logging-observability

あなたは、可観測性を専門とするシニア .NET アーキテクトです。Razor Pages アプリケーションでログ記録とモニタリングを実装する際は、これらのパターンに従って、本番環境に対応した可観測性、トラブルシューティング機能、モニタリングシステムとの連携を実現してください。対象は .NET 8 以降で、null 許容参照型を有効にします。

理由

効果的な可観測性は、本番アプリケーションにとって重要です。ログ記録が不十分だとデバッグが不可能になり、相関 ID がないとサービスをまたいでリクエストを追跡するのが困難になります。これらのパターンを使うと、トラブルシューティングに必要なコンテキストを含む、構造化された検索可能なログを記録できます。

基本原則

1. 構造化ログ：機械で解析できる構造化形式（JSON）を使用します
2. 相関 ID：エンドツーエンドのトレースに備えて、すべてのリクエストに一意の ID を付与します
3. コンテキストの付加：ログに関連するコンテキスト（ユーザー、エンドポイント、所要時間）を含めます
4. ログレベル：適切なレベル（Debug、Info、Warning、Error、Fatal）を使用します
5. 外部シンク：ログを一元管理システム（Seq、Datadog、CloudWatch）に送信します

パターン 1：Serilog の構成

NuGet パッケージ

<PackageReference Include="Serilog.AspNetCore" Version="8.0.*" />
<PackageReference Include="Serilog.Expressions" Version="4.0.*" />
<PackageReference Include="Serilog.Sinks.Seq" Version="7.0.*" /> 

ブートストラップロガー（Program.cs の開始時）

using Serilog;
using Serilog.Debugging;

// Enable Serilog self-logging for diagnostics
SelfLog.Enable(msg => Console.Error.WriteLine($"[SERILOG] {msg}"));

// Create bootstrap logger for startup errors
Log.Logger = new LoggerConfiguration()
    .MinimumLevel.Override("Microsoft", LogEventLevel.Information)
    .Enrich.FromLogContext()
    .WriteTo.Console(formatProvider: CultureInfo.CurrentCulture)
    .CreateBootstrapLogger();

try
{
    Log.Information("Starting web application...");
    var builder = WebApplication.CreateBuilder(args);
    
    // ... configure services ...
    
    var app = builder.Build();
    await app.RunAsync();
}
catch (Exception ex)
{
    Log.Fatal(ex, "Application terminated unexpectedly");
}
finally
{
    await Log.CloseAndFlushAsync();
}

サービスの登録

public static class LoggingServiceRegistration
{
    public static IServiceCollection ConfigureSerilog(
        this IServiceCollection services, 
        IConfiguration configuration)
    {
        services.AddSerilog((services, lc) => lc
            .ReadFrom.Configuration(configuration)
            .Enrich.FromLogContext()
            .Enrich.WithMachineName()
            .Enrich.WithEnvironmentName()
            .WriteTo.Console(formatter: new ExpressionTemplate(
                "[{@t:hh:mm:ss.fff tt} {@l:u3}] {SourceContext} - {CorrelationId} - {@m}\n{@x}",
                theme: TemplateTheme.Code))
            .WriteTo.Seq(
                configuration["Seq:ServerUrl"] ?? "http://localhost:5341",
                apiKey: configuration["Seq:ApiKey"],
                formatProvider: CultureInfo.CurrentCulture)
        );

        return services;
    }
}

appsettings.json の構成

{
  "Serilog": {
    "Using": ["Serilog.Sinks.Console", "Serilog.Sinks.Seq"],
    "MinimumLevel": {
      "Default": "Information",
      "Override": {
        "Microsoft": "Warning",
        "Microsoft.AspNetCore": "Warning",
        "System": "Warning"
      }
    },
    "WriteTo": [
      {
        "Name": "Console",
        "Args": {
          "outputTemplate": "[{Timestamp:HH:mm:ss} {Level:u3}] {Message:lj}{NewLine}{Exception}"
        }
      }
    ],
    "Enrich": ["FromLogContext", "WithMachineName", "WithThreadId"],
    "Properties": {
      "Application": "MyApp"
    }
  }
}

パターン 2：相関 ID ミドルウェア

public class RequestContextLoggingMiddleware
{
    private readonly RequestDelegate _next;
    private const string CorrelationHeaderName = "X-Correlation-Id";

    public RequestContextLoggingMiddleware(RequestDelegate next)
    {
        _next = next;
    }

    public async Task Invoke(HttpContext httpContext)
    {
        var correlationId = GetCorrelationId(httpContext);
        
        // Add to response headers
        httpContext.Response.OnStarting(() =>
        {
            httpContext.Response.Headers[CorrelationHeaderName] = correlationId;
            return Task.CompletedTask;
        });

        using (LogContext.PushProperty("CorrelationId", correlationId))
        using (LogContext.PushProperty("RequestPath", httpContext.Request.Path))
        using (LogContext.PushProperty("RequestMethod", httpContext.Request.Method))
        {
            await _next.Invoke(httpContext);
        }
    }

    private static string GetCorrelationId(HttpContext httpContext)
    {
        httpContext.Request.Headers.TryGetValue(
            CorrelationHeaderName, out var correlationId);

        return correlationId.FirstOrDefault() ?? httpContext.TraceIdentifier;
    }
}

// Extension method for easy registration
public static class RequestContextLoggingExtensions
{
    public static IApplicationBuilder UseRequestContextLogging(
        this IApplicationBuilder app)
    {
        return app.UseMiddleware<RequestContextLoggingMiddleware>();
    }
}

登録

// Program.cs
var app = builder.Build();

// Place early in pipeline
app.UseRequestContextLogging();
app.UseSerilogRequestLogging(); // Logs each request

パターン 3：Serilog を使ったリクエストのログ記録

// Program.cs
app.UseSerilogRequestLogging(options =>
{
    options.EnrichDiagnosticContext = (diagnosticContext, httpContext) =>
    {
        diagnosticContext.Set("UserId", httpContext.User.Identity?.Name ?? "anonymous");
        diagnosticContext.Set("ClientIp", httpContext.Connection.RemoteIpAddress?.ToString());
        diagnosticContext.Set("UserAgent", httpContext.Request.Headers["User-Agent"].ToString());
    };
    
    options.GetLevel = (httpContext, elapsed, ex) =>
    {
        // Log 5xx as Error, slow requests as Warning
        if (ex != null || httpContext.Response.StatusCode > 499)
            return LogEventLevel.Error;
        
        if (elapsed > 1000)
            return LogEventLevel.Warning;
        
        return LogEventLevel.Information;
    };
});

カスタムリクエストログ（PageModel）

public class OrderDetailsModel(ILogger<OrderDetailsModel> logger) : PageModel
{
    public async Task OnGetAsync(Guid orderId)
    {
        // Push contextual properties
        using (logger.BeginScope(new Dictionary<string, object>
        {
            ["OrderId"] = orderId,
            ["UserId"] = User.Identity?.Name ?? "anonymous"
        }))
        {
            logger.LogInformation("Loading order details");
            
            try
            {
                var order = await _orderService.GetAsync(orderId);
                
                if (order == null)
                {
                    logger.LogWarning("Order not found");
                    return NotFound();
                }
                
                logger.LogInformation("Order loaded successfully");
            }
            catch (Exception ex)
            {
                logger.LogError(ex, "Failed to load order");
                throw;
            }
        }
    }
}

パターン 4：MediatR パイプラインのログ記録

public class RequestLoggingBehavior<TRequest, TResponse>(ILogger<RequestLoggingBehavior<TRequest, TResponse>> logger)
    : IPipelineBehavior<TRequest, TResponse>
    where TRequest : IRequest<TResponse>
{
    private static readonly string RequestName = typeof(TRequest).Name;
    private static readonly TimeSpan SlowRequestThreshold = TimeSpan.FromMilliseconds(500);

    public async Task<TResponse> Handle(
        TRequest request,
        RequestHandlerDelegate<TResponse> next,
        CancellationToken cancellationToken)
    {
        logger.LogInformation("Handling {RequestName}", RequestName);

        var stopwatch = Stopwatch.StartNew();

        try
        {
            var response = await next(cancellationToken);

            stopwatch.Stop();
            var elapsed = stopwatch.Elapsed;

            if (elapsed > SlowRequestThreshold)
            {
                logger.LogWarning(
                    "Handled {RequestName} successfully in {ElapsedMs}ms (exceeds {ThresholdMs}ms threshold)",
                    RequestName,
                    elapsed.TotalMilliseconds,
                    SlowRequestThreshold.TotalMilliseconds);
            }
            else
            {
                logger.LogInformation(
                    "Handled {RequestName} successfully in {ElapsedMs}ms",
                    RequestName,
                    elapsed.TotalMilliseconds);
            }

            return response;
        }
        catch (Exception ex)
        {
            stopwatch.Stop();

            logger.LogError(
                ex,
                "Error handling {RequestName} after {ElapsedMs}ms",
                RequestName,
                stopwatch.Elapsed.TotalMilliseconds);

            throw;
        }
    }
}

登録

builder.Services.AddMediatR(cfg =>
{
    cfg.RegisterServicesFromAssemblyContaining<Program>();
    cfg.AddBehavior(typeof(IPipelineBehavior<,>), typeof(RequestLoggingBehavior<,>));
});

パターン 5：ヘルスチェック

基本構成

// Program.cs
builder.Services.AddHealthChecks()
    .AddDbContextCheck<AppDbContext>(
        name: "database",
        tags: new[] { "db", "critical" })
    .AddCheck<EmailServiceHealthCheck>(
        name: "email-service",
        tags: new[] { "external", "email" })
    .AddRedis(
        name: "redis",
        tags: new[] { "cache", "distributed" });

var app = builder.Build();

// Simple health endpoint
app.MapHealthChecks("/up");

// Detailed health with authentication
app.MapHealthChecks("/health", new HealthCheckOptions
{
    ResponseWriter = HealthCheckResponseWriter.WriteResponse,
    AllowCachingResponses = false
}).RequireAuthorization("HealthCheckPolicy");

カスタムヘルスチェック

public class EmailServiceHealthCheck(IEmailServiceAgent emailService) : IHealthCheck
{
    public async Task<HealthCheckResult> CheckHealthAsync(
        HealthCheckContext context,
        CancellationToken cancellationToken = default)
    {
        try
        {
            var canConnect = await emailService.PingAsync(cancellationToken);
            
            if (canConnect)
            {
                return HealthCheckResult.Healthy("Email service is accessible");
            }
            
            return HealthCheckResult.Unhealthy("Cannot connect to email service");
        }
        catch (Exception ex)
        {
            return HealthCheckResult.Unhealthy(
                "Email service health check failed", ex);
        }
    }
}

カスタムレスポンスライター

public static class HealthCheckResponseWriter
{
    public static Task WriteResponse(
        HttpContext context, 
        HealthReport report)
    {
        context.Response.ContentType = "application/json";

        var response = new
        {
            Status = report.Status.ToString(),
            TotalDuration = report.TotalDuration.TotalMilliseconds,
            Checks = report.Entries.Select(e => new
            {
                Name = e.Key,
                Status = e.Value.Status.ToString(),
                Duration = e.Value.Duration.TotalMilliseconds,
                Exception = e.Value.Exception?.Message,
                Data = e.Value.Data
            })
        };

        return context.Response.WriteAsJsonAsync(response);
    }
}

パターン 6：LoggerMessage パターン（高パフォーマンス）

ホットパスでは、文字列補間のオーバーヘッドを避けるために、コンパイル時ログ記録を使用します。

public static partial class LoggerMessages
{
    // Information
    [LoggerMessage(
        EventId = 1001,
        Level = LogLevel.Information,
        Message = "User {UserId} signed up successfully")]
    public static partial void UserSignedUp(ILogger logger, string userId);

    // Warning
    [LoggerMessage(
        EventId = 2001,
        Level = LogLevel.Warning,
        Message = "Slow database query detected: {QueryName} took {ElapsedMs}ms")]
    public static partial void SlowQueryDetected(
        ILogger logger, string queryName, long elapsedMs);

    // Error
    [LoggerMessage(
        EventId = 3001,
        Level = LogLevel.Error,
        Message = "Failed to send email to {EmailAddress}")]
    public static partial void EmailSendFailed(
        ILogger logger, Exception ex, string emailAddress);

    // With scopes
    [LoggerMessage(
        EventId = 1002,
        Level = LogLevel.Information,
        Message = "Order {OrderId} processed")]
    public static partial void OrderProcessed(ILogger logger, Guid orderId);
}

// Usage
public class SignUpHandler(ILogger<SignUpHandler> logger)
{
    public async Task<SignUpResponse> Handle(SignUpRequest request)
    {
        // ... process signup ...
        
        LoggerMessages.UserSignedUp(logger, user.Id);
        
        return new SignUpResponse { Success = true };
    }
}

パターン 7：OpenTelemetry との連携

構成

// NuGet: OpenTelemetry.Extensions.Hosting, OpenTelemetry.Instrumentation.AspNetCore

builder.Services.AddOpenTelemetry()
    .WithTracing(tracing =>
    {
        tracing
            .AddAspNetCoreInstrumentation()
            .AddHttpClientInstrumentation()
            .AddEntityFrameworkCoreInstrumentation()
            .AddSource("MyApp") // Custom activity source
            .AddOtlpExporter(options =>
            {
                options.Endpoint = new Uri("http://localhost:4317");
            });
    })
    .WithMetrics(metrics =>
    {
        metrics
            .AddAspNetCoreInstrumentation()
            .AddRuntimeInstrumentation()
            .AddPrometheusExporter();
    });

var app = builder.Build();
app.UseOpenTelemetryPrometheusScrapingEndpoint();

カスタムアクティビティ

public class OrderProcessingService
{
    private static readonly ActivitySource ActivitySource = new("MyApp.Orders");

    public async Task ProcessOrderAsync(Order order)
    {
        using var activity = ActivitySource.StartActivity("ProcessOrder");
        activity?.SetTag("order.id", order.Id);
        activity?.SetTag("order.total", order.Total);

        try
        {
            activity?.AddEvent(new ActivityEvent("ValidatingOrder"));
            await ValidateOrderAsync(order);

            activity?.AddEvent(new ActivityEvent("ChargingPayment"));
            await ChargePaymentAsync(order);

            activity?.AddEvent(new ActivityEvent("SendingConfirmation"));
            await SendConfirmationAsync(order);

            activity?.SetStatus(ActivityStatusCode.Ok);
        }
        catch (Exception ex)
        {
            activity?.SetStatus(ActivityStatusCode.Error, ex.Message);
            throw;
        }
    }
}

パターン 8：Razor Pages のエラーログ記録

[ResponseCache(Duration = 0, Location = ResponseCacheLocation.None, NoStore = true)]
[IgnoreAntiforgeryToken]
public class ErrorModel(ILogger<ErrorModel> logger) : PageModel
{
    public string? RequestId { get; set; }
    public bool ShowRequestId => !string.IsNullOrEmpty(RequestId);
    public string? ErrorMessage { get; set; }

    public void OnGet()
    {
        RequestId = Activity.Current?.Id ?? HttpContext.TraceIdentifier;
        
        // Log the error that brought us here
        var exceptionHandlerPathFeature = HttpContext.Features.Get<IExceptionHandlerPathFeature>();
        
        if (exceptionHandlerPathFeature?.Error != null)
        {
            var ex = exceptionHandlerPathFeature.Error;
            var path = exceptionHandlerPathFeature.Path;
            
            logger.LogError(ex, 
                "Unhandled exception at {Path}. RequestId: {RequestId}", 
                path, RequestId);
            
            // Only show details in development
            if (HttpContext.RequestServices.GetRequiredService<IWebHostEnvironment>().IsDevelopment())
            {
                ErrorMessage = ex.ToString();
            }
        }
    }
}

アンチパターン

静的ロガーへのアクセス

// ❌ BAD: Static logger access
public class OrderService
{
    private static readonly Logger Logger = Log.ForContext<OrderService>();
}

// ✅ GOOD: Inject ILogger<T>
public class OrderService(ILogger<OrderService> logger)
{
}

ログ内での文字列補間

// ❌ BAD: String interpolation evaluated even if log level is disabled
_logger.LogInformation($"Processing order {orderId} for user {userId}");

// ✅ GOOD: Structured logging with parameters
_logger.LogInformation("Processing order {OrderId} for user {UserId}", orderId, userId);

// ✅ BETTER: Use LoggerMessage for hot paths
[LoggerMessage(EventId = 1001, Level = LogLevel.Information, Message = "Processing order {OrderId} for user {UserId}")]
public static partial void ProcessingOrder(ILogger logger, Guid orderId, string userId);

例外を捕捉して握りつぶす

// ❌ BAD: Silent failures
try
{
    await _service.DoWorkAsync();
}
catch (Exception ex)
{
    // Nothing logged!
}

// ✅ GOOD: Always log exceptions
catch (Exception ex)
{
    _logger.LogError(ex, "Failed to complete work");
    throw; // Re-throw if you can't handle it
}

参考資料

• Serilog：https://serilog.net/
• Serilog.AspNetCore：https://github.com/serilog/serilog-aspnetcore
• ヘルスチェック：https://learn.microsoft.com/en-us/aspnet/core/host-and-deploy/health-checks
• OpenTelemetry：https://opentelemetry.io/docs/instrumentation/net/
• LoggerMessage ソースジェネレーター：https://learn.microsoft.com/en-us/dotnet/core/extensions/logger-message-generator