logging

---

name: logging description: Comprehensive logging standards for applications including structured logging, log levels, message formatting, and observability guidelines version: 1.0.0 author: {{PROJECT_NAME}} Team created: 2026-01-26 updated: 2026-01-26 tags:

• logging
• structured-logging
• observability
• debugging
• monitoring applies_to:
• "**/*.{{FILE_EXTENSION}}"
• "services/**/*"
• "api/**/*"
• "packages/**/*"
• "apps/**/*"

---

Logging Standards

Comprehensive logging guidelines for consistent, structured, and maintainable logging across the monorepo.

Core Principles

1. Structured Logging

All logs MUST be structured (JSON-serializable) for efficient querying and analysis.

2. Contextual Information

Include relevant context (request ID, user ID, session ID) in every log.

3. Appropriate Levels

Use correct log levels to enable effective filtering.

4. Performance Aware

Avoid expensive operations in log statements.

Log Levels

Level	When to Use	Examples
ERROR	Application errors requiring attention	Unhandled exceptions, failed critical operations
WARN	Unexpected but recoverable situations	Retry attempts, deprecated usage, rate limiting
INFO	Significant application events	Request handling, job completion, config changes
DEBUG	Detailed diagnostic information	Variable values, execution paths, timing
TRACE	Very detailed debugging	Loop iterations, method entry/exit

Message Format Pattern

REQUIRED: All log messages MUST follow this pattern:

[SERVICE_OR_CLASS]: MESSAGE

The prefix should be the actual class name or service name enclosed in square brackets.

Language-Specific Examples

TypeScript/JavaScript

import { logger } from "@{{PROJECT_NAME}}/logging";

// ✅ GOOD - Structured with context
logger.info("[UserService]: User created", {
	userId: user.id,
	email: user.email,
	source: "registration",
});

// ✅ GOOD - Error with stack trace
logger.error("[PaymentService]: Payment processing failed", {
	error: err.message,
	stack: err.stack,
	orderId: order.id,
	amount: order.total,
});

// ❌ BAD - String interpolation loses structure
logger.info(`User ${userId} created with email ${email}`);

// ❌ BAD - Missing context
logger.error("Payment failed");

Python

import structlog

logger = structlog.get_logger()

# ✅ GOOD - Structured with context
logger.info("[UserService]: User created",
    user_id=user.id,
    email=user.email,
    source="registration"
)

# ✅ GOOD - Error with exception
logger.error("[PaymentService]: Payment processing failed",
    error=str(e),
    order_id=order.id,
    amount=order.total,
    exc_info=True
)

# ❌ BAD - f-string loses structure
logger.info(f"User {user_id} created with email {email}")

C# / .NET

// ✅ GOOD - Structured logging with named parameters
_logger.LogInformation("[UserService]: User created - UserId: {UserId}, Email: {Email}",
    user.Id, user.Email);

// ✅ GOOD - Error with exception
_logger.LogError(ex, "[PaymentService]: Payment failed - OrderId: {OrderId}, Amount: {Amount}",
    order.Id, order.Total);

// ❌ BAD - String interpolation
_logger.LogInformation($"User {userId} created with email {email}");

Go

// ✅ GOOD - Structured with context
logger.Info("[UserService]: User created",
    zap.String("userId", user.ID),
    zap.String("email", user.Email),
    zap.String("source", "registration"),
)

// ✅ GOOD - Error with stack
logger.Error("[PaymentService]: Payment failed",
    zap.Error(err),
    zap.String("orderId", order.ID),
    zap.Float64("amount", order.Total),
)

// ❌ BAD - Printf style
logger.Info(fmt.Sprintf("User %s created", userId))

Contextual Logging

Request Context

Always include request-scoped identifiers:

// Middleware sets context
const requestContext = {
	requestId: req.headers["x-request-id"] || uuid(),
	userId: req.user?.id,
	sessionId: req.session?.id,
	traceId: req.headers["x-trace-id"],
};

// All logs in request include context
logger.info("[OrderController]: Order submitted", {
	...requestContext,
	orderId: order.id,
	itemCount: order.items.length,
});

Correlation IDs

For async operations, propagate correlation:

async function processOrder(order: Order, correlationId: string) {
	logger.info("[OrderProcessor]: Starting order processing", {
		correlationId,
		orderId: order.id,
	});

	// Pass to downstream services
	await inventoryService.reserve(order.items, { correlationId });
	await paymentService.charge(order.total, { correlationId });

	logger.info("[OrderProcessor]: Order processing complete", {
		correlationId,
		orderId: order.id,
		duration: Date.now() - startTime,
	});
}

Performance Logging

Timing Operations

const startTime = performance.now();

await expensiveOperation();

const duration = performance.now() - startTime;
logger.info("[DataProcessor]: Operation completed", {
	operationType: "batch-import",
	recordCount: records.length,
	durationMs: Math.round(duration),
	recordsPerSecond: Math.round(records.length / (duration / 1000)),
});

Conditional Debug Logging

// Avoid expensive serialization unless needed
if (logger.isDebugEnabled()) {
	logger.debug("[CacheService]: Cache contents", {
		keys: cache.keys(),
		size: cache.size,
		hitRate: cache.stats.hitRate,
	});
}

Error Logging Best Practices

Include Full Context

try {
	await processPayment(order);
} catch (error) {
	logger.error("[PaymentService]: Payment processing failed", {
		error: error.message,
		stack: error.stack,
		errorCode: error.code,
		// Business context
		orderId: order.id,
		userId: order.userId,
		amount: order.total,
		paymentMethod: order.paymentMethod,
		// Request context
		requestId: context.requestId,
		// Recovery info
		retryable: isRetryableError(error),
		attemptNumber: attempt,
	});
	throw error;
}

Categorize Errors

// Tag errors for monitoring/alerting
logger.error("[AuthService]: Authentication failed", {
	errorCategory: "SECURITY",
	errorType: "INVALID_CREDENTIALS",
	userId: attemptedUserId,
	ipAddress: request.ip,
	userAgent: request.headers["user-agent"],
});

Sensitive Data

Never Log

• Passwords or secrets
• Full credit card numbers
• Social security numbers
• API keys or tokens
• Personal health information

Masking Patterns

const maskEmail = (email: string) => {
	const [local, domain] = email.split("@");
	return `${local.slice(0, 2)}***@${domain}`;
};

const maskCardNumber = (card: string) => {
	return `****-****-****-${card.slice(-4)}`;
};

logger.info("[PaymentService]: Payment authorized", {
	email: maskEmail(user.email), // "jo***@example.com"
	cardNumber: maskCardNumber(card), // "****-****-****-1234"
	amount: order.total,
});

Monorepo Logger Package

Create a shared logging package for consistency:

packages/
└── logging/
    ├── src/
    │   └── index.ts
    ├── package.json
    └── README.md

// packages/logging/src/index.ts
import pino from "pino";

export const createLogger = (service: string) => {
	return pino({
		name: service,
		level: process.env.LOG_LEVEL || "info",
		formatters: {
			level: (label) => ({ level: label }),
		},
		base: {
			service,
			environment: process.env.NODE_ENV,
			version: process.env.APP_VERSION,
		},
	});
};

// Usage in apps
// apps/{{APP_NAME_1}}/src/index.ts
import { createLogger } from "@{{PROJECT_NAME}}/logging";
const logger = createLogger("{{APP_NAME_1}}");

Environment Configuration

# .env
LOG_LEVEL=info              # Minimum log level
LOG_FORMAT=json             # json | pretty
LOG_DESTINATION=stdout      # stdout | file | service
LOG_SERVICE_URL=            # External logging service URL

Observability Integration

OpenTelemetry Context

import { trace, context } from "@opentelemetry/api";

function logWithTrace(level: string, message: string, data: object) {
	const span = trace.getSpan(context.active());
	const traceContext = span
		? {
				traceId: span.spanContext().traceId,
				spanId: span.spanContext().spanId,
			}
		: {};

	logger[level](message, { ...data, ...traceContext });
}

Quick Reference

Log Level Decision Tree

Is it an error that needs attention? → ERROR
Is it unexpected but handled? → WARN
Is it a significant business event? → INFO
Is it helpful for debugging? → DEBUG
Is it very detailed/verbose? → TRACE

Checklist Before Logging

• Using structured format (not string interpolation)
• Service/class prefix in brackets
• Relevant context included
• Sensitive data masked
• Appropriate log level
• No expensive operations in log statement

Related Skills

• See .agents/skills/systematic-debugging/ for debugging workflows
• See .agents/skills/verification-before-completion/ for testing logs