Do you use Structured Logging with correlation IDs?

// Plain text - hard to query
_logger.LogInformation($"User {userId} placed order {orderId} for ${amount}");
// Output: "User 123 placed order 456 for $99.99"
// How do you find all orders over $50? All orders for user 123? Impossible to query!

// Structured - easily queryable
_logger.LogInformation(
    "User {UserId} placed order {OrderId} for {Amount:C}",
    userId, orderId, amount);
// Output: { "UserId": 123, "OrderId": 456, "Amount": 99.99, "Message": "User 123 placed order 456 for $99.99" }
// Now you can query: WHERE Amount > 50 AND UserId = 123

// Program.cs
var builder = WebApplication.CreateBuilder(args);

builder.Host.UseSerilog((context, configuration) =>
{
    configuration
        .ReadFrom.Configuration(context.Configuration)
        .Enrich.FromLogContext()
        .Enrich.WithMachineName()
        .Enrich.WithEnvironmentName()
        .WriteTo.Console()
        .WriteTo.Seq("http://localhost:5341"); // Or Application Insights
});

var app = builder.Build();

// Add request logging
app.UseSerilogRequestLogging();

// appsettings.json
{
  "Serilog": {
    "MinimumLevel": {
      "Default": "Information",
      "Override": {
        "Microsoft.AspNetCore": "Warning",
        "Microsoft.EntityFrameworkCore": "Warning"
      }
    }
  }
}

// Middleware to add correlation ID
public class CorrelationIdMiddleware
{
    private readonly RequestDelegate _next;
    private const string CorrelationIdHeader = "X-Correlation-ID";

    public CorrelationIdMiddleware(RequestDelegate next) => _next = next;

    public async Task InvokeAsync(HttpContext context)
    {
        var correlationId = context.Request.Headers[CorrelationIdHeader].FirstOrDefault()
            ?? Guid.NewGuid().ToString();

        context.Response.Headers[CorrelationIdHeader] = correlationId;

        using (LogContext.PushProperty("CorrelationId", correlationId))
        {
            await _next(context);
        }
    }
}

// Program.cs
app.UseMiddleware<CorrelationIdMiddleware>();

// When calling other APIs, pass the correlation ID
public class OrderService
{
    private readonly HttpClient _httpClient;
    private readonly IHttpContextAccessor _httpContextAccessor;

    public async Task<ShippingResult> CalculateShippingAsync(Order order)
    {
        var correlationId = _httpContextAccessor.HttpContext?
            .Response.Headers["X-Correlation-ID"].FirstOrDefault();

        var request = new HttpRequestMessage(HttpMethod.Post, "/api/shipping/calculate");
        request.Headers.Add("X-Correlation-ID", correlationId);

        var response = await _httpClient.SendAsync(request);
        // Shipping service logs will have the same CorrelationId
        return await response.Content.ReadFromJsonAsync<ShippingResult>();
    }
}

_logger.LogDebug("Cache lookup for key {CacheKey}", key);           // Development only
_logger.LogInformation("Order {OrderId} created", orderId);          // Normal operations
_logger.LogWarning("Retry attempt {Attempt} for {Service}", n, svc); // Potential issues
_logger.LogError(ex, "Failed to process order {OrderId}", orderId);  // Errors
_logger.LogCritical("Database connection lost");                     // System failures

// NEVER DO THIS
_logger.LogInformation("User login: {Email} with password {Password}", email, password);
_logger.LogInformation("Payment processed: {CreditCard}", cardNumber);

// Instead, mask or exclude sensitive data
_logger.LogInformation("User login: {Email}", email);
_logger.LogInformation("Payment processed for card ending in {LastFour}", card[^4..]);

public async Task ProcessOrderAsync(Order order)
{
    using (_logger.BeginScope(new Dictionary<string, object>
    {
        ["OrderId"] = order.Id,
        ["CustomerId"] = order.CustomerId,
        ["OrderTotal"] = order.Total
    }))
    {
        _logger.LogInformation("Starting order processing");
        await ValidateInventoryAsync(order);
        _logger.LogInformation("Inventory validated");
        await ProcessPaymentAsync(order);
        _logger.LogInformation("Payment processed");
        // All logs in this scope automatically include OrderId, CustomerId, OrderTotal
    }
}

-- Find all failed orders over $100
SELECT * FROM logs
WHERE OrderTotal > 100
AND Level = 'Error'
AND Message LIKE '%order%'

-- Find slow requests for a specific user
SELECT * FROM logs
WHERE UserId = 'user123'
AND Elapsed > 1000

-- Trace a request across services
SELECT * FROM logs
WHERE CorrelationId = 'abc-123-def'
ORDER BY Timestamp