Exchange rate provider - Diego Fernandez Pandiello

.gitignore

@@ -21,3 +21,5 @@
 node_modules
 bower_components
 npm-debug.log
+.vs/
+**/.vs/

jobs/Backend/Task/Domain/Currency.cs

@@ -0,0 +1,24 @@
+using System;
+
+namespace ExchangeRateUpdater.Domain
+{
+    public record Currency
+    {
+        public string Code { get; }
+
+        public Currency(string code)
+        {
+            if (string.IsNullOrWhiteSpace(code))
+            {
+                throw new ArgumentException("Currency code cannot be null or empty.", nameof(code));
+            }
+
+            Code = code.Trim().ToUpperInvariant();
+        }
+
+        public override string ToString()
+        {
+            return Code;
+        }
+    }
+}

jobs/Backend/Task/Domain/ExchangeRate.cs

@@ -0,0 +1,10 @@
+namespace ExchangeRateUpdater.Domain
+{
+    public record ExchangeRate(Currency SourceCurrency, Currency TargetCurrency, decimal Value)
+    {
+        public override string ToString()
+        {
+            return $"{SourceCurrency}/{TargetCurrency}={Value}";
+        }
+    }
+}

jobs/Backend/Task/Domain/ExchangeRateProvider.cs

@@ -0,0 +1,25 @@
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+
+namespace ExchangeRateUpdater.Domain
+{
+    internal sealed class ExchangeRateProvider(IExchangeRateFetcher exchangeRateFetcher)
+    {
+        private readonly IExchangeRateFetcher exchangeRateFetcher = exchangeRateFetcher;
+
+        /// <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 async Task<IEnumerable<ExchangeRate>> GetExchangeRates(IEnumerable<Currency> currencies)
+        {
+            var currenciesSet = new HashSet<Currency>(currencies);
+
+            var rates = await exchangeRateFetcher.GetExchangeRates();
+            return rates.Where(r => currenciesSet.Contains(r.TargetCurrency));
+        }
+    }
+}

jobs/Backend/Task/Domain/IExchangeRateFetcher.cs

@@ -0,0 +1,11 @@
+using System.Collections.Generic;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace ExchangeRateUpdater.Domain
+{
+    internal interface IExchangeRateFetcher
+    {
+        Task<IEnumerable<ExchangeRate>> GetExchangeRates(CancellationToken cancellationToken = default);
+    }
+}

jobs/Backend/Task/ExchangeRateUpdater.csproj

@@ -2,7 +2,34 @@
 
   <PropertyGroup>
     <OutputType>Exe</OutputType>
-    <TargetFramework>net6.0</TargetFramework>
+    <TargetFramework>net9.0</TargetFramework>
+    <UserSecretsId>8bbe5d99-db8b-403b-9849-2f28ed7c8031</UserSecretsId>
   </PropertyGroup>
 
+  <ItemGroup>
+    <PackageReference Include="CsvHelper" Version="33.0.1" />
+    <PackageReference Include="Microsoft.Extensions.Caching.Memory" Version="9.0.5" />
+    <PackageReference Include="Microsoft.Extensions.Configuration.Abstractions" Version="9.0.5" />
+    <PackageReference Include="Microsoft.Extensions.Configuration.Json" Version="9.0.5" />
+    <PackageReference Include="Microsoft.Extensions.DependencyInjection" Version="9.0.5" />
+    <PackageReference Include="Microsoft.Extensions.Hosting" Version="9.0.5" />
+    <PackageReference Include="Microsoft.Extensions.Http" Version="9.0.5" />
+    <PackageReference Include="Microsoft.Extensions.Http.Polly" Version="9.0.5" />
+    <PackageReference Include="Microsoft.Extensions.Logging" Version="9.0.5" />
+    <PackageReference Include="OpenTelemetry" Version="1.12.0" />
+    <PackageReference Include="OpenTelemetry.Exporter.Console" Version="1.12.0" />
+    <PackageReference Include="OpenTelemetry.Extensions.Hosting" Version="1.12.0" />
+    <PackageReference Include="OpenTelemetry.Instrumentation.Http" Version="1.12.0" />
+  </ItemGroup>
+    <ItemGroup>
+		<None Update="appsettings.json">
+			<CopyToOutputDirectory>Always</CopyToOutputDirectory>
+		</None>
+	</ItemGroup>
+
+	<ItemGroup>
+		<InternalsVisibleTo Include="ExchangeRateUpdaterTests" />
+		<InternalsVisibleTo Include="DynamicProxyGenAssembly2" />		
+	</ItemGroup>
+
 </Project>

jobs/Backend/Task/ExchangeRateUpdater.sln

@@ -1,10 +1,12 @@
 
 Microsoft Visual Studio Solution File, Format Version 12.00
-# Visual Studio 14
-VisualStudioVersion = 14.0.25123.0
+# Visual Studio Version 17
+VisualStudioVersion = 17.13.35931.197 d17.13
 MinimumVisualStudioVersion = 10.0.40219.1
 Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "ExchangeRateUpdater", "ExchangeRateUpdater.csproj", "{7B2695D6-D24C-4460-A58E-A10F08550CE0}"
 EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "ExchangeRateUpdaterTests", "..\Tests\ExchangeRateUpdaterTests\ExchangeRateUpdaterTests.csproj", "{1E427E08-DC0D-4B92-9B2F-2B880C2B1809}"
+EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
 		Debug|Any CPU = Debug|Any CPU
@@ -15,8 +17,15 @@ Global
 		{7B2695D6-D24C-4460-A58E-A10F08550CE0}.Debug|Any CPU.Build.0 = Debug|Any CPU
 		{7B2695D6-D24C-4460-A58E-A10F08550CE0}.Release|Any CPU.ActiveCfg = Release|Any CPU
 		{7B2695D6-D24C-4460-A58E-A10F08550CE0}.Release|Any CPU.Build.0 = Release|Any CPU
+		{1E427E08-DC0D-4B92-9B2F-2B880C2B1809}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{1E427E08-DC0D-4B92-9B2F-2B880C2B1809}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{1E427E08-DC0D-4B92-9B2F-2B880C2B1809}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{1E427E08-DC0D-4B92-9B2F-2B880C2B1809}.Release|Any CPU.Build.0 = Release|Any CPU
 	EndGlobalSection
 	GlobalSection(SolutionProperties) = preSolution
 		HideSolutionNode = FALSE
 	EndGlobalSection
+	GlobalSection(ExtensibilityGlobals) = postSolution
+		SolutionGuid = {02958319-B4BB-49EE-92B0-3B8A23C3A4EE}
+	EndGlobalSection
 EndGlobal

jobs/Backend/Task/Infrastructure/CNB/CNBExchangeRateFetcher.cs

@@ -0,0 +1,44 @@
+using System.Collections.Generic;
+using System.Net.Http;
+using System.Threading;
+using System.Threading.Tasks;
+using ExchangeRateUpdater.Domain;
+using Microsoft.Extensions.Logging;
+using Polly.CircuitBreaker;
+
+namespace ExchangeRateUpdater.Infrastructure.CNB
+{
+    internal sealed class CNBExchangeRateFetcher(
+        HttpClient httpClient, 
+        IExchangeRateParser parser,
+        ILogger<CNBExchangeRateFetcher> logger) : IExchangeRateFetcher
+    {
+        private const string RatesSegment = "daily.txt";
+
+        private readonly HttpClient httpClient = httpClient;
+        private readonly IExchangeRateParser parser = parser;
+
+        public async Task<IEnumerable<ExchangeRate>> GetExchangeRates(CancellationToken cancellationToken = default)
+        {
+            logger.LogTrace("Getting exchange rates from CNB");
+
+            try
+            {
+                var response = await httpClient.GetAsync(RatesSegment, cancellationToken);
+                response.EnsureSuccessStatusCode();
+
+                var content = await response.Content.ReadAsStringAsync(cancellationToken);
+
+                return parser.Parse(content).Records;
+            }
+            catch (BrokenCircuitException)
+            {
+                // We would want to retrieve the info from other source or use "old" data. 
+                // Returning empty collection for simplicy here.
+                logger.LogWarning("Circuit open when fetching data. Using alternative path");
+
+                return [];
+            }
+        }
+    }
+}

jobs/Backend/Task/Infrastructure/CNB/CNBExchangeRateParser.cs

@@ -0,0 +1,77 @@
+using System;
+using System.Collections.Generic;
+using System.Globalization;
+using System.IO;
+using System.Linq;
+using CsvHelper;
+using CsvHelper.Configuration;
+using ExchangeRateUpdater.Domain;
+using Microsoft.Extensions.Logging;
+
+namespace ExchangeRateUpdater.Infrastructure.CNB
+{
+    internal interface IExchangeRateParser
+    {
+        (ExchangeRateMetadata Metadata, IEnumerable<ExchangeRate> Records) Parse(string exchangeRate);
+    }
+
+    internal record ExchangeRateMetadata(DateTime DateTime, int Identifier);
+
+    internal sealed class CNBExchangeRateParser(ILogger<CNBExchangeRateParser> logger) : IExchangeRateParser
+    {
+        private const string CheckCurrencyCode = "CZK";
+        private readonly CsvConfiguration config = new CsvConfiguration(CultureInfo.InvariantCulture)
+        {
+            PrepareHeaderForMatch = args => args.Header.Trim().ToLower(),
+            Delimiter = "|",            
+        };
+        private readonly ILogger<CNBExchangeRateParser> logger = logger;
+
+        public (ExchangeRateMetadata Metadata, IEnumerable<ExchangeRate> Records) Parse(string exchangeRate)
+        {
+            logger.LogTrace("Parsing exchange rate data");
+
+            using (var reader = new StringReader(exchangeRate))
+            using (var csvReader = new CsvReader(reader, config))
+            {                
+                var dateInfo = reader.ReadLine();
+                var metadata = ParseMetadata(dateInfo);
+
+                logger.LogTrace("Parsed metadata: {Date} {Identifier}", metadata.DateTime, metadata.Identifier);
+
+                var records = csvReader.GetRecords<ExchangeRateRecord>();
+                return (metadata, records.Select(records => new ExchangeRate(
+                    new Currency(CheckCurrencyCode),
+                    new Currency(records.Code),
+                    records.Rate / records.Amount))
+                    .ToArray());
+            }
+        }
+
+        private static ExchangeRateMetadata ParseMetadata(string dateInfo)
+        {
+            // Example: dateInfo = "16 May 2025 #93"
+            DateTime? parsedDate = null;
+            int? identifier = null;
+
+            if (!string.IsNullOrWhiteSpace(dateInfo))
+            {
+                var parts = dateInfo.Split('#');
+                var datePart = parts[0].Trim();
+
+                if (DateTime.TryParseExact(datePart, "d MMM yyyy", CultureInfo.InvariantCulture, DateTimeStyles.None, out var dt))
+                    parsedDate = dt;
+
+                if (parts.Length > 1 && int.TryParse(parts[1].Trim(), out var id))
+                    identifier = id;
+
+                return new ExchangeRateMetadata(parsedDate ?? DateTime.MinValue, identifier ?? 0);
+            }
+
+            return new ExchangeRateMetadata(DateTime.MinValue, 0);
+
+        }
+
+        private record class ExchangeRateRecord(string Country, string Currency, decimal Amount, string Code, decimal Rate);
+    }
+}

jobs/Backend/Task/Infrastructure/CNB/CNBOptions.cs

@@ -0,0 +1,11 @@
+using System;
+
+namespace ExchangeRateUpdater.Infrastructure.CNB
+{
+    internal record CNBOptions
+    {
+        public string BaseUrl { get; set; }
+
+        public TimeOnly NewDataSchedule { get; set; }
+    }
+}

jobs/Backend/Task/Infrastructure/CNB/CachedExchangeRateFetcher.cs

@@ -0,0 +1,46 @@
+using System;
+using System.Collections.Generic;
+using System.Text.Json;
+using System.Threading;
+using System.Threading.Tasks;
+using ExchangeRateUpdater.Domain;
+using Microsoft.Extensions.Caching.Distributed;
+
+namespace ExchangeRateUpdater.Infrastructure.CNB
+{
+    // We could use a basic dictionary for caching, but we will use a distributed cache
+    // to allow for better scalability and to avoid issues with multiple instances of the application.
+    // We assumed 5 minutes of sliding expiration, but this could be changed to a more suitable value.
+    internal sealed class CachedExchangeRateFetcher(
+        IDistributedCache cache, 
+        IExchangeRateFetcher underlying) : IExchangeRateFetcher
+    {
+        private readonly IDistributedCache cache = cache;
+        private readonly IExchangeRateFetcher underlying = underlying;
+
+        public async Task<IEnumerable<ExchangeRate>> GetExchangeRates(CancellationToken cancellationToken = default)
+        {
+            const string cacheKey = "exchange-rates";
+            var cached = await cache.GetStringAsync(cacheKey, cancellationToken);
+
+            if (!string.IsNullOrEmpty(cached))
+            {
+                return JsonSerializer.Deserialize<IEnumerable<ExchangeRate>>(cached)!;
+            }
+
+            var rates = await underlying.GetExchangeRates(cancellationToken);
+            var serialized = JsonSerializer.Serialize(rates);
+
+            await cache.SetStringAsync(
+                cacheKey, 
+                serialized, 
+                new DistributedCacheEntryOptions
+                {                    
+                    SlidingExpiration = TimeSpan.FromMinutes(5),
+                }, 
+                cancellationToken);
+
+            return rates;
+        }
+    }
+}