Structured Logging System Design

Core Principles

• Logs are data — Treat every log as a queryable, structured data point
• Single source of truth — One logging configuration, used everywhere
• Context is king — Every log must be traceable to its source and request
• Human + Machine readable — Structured for parsing, clear for debugging
• Progressive enhancement — Start simple, add fields as needed
• No secrets — Never log passwords, tokens, or PII without masking

System Architecture

The Golden Rule

Configure once, use everywhere. Never instantiate loggers directly in business code.

Architecture Overview

┌─────────────────────────────────────────────────────────────────┐
│                     APPLICATION LAYER                           │
│  ┌──────────┐  ┌──────────┐  ┌──────────┐  ┌──────────┐        │
│  │ Service A│  │ Service B│  │ Service C│  │ Handler D│        │
│  └────┬─────┘  └────┬─────┘  └────┬─────┘  └────┬─────┘        │
│       │             │             │             │               │
│       └─────────────┴──────┬──────┴─────────────┘               │
│                            ▼                                    │
│  ┌─────────────────────────────────────────────────────────┐   │
│  │              LOGGING INFRASTRUCTURE                      │   │
│  │  ┌─────────────┐  ┌─────────────┐  ┌─────────────────┐  │   │
│  │  │ Logger      │  │ Context     │  │ Formatters &    │  │   │
│  │  │ Factory     │  │ Provider    │  │ Transformers    │  │   │
│  │  └─────────────┘  └─────────────┘  └─────────────────┘  │   │
│  └─────────────────────────────────────────────────────────┘   │
│                            │                                    │
└────────────────────────────┼────────────────────────────────────┘
                             ▼
┌─────────────────────────────────────────────────────────────────┐
│                      OUTPUT TARGETS                             │
│  ┌──────────┐  ┌──────────┐  ┌──────────┐  ┌──────────┐        │
│  │ Console  │  │   File   │  │ Log Agg. │  │  Metrics │        │
│  │ (Dev)    │  │ (Local)  │  │ (Prod)   │  │ (Alerts) │        │
│  └──────────┘  └──────────┘  └──────────┘  └──────────┘        │
└─────────────────────────────────────────────────────────────────┘

Project Structure

src/
├── lib/
│   └── logging/                    # Centralized logging infrastructure
│       ├── index.ts                # Public API exports
│       ├── logger.ts               # Core logger implementation
│       ├── context.ts              # Request context management
│       ├── formatters.ts           # Output formatters (JSON, pretty)
│       ├── transports.ts           # Output destinations
│       ├── middleware.ts           # Framework integrations
│       ├── decorators.ts           # Method decorators (optional)
│       └── types.ts                # Type definitions
├── modules/
│   ├── users/
│   │   └── user.service.ts         # Uses: import { logger } from '@/lib/logging'
│   └── orders/
│       └── order.service.ts        # Uses: import { logger } from '@/lib/logging'
└── main.ts                         # Initialize logging once at startup

Anti-Patterns to Avoid

// ❌ BAD: Creating loggers everywhere
class UserService {
  private logger = new Logger({ service: 'user' });  // Don't do this
}

class OrderService {
  private logger = new Logger({ service: 'order' }); // Duplicated config
}

// ❌ BAD: Inconsistent configuration
const logger1 = winston.createLogger({ format: json() });
const logger2 = winston.createLogger({ format: simple() }); // Different format!

// ❌ BAD: Direct console usage in production code
console.log('User created:', userId);  // No structure, no context

// ❌ BAD: Passing logger instances around
function processOrder(order: Order, logger: Logger) { } // Don't pass loggers

Correct Patterns

// ✅ GOOD: Single configuration, exported singleton
// lib/logging/index.ts
export const logger = createLogger(config);
export { getContextLogger } from './context';

// ✅ GOOD: Import from central location
// modules/users/user.service.ts
import { logger, getContextLogger } from '@/lib/logging';

class UserService {
  async createUser(data: CreateUserInput) {
    const log = getContextLogger();  // Gets context automatically
    log.info('Creating user', { email: data.email });
    // ...
  }
}

// ✅ GOOD: Configure once at application startup
// main.ts
import { initializeLogging } from '@/lib/logging';

initializeLogging({
  service: 'my-app',
  environment: process.env.NODE_ENV,
  level: process.env.LOG_LEVEL || 'info',
});

Log Record Schema

Tier 1: Essential Fields (Always Required)

{
  "timestamp": "2025-12-16T10:30:00.123Z",
  "level": "INFO",
  "message": "Payment processed successfully",
  "service": "payment-service"
}

Tier 2: Tracing Fields (Distributed Systems)

{
  "timestamp": "2025-12-16T10:30:00.123Z",
  "level": "INFO",
  "message": "Order created",
  "service": "order-service",
  "trace_id": "7b2e4f1a9c3d8e5b6a1f2c3d4e5f6a7b",
  "span_id": "1a2b3c4d5e6f7890",
  "request_id": "req_abc123"
}

Tier 3: Context Fields (Debugging & Analysis)

Tier 4: Environment Fields (Operations)

Log Levels

When to Use Each Level

Level Selection Guide

DEBUG: Variable states, method entry/exit, detailed flow
       → "Processing item 5 of 10"
       → "Cache miss for key: user:123"

INFO:  Business milestones, successful operations
       → "User registered successfully"
       → "Order #123 shipped"
       → "Scheduled job completed: 50 records processed"

WARN:  Degraded service, retries, approaching limits
       → "Database connection slow (>2s), retrying"
       → "Rate limit at 80%, throttling soon"
       → "Deprecated API called, migrate to v2"

ERROR: Operation failures, exceptions, data issues
       → "Payment failed: card declined"
       → "Database query timeout after 3 retries"
       → "Invalid data format in message queue"

FATAL: Unrecoverable state, shutdown imminent
       → "Database connection lost, shutting down"
       → "Out of memory, cannot allocate"
       → "Critical configuration missing"

Implementation Patterns

Pattern 1: Basic Logger Setup

// logger.ts
interface LogEntry {
  timestamp: string;
  level: string;
  message: string;
  service: string;
  trace_id?: string;
  span_id?: string;
  [key: string]: unknown;
}

class Logger {
  constructor(
    private service: string,
    private context: Record<string, unknown> = {}
  ) {}

  private log(level: string, message: string, data?: Record<string, unknown>) {
    const entry: LogEntry = {
      timestamp: new Date().toISOString(),
      level,
      message,
      service: this.service,
      ...this.context,
      ...data
    };
    console.log(JSON.stringify(entry));
  }

  debug(message: string, data?: Record<string, unknown>) {
    this.log('DEBUG', message, data);
  }

  info(message: string, data?: Record<string, unknown>) {
    this.log('INFO', message, data);
  }

  warn(message: string, data?: Record<string, unknown>) {
    this.log('WARN', message, data);
  }

  error(message: string, data?: Record<string, unknown>) {
    this.log('ERROR', message, data);
  }

  child(context: Record<string, unknown>): Logger {
    return new Logger(this.service, { ...this.context, ...context });
  }
}

export const logger = new Logger('my-service');

Pattern 2: Request Context Propagation

// middleware/logging.ts
import { AsyncLocalStorage } from 'async_hooks';
import { v4 as uuid } from 'uuid';

interface RequestContext {
  trace_id: string;
  span_id: string;
  request_id: string;
  user_id?: string;
}

export const requestContext = new AsyncLocalStorage<RequestContext>();

// Express middleware
export function loggingMiddleware(req, res, next) {
  const context: RequestContext = {
    trace_id: req.headers['x-trace-id'] || uuid().replace(/-/g, ''),
    span_id: uuid().substring(0, 16),
    request_id: req.headers['x-request-id'] || `req_${uuid().substring(0, 8)}`,
    user_id: req.user?.id
  };

  // Propagate to downstream services
  res.setHeader('x-trace-id', context.trace_id);
  res.setHeader('x-request-id', context.request_id);

  requestContext.run(context, () => next());
}

// Context-aware logger
export function getLogger() {
  const ctx = requestContext.getStore();
  return logger.child(ctx || {});
}

Pattern 3: Operation Logging with Duration

// utils/operation-logger.ts
export async function logOperation<T>(
  name: string,
  operation: () => Promise<T>,
  metadata?: Record<string, unknown>
): Promise<T> {
  const log = getLogger();
  const startTime = performance.now();

  log.info(`${name} started`, { operation: name, ...metadata });

  try {
    const result = await operation();
    const duration_ms = Math.round(performance.now() - startTime);

    log.info(`${name} completed`, {
      operation: name,
      duration_ms,
      status: 'success',
      ...metadata
    });

    return result;
  } catch (error) {
    const duration_ms = Math.round(performance.now() - startTime);

    log.error(`${name} failed`, {
      operation: name,
      duration_ms,
      status: 'failure',
      error_code: error.code || 'UNKNOWN',
      error_message: error.message,
      stack_trace: error.stack,
      ...metadata
    });

    throw error;
  }
}

// Usage
await logOperation('create_order', async () => {
  return orderService.create(orderData);
}, { order_id: orderData.id, user_id: userId });

Pattern 4: Structured Error Logging

// errors/logging.ts
interface ErrorContext {
  error_code: string;
  error_message: string;
  error_type: string;
  stack_trace?: string;
  original_error?: unknown;
}

export function logError(error: unknown, context?: Record<string, unknown>) {
  const log = getLogger();
  const errorContext: ErrorContext = normalizeError(error);

  log.error(errorContext.error_message, {
    ...errorContext,
    ...context
  });
}

function normalizeError(error: unknown): ErrorContext {
  if (error instanceof AppError) {
    return {
      error_code: error.code,
      error_message: error.message,
      error_type: error.constructor.name,
      stack_trace: error.stack
    };
  }

  if (error instanceof Error) {
    return {
      error_code: 'INTERNAL_ERROR',
      error_message: error.message,
      error_type: error.constructor.name,
      stack_trace: error.stack
    };
  }

  return {
    error_code: 'UNKNOWN_ERROR',
    error_message: String(error),
    error_type: 'Unknown',
    original_error: error
  };
}

Best Practices

DO ✅

// Structured, queryable data
log.info('Order created', {
  order_id: 'ord_123',
  user_id: 'usr_456',
  total_amount: 99.99,
  currency: 'USD',
  items_count: 3
});

// Consistent field naming across services
// user_id (not userId, user-id, or uid)
// duration_ms (not duration, time_ms, or elapsed)

// Include correlation IDs in all logs
log.info('Processing payment', { trace_id, span_id, order_id });

// Use appropriate log levels
log.warn('Rate limit approaching', { current: 80, limit: 100 });
log.error('Payment failed', { error_code: 'CARD_DECLINED' });

// Log at boundaries (entry/exit points)
log.info('API request received', { method: 'POST', path: '/orders' });
log.info('API response sent', { status: 201, duration_ms: 145 });

DON'T ❌

// String concatenation (not queryable)
log.info(`User ${userId} created order ${orderId}`);

// Logging sensitive data
log.info('Login', { password: '...', credit_card: '...' });

// Inconsistent field names
log.info('Order', { orderId: '...' });  // another file uses order_id
log.info('Order', { order_id: '...' }); // inconsistent!

// Wrong log levels
log.error('Cache miss');  // Should be DEBUG
log.debug('Payment failed'); // Should be ERROR

// Missing context
log.error('Something went wrong'); // What? Where? Why?

// Logging entire objects (may contain secrets)
log.info('User data', { user }); // Might log password hash

Extended Reference

Detailed material starting at ## Sensitive Data Handling has been moved to reference/extended.md to keep this skill concise. Load that reference when the task requires the moved examples, command catalogs, checklists, platform details, or implementation templates.