Serilog - Structured Logging for Production

// dotnet add package Serilog.AspNetCore
// dotnet add package Serilog.Sinks.Seq          (local dev)
// dotnet add package Serilog.Sinks.Elasticsearch (production)
// dotnet add package Serilog.Enrichers.CorrelationId

// ━━ Program.cs - configure Serilog FIRST (before builder.Build()) ━━
Log.Logger = new LoggerConfiguration()
    .MinimumLevel.Override("Microsoft", LogEventLevel.Warning)    // suppress noisy framework logs
    .MinimumLevel.Override("System", LogEventLevel.Warning)
    .Enrich.FromLogContext()          // picks up BeginScope properties
    .Enrich.WithEnvironmentName()
    .Enrich.WithMachineName()
    .Enrich.WithProperty("Application", "MyApp")
    .Enrich.WithProperty("Version", "1.0.0")
    .WriteTo.Console(new Serilog.Formatting.Compact.CompactJsonFormatter()) // JSON in prod
    .WriteTo.Seq("http://localhost:5341")              // query logs in Seq UI
    .WriteTo.Elasticsearch(new ElasticsearchSinkOptions(new Uri("http://elastic:9200"))
    {
        IndexFormat = "myapp-logs-{0:yyyy-MM}",
        AutoRegisterTemplate = true,
        OverwriteTemplate = true,
    })
    .CreateBootstrapLogger(); // early logger (before host is built)

builder.Host.UseSerilog((ctx, services, config) =>
    config.ReadFrom.Configuration(ctx.Configuration)   // appsettings.json overrides
          .ReadFrom.Services(services)                 // IEnricher from DI
          .Enrich.FromLogContext());

// ━━ Request logging middleware ━━
app.UseSerilogRequestLogging(opts =>
{
    opts.MessageTemplate = "{RequestMethod} {RequestPath} → {StatusCode} in {Elapsed:0.0}ms";
    opts.EnrichDiagnosticContext = (diagnosticContext, httpContext) =>
    {
        diagnosticContext.Set("RequestHost", httpContext.Request.Host.Value);
        diagnosticContext.Set("UserAgent", httpContext.Request.Headers.UserAgent.ToString());
        diagnosticContext.Set("UserId", httpContext.User.FindFirst("sub")?.Value ?? "anonymous");
        diagnosticContext.Set("CorrelationId", httpContext.Items["CorrelationId"]);
    };
});

// ━━ Structured logging in services ━━
public class ProductService(ILogger<ProductService> logger)
{
    public async Task<Product?> GetByIdAsync(Guid id)
    {
        // GOOD: structured - {ProductId} is a searchable property, not just text
        logger.LogInformation("Fetching product {ProductId} for user {UserId}", id, GetUserId());

        var product = await FetchProduct(id);

        if (product is null)
            logger.LogWarning("Product {ProductId} not found - returning 404", id);
        else
            logger.LogDebug("Product {ProductId} loaded from DB in {Category}", id, product.Category);

        return product;
    }

    // Log scope - ALL logs in this scope automatically inherit these properties
    public async Task ProcessOrderAsync(Guid orderId)
    {
        using (logger.BeginScope(new Dictionary<string, object>
               { ["OrderId"] = orderId, ["Operation"] = "ProcessOrder" }))
        {
            logger.LogInformation("Order processing started");    // has OrderId, Operation
            // ... processing ...
            logger.LogInformation("Order processing complete");   // has OrderId, Operation
        }
    }

    private static Guid GetUserId() => Guid.NewGuid();
    private static Task<Product?> FetchProduct(Guid id) => Task.FromResult<Product?>(null);
}

// ━━ appsettings.json - override log levels per environment ━━
// {
//   "Serilog": {
//     "MinimumLevel": {
//       "Default": "Information",
//       "Override": {
//         "Microsoft.EntityFrameworkCore.Database.Command": "Warning",
//         "Microsoft.AspNetCore.Routing": "Warning"
//       }
//     }
//   }
// }

class Product { public string Category { get; set; } = ""; }