E.g. if the source contains "CZK/USD" but not "USD/CZK",
+    /// do not return exchange rate "USD/CZK" with value calculated as 1 / "CZK/USD". If the source does not provide
+    /// some of the currencies, ignore them.
+    /// </summary>
+    public Task<Result<IEnumerable<ExchangeRate>>> GetExchangeRates(string[] currencies);
+}

jobs/Backend/Task/ExchangeRateUpdater/Business/Processors/CzechNationalBankProcessor.cs

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+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+using ExchangeRateUpdater.Infrastructure;
+using ExchangeRateUpdater.Mapping;
+using ExchangeRateUpdater.Models;
+using ExchangeRateUpdater.Models.CzechNationalBank;
+using ExchangeRateUpdater.Models.CzechNationalBankResponse;
+using Microsoft.Extensions.Logging;
+
+namespace ExchangeRateUpdater.Business.Processors;
+
+public class CzechNationalBankProcessor : IExchangeRateProcessor 
+{
+    private readonly IExchangeRateClient<Response> _client;
+    private readonly IExchangeRateMapper<RecordedRate> _mapper;
+    private readonly ILogger<CzechNationalBankProcessor> _logger;
+
+    public CzechNationalBankProcessor(
+        IExchangeRateClient<Response> client,
+        IExchangeRateMapper<RecordedRate> mapper,
+        ILogger<CzechNationalBankProcessor> logger)
+    {
+        _client = client;
+        _mapper = mapper;
+        _logger = logger;
+    }
+
+    public async Task<Result<IEnumerable<ExchangeRate>>> ProcessExchangeRates(string[] currenciesToProcess)
+    { 
+        _logger.LogInformation("CzechNationalBankProcessor_ProcessingCurrencies");
+
+        var processedRates = new List<ExchangeRate>();
+       var responseToProcess = await _client.RetrieveExchangeRatesAsync();
+
+       if (responseToProcess is EmptyResponse)
+       {
+           return Result<IEnumerable<ExchangeRate>>.Fail("CzechNationalBankProcessor_UnexpectedClientError");
+       }
+
+       var dictionaryOfReturnedRates =
+           responseToProcess.Rates.ToDictionary(key => key.CurrencyCode, StringComparer.OrdinalIgnoreCase);
+
+       foreach (var desiredCurrency in currenciesToProcess)
+       {
+           var haveFoundEntry = dictionaryOfReturnedRates.TryGetValue(desiredCurrency, out var value);
+           if (!haveFoundEntry)
+           {
+               _logger.LogWarning("CzechNationalBankProcessor_CurrencyNotFound_{DesiredCurrency}", desiredCurrency);
+               continue;
+           }
+
+           var mappedExchangeRate = _mapper.MapToExchangeRate(value);
+           processedRates.Add(mappedExchangeRate);
+       }
+
+       _logger.LogInformation("CzechNationalBankProcessor_FinishedProcessingCurrencies");
+       return Result<IEnumerable<ExchangeRate>>.Ok(processedRates);
+    }
+}

jobs/Backend/Task/ExchangeRateUpdater/Business/Processors/IExchangeRateProcessor.cs

@@ -0,0 +1,15 @@
+using System.Collections.Generic;
+using System.Threading.Tasks;
+using ExchangeRateUpdater.Models;
+
+namespace ExchangeRateUpdater.Business.Processors;
+
+public interface IExchangeRateProcessor
+{
+    /// <summary>
+    /// Should return exchange rates among the specified currencies that are defined by the source. But only those defined
+    /// </summary>
+    /// <param name="currenciesToProcess"></param>
+    /// <returns>List of <see cref="ExchangeRate"/></returns>
+    Task<Result<IEnumerable<ExchangeRate>>> ProcessExchangeRates(string[] currenciesToProcess);
+}

jobs/Backend/Task/ExchangeRateUpdater/Controllers/ExchangeRatesController.cs

@@ -0,0 +1,59 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Net;
+using System.Threading.Tasks;
+using ExchangeRateUpdater.Business;
+using ExchangeRateUpdater.Models;
+using FluentValidation;
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.Extensions.Logging;
+
+namespace ExchangeRateUpdater.Controllers;
+
+[ApiController]
+[Route("api/v1/exchange-rates")]
+[Produces("application/json")]
+public class ExchangeRatesController : ControllerBase
+{
+    private readonly IExchangeRateProvider _exchangeRateProvider;
+    private readonly IValidator<string[]> _currencyCodeValidator;
+    private readonly ILogger<ExchangeRatesController> _logger;
+
+    public ExchangeRatesController(IExchangeRateProvider exchangeRateProvider, IValidator<string[]> currencyCodeValidator, ILogger<ExchangeRatesController> logger)
+    {
+        _exchangeRateProvider = exchangeRateProvider;
+        _currencyCodeValidator = currencyCodeValidator;
+        _logger = logger;
+    }
+
+    ///<summary>
+    /// Fetches the desired currencies and returns the <see cref="ExchangeRate"/> based on the Source Currency for the current business day.
+    /// </summary>>
+    /// <param name="currencies" example="{'USD', 'EUR', CZK'}">Desired Currencies</param>
+    [HttpGet("daily")]
+    [ProducesResponseType(typeof(ActionResult<List<ExchangeRate>>),(int)HttpStatusCode.OK)]
+    [ProducesResponseType(typeof(ActionResult<string>),(int)HttpStatusCode.BadRequest)]
+    [ProducesResponseType((int)HttpStatusCode.InternalServerError)]
+    public async Task<ActionResult> GetSelectedCurrencyCodesDaily([FromQuery] string[] currencies = null)
+    {
+        var validationResult = await _currencyCodeValidator.ValidateAsync(currencies);
+
+        if (!validationResult.IsValid)
+        {
+            var errors = validationResult.Errors.Select(x => x.ErrorMessage);
+            _logger.LogError("ExchangeRatesController_ValidationFailure {Errors}", errors);
+            return BadRequest(validationResult.Errors.Select(x => x.ErrorMessage));
+        }
+
+        var result = await _exchangeRateProvider.GetExchangeRates(currencies);
+
+        if (!result.Success)
+        {
+            _logger.LogError("ExchangeRatesController_ProviderError {Error}", result.Error);
+            return BadRequest(result.Error);
+        }
+
+        return Ok(result.Value);
+    }
+}

jobs/Backend/Task/ExchangeRateUpdater/ExchangeRateUpdater.csproj

@@ -0,0 +1,18 @@
+<Project Sdk="Microsoft.NET.Sdk.Web">
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="FluentValidation" Version="12.1.1" />
+    <PackageReference Include="Polly" Version="8.6.5" />
+    <PackageReference Include="Swashbuckle.AspNetCore" Version="10.1.0" />
+    <PackageReference Include="Swashbuckle.AspNetCore.Swagger" Version="10.1.0" />
+    <PackageReference Include="Swashbuckle.AspNetCore.SwaggerUI" Version="10.1.0" />
+  </ItemGroup>
+
+    <PropertyGroup>
+        <GenerateDocumentationFile>true</GenerateDocumentationFile>
+        <NoWarn>$(NoWarn);1591</NoWarn>
+    </PropertyGroup>
+</Project>