Logging & Observability

Core Principles

1. Structured logging with Serilog — Every log entry is a structured event with named properties, not a formatted string. This enables searching, filtering, and alerting.
2. OpenTelemetry for distributed tracing — Traces connect requests across services. Metrics track system health over time.
3. Health checks for operational readiness — Every service exposes /health endpoints for load balancers and orchestrators.
4. Correlation IDs for request tracing — Every request gets a unique ID that flows through all log entries and downstream service calls.

Patterns

Serilog Setup

// Program.cs
builder.Host.UseSerilog((context, loggerConfig) =>
{
    loggerConfig
        .ReadFrom.Configuration(context.Configuration)
        .Enrich.FromLogContext()
        .Enrich.WithMachineName()
        .Enrich.WithProperty("Application", "MyApp.Api")
        .WriteTo.Console(outputTemplate:
            "[{Timestamp:HH:mm:ss} {Level:u3}] {Message:lj} {Properties:j}{NewLine}{Exception}")
        .WriteTo.Seq(context.Configuration["Seq:Url"] ?? "http://localhost:5341");
});

// After building the app
app.UseSerilogRequestLogging(options =>
{
    options.EnrichDiagnosticContext = (diagnosticContext, httpContext) =>
    {
        diagnosticContext.Set("UserId",
            httpContext.User.FindFirstValue(ClaimTypes.NameIdentifier) ?? "anonymous");
    };
});

Structured Logging (Correct Usage)

// GOOD — structured logging with message template
logger.LogInformation("Processing order {OrderId} for customer {CustomerId}",
    orderId, customerId);

// GOOD — include relevant context
logger.LogWarning("Payment failed for order {OrderId}. Attempt {Attempt} of {MaxAttempts}",
    orderId, attempt, maxAttempts);

// GOOD — log exceptions with structured data
logger.LogError(exception, "Failed to process order {OrderId}", orderId);

Correlation IDs

// Middleware to set correlation ID
public class CorrelationIdMiddleware(RequestDelegate next)
{
    private const string CorrelationIdHeader = "X-Correlation-Id";

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

        context.Items["CorrelationId"] = correlationId;
        context.Response.Headers[CorrelationIdHeader] = correlationId;

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

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

OpenTelemetry Integration

For full OpenTelemetry setup (metrics, tracing, OTLP export), see the opentelemetry skill.
The logging skill focuses on structured logging with Serilog. OpenTelemetry handles the export pipeline.

Health Checks

// Program.cs
builder.Services.AddHealthChecks()
    .AddNpgSql(builder.Configuration.GetConnectionString("Default")!,
        name: "database", tags: ["ready"])
    .AddRedis(builder.Configuration.GetConnectionString("Redis")!,
        name: "redis", tags: ["ready"])
    .AddRabbitMQ(builder.Configuration.GetConnectionString("RabbitMq")!,
        name: "rabbitmq", tags: ["ready"]);

// Map endpoints
app.MapHealthChecks("/health/live", new HealthCheckOptions
{
    Predicate = _ => false // No dependency checks — just "am I running?"
});

app.MapHealthChecks("/health/ready", new HealthCheckOptions
{
    Predicate = check => check.Tags.Contains("ready")
});

Anti-patterns

Don't Use String Interpolation in Log Messages

// BAD — allocates string even if level is disabled, breaks structured logging
logger.LogInformation($"Order {orderId} created for {customerId}");

// GOOD — message template with named parameters
logger.LogInformation("Order {OrderId} created for {CustomerId}", orderId, customerId);

Don't Log Sensitive Data

// BAD — logging credentials
logger.LogInformation("User logged in: {Email} with password {Password}", email, password);

// GOOD — never log secrets, passwords, tokens, or PII
logger.LogInformation("User logged in: {Email}", email);

Don't Skip Health Check Tags

// BAD — all checks run for liveness AND readiness
app.MapHealthChecks("/health");

// GOOD — separate liveness (am I running?) from readiness (can I serve traffic?)
app.MapHealthChecks("/health/live", new() { Predicate = _ => false });
app.MapHealthChecks("/health/ready", new() { Predicate = c => c.Tags.Contains("ready") });

Decision Guide