# Logging Patterns

> Structured logging standards — JSON logs, trace IDs, log levels, PII redaction, boundary logging with pino/winston

- Skill: `nadav011/logging-patterns` (Agent Skill)
- Install (CLI): `npx skillmds@latest add nadav011/logging-patterns`
- Raw SKILL.md: https://api.skillmd.com/api/skills/nadav011/logging-patterns/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: Nadav011 (https://skillmd.com/u/nadav011)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/nadav011/logging-patterns

---

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# Logging Patterns

---

## Why Structured JSON Logs

`console.log` strings become unsearchable noise at scale. Structured logs are queryable, alertable, and machine-parseable.

```typescript
// ❌ WRONG — unstructured, unsearchable, no context
console.log('User logged in: ' + userId);
console.error('DB error: ' + err.message);

// ✅ CORRECT — structured, every field indexable
logger.info({ userId, action: 'login' }, 'User authenticated');
logger.error({ err, query, durationMs }, 'Database query failed');
```

A structured log entry looks like this on the wire:
```json
{
  "level": "info",
  "time": 1714000000000,
  "traceId": "01HX4ZK9R3BVNMJ2QPFW5T8CDE",
  "service": "api",
  "userId": "usr_abc123",
  "action": "login",
  "msg": "User authenticated"
}
```

Every field is independently filterable in any log aggregator (Loki, Datadog, CloudWatch, Splunk).

---

## Log Levels

Use the right level. Miscalibrated levels create alert fatigue (too much noise) or blind spots (too quiet).

| Level | When to use | Examples |
|-------|-------------|---------|
| `trace` | Very fine-grained detail, only for local debugging | Entering/leaving a function, raw SQL parameters |
| `debug` | Developer diagnostics, disabled in production | Parsed config values, cache hit/miss detail |
| `info` | Normal business events worth recording | User logged in, order placed, cron job started |
| `warn` | Unexpected but non-fatal condition | Retry attempt 2/3, deprecated API path called, slow query |
| `error` | Something failed that shouldn't have | Unhandled exception, DB write failed, third-party API error |
| `fatal` | System cannot continue | Missing required config, port already in use on startup |

**Rule:** Production log level should be `info` by default. Never ship with `debug` or `trace` enabled in production — they produce excessive volume and may log sensitive data.

---

## Setup: Pino (Recommended)

Pino is the fastest production logger for Node.js with minimal overhead.

```typescript
// logger.ts
import pino from 'pino';

export const logger = pino({
  level: process.env.LOG_LEVEL ?? 'info',
  // Redact PII fields wherever they appear in the log object
  redact: {
    paths: [
      'req.headers.authorization',
      'req.headers.cookie',
      'body.password',
      'body.token',
      'body.creditCard',
      '*.email',       // redact email in any nested object
      '*.ssn',
      '*.phone',
    ],
    censor: '[REDACTED]',
  },
  // In development: pretty-print. In production: JSON.
  transport: process.env.NODE_ENV === 'development'
    ? { target: 'pino-pretty', options: { colorize: true } }
    : undefined,
});
```

---

## Setup: Winston (Alternative)

```typescript
// logger.ts
import winston from 'winston';

export const logger = winston.createLogger({
  level: process.env.LOG_LEVEL ?? 'info',
  format: winston.format.combine(
    winston.format.timestamp(),
    winston.format.errors({ stack: true }),
    winston.format.json(),
  ),
  transports: [
    new winston.transports.Console(),
    // Add file/remote transports as needed
  ],
});
```

---

## Trace ID Propagation

Every log entry for a single request must share the same trace ID. This lets you reconstruct a full request trace from fragmented log lines.

```typescript
// trace-context.ts — using AsyncLocalStorage (Node 16+)
import { AsyncLocalStorage } from 'node:async_hooks';

interface TraceContext {
  traceId: string;
  spanId?: string;
}

const storage = new AsyncLocalStorage<TraceContext>();

export const getTraceContext = (): TraceContext | undefined => storage.getStore();

export const runWithTrace = <T>(ctx: TraceContext, fn: () => T): T =>
  storage.run(ctx, fn);
```

```typescript
// middleware/trace.ts — attach at request entry point
import { randomUUID } from 'node:crypto';
import { runWithTrace } from '../trace-context.js';

export function traceMiddleware(
  req: Request,
  res: Response,
  next: NextFunction,
): void {
  // Accept incoming trace ID from upstream (e.g. load balancer) or generate new
  const traceId =
    (req.headers['x-trace-id'] as string) ?? randomUUID();

  // Propagate to downstream services
  res.setHeader('x-trace-id', traceId);

  runWithTrace({ traceId }, next);
}
```

```typescript
// logger.ts — inject trace ID into every log entry automatically
import pino from 'pino';
import { getTraceContext } from './trace-context.js';

const baseLogger = pino({ level: process.env.LOG_LEVEL ?? 'info' });

export const logger = {
  trace: (obj: object, msg: string) =>
    baseLogger.trace({ ...getTraceContext(), ...obj }, msg),
  debug: (obj: object, msg: string) =>
    baseLogger.debug({ ...getTraceContext(), ...obj }, msg),
  info: (obj: object, msg: string) =>
    baseLogger.info({ ...getTraceContext(), ...obj }, msg),
  warn: (obj: object, msg: string) =>
    baseLogger.warn({ ...getTraceContext(), ...obj }, msg),
  error: (obj: object, msg: string) =>
    baseLogger.error({ ...getTraceContext(), ...obj }, msg),
  fatal: (obj: object, msg: string) =>
    baseLogger.fatal({ ...getTraceContext(), ...obj }, msg),
} as const;
```

Now every `logger.info(...)` call in any downstream function automatically includes `traceId` without threading it manually through every argument.

---

## Log at Boundaries

Log at the edges of your system, not in the middle of business logic.

### HTTP Request / Response

```typescript
// middleware/request-logger.ts
export function requestLoggerMiddleware(
  req: Request,
  res: Response,
  next: NextFunction,
): void {
  const start = Date.now();

  logger.info({
    method: req.method,
    url: req.url,
    userAgent: req.headers['user-agent'],
  }, 'HTTP request received');

  res.on('finish', () => {
    const durationMs = Date.now() - start;
    const level = res.statusCode >= 500 ? 'error'
      : res.statusCode >= 400 ? 'warn'
      : 'info';

    logger[level]({
      method: req.method,
      url: req.url,
      statusCode: res.statusCode,
      durationMs,
    }, 'HTTP request completed');
  });

  next();
}
```

### Database Calls

```typescript
// db/query.ts
export async function query<T>(sql: string, params: unknown[]): Promise<T[]> {
  const start = Date.now();
  try {
    const rows = await pool.query<T>(sql, params);
    const durationMs = Date.now() - start;

    if (durationMs > 200) {
      logger.warn({ durationMs, sql: sanitizeSql(sql) }, 'Slow query detected');
    } else {
      logger.debug({ durationMs, rowCount: rows.length }, 'DB query OK');
    }

    return rows;
  } catch (err) {
    logger.error({ err, durationMs: Date.now() - start }, 'DB query failed');
    throw err;
  }
}

// Never log raw SQL with parameter values — they may contain PII or secrets.
// Log a sanitized version (query shape only) or a query name/identifier.
function sanitizeSql(sql: string): string {
  return sql.replace(/\s+/g, ' ').substring(0, 200);
}
```

### External API Calls

```typescript
// lib/http-client.ts
export async function callExternalApi(
  url: string,
  options: RequestInit,
): Promise<Response> {
  const start = Date.now();
  logger.info({ url: redactUrl(url), method: options.method }, 'External API call started');

  try {
    const response = await fetch(url, options);
    const durationMs = Date.now() - start;

    if (!response.ok) {
      logger.warn({
        url: redactUrl(url),
        statusCode: response.status,
        durationMs,
      }, 'External API returned non-2xx');
    } else {
      logger.info({ url: redactUrl(url), statusCode: response.status, durationMs }, 'External API call OK');
    }

    return response;
  } catch (err) {
    logger.error({ err, url: redactUrl(url), durationMs: Date.now() - start }, 'External API call failed');
    throw err;
  }
}

// Strip tokens and API keys from URLs before logging
function redactUrl(url: string): string {
  return url.replace(/([?&])(key|token|secret|api_key)=[^&]*/gi, '$1$2=[REDACTED]');
}
```

---

## Avoid Logging PII

PII includes: email addresses, phone numbers, names, IP addresses (in some jurisdictions), passwords, payment card numbers, health data, location data.

```typescript
// ❌ WRONG — PII in log
logger.info({ email: user.email, name: user.name }, 'User registered');

// ✅ CORRECT — use internal identifiers only
logger.info({ userId: user.id, plan: user.plan }, 'User registered');
```

Use Pino's `redact` (shown in Setup above) as a safety net, but don't rely on it as the primary defense — don't pass PII into logs in the first place.

**Minimum redact paths for any service handling users:**
```typescript
redact: [
  'req.headers.authorization',
  'req.headers.cookie',
  '*.password',
  '*.token',
  '*.secret',
  '*.creditCard',
  '*.cardNumber',
  '*.ssn',
  '*.email',
  '*.phone',
]
```

---

## Child Loggers for Scoped Context

Pino child loggers bind context fields once, so you don't repeat them on every call.

```typescript
// Add a request-scoped child logger to the request object
app.use((req: Request, _res: Response, next: NextFunction) => {
  req.log = logger.child({
    requestId: req.headers['x-request-id'],
    method: req.method,
    path: req.path,
  });
  next();
});

// In a route handler — no need to repeat method/path/requestId
router.get('/users/:id', async (req, res) => {
  req.log.info({ userId: req.params.id }, 'Fetching user');
  // ...
});
```

---

## Checklist

- [ ] No bare `console.log` / `console.error` — replaced with structured logger
- [ ] Trace ID attached to every request and propagated downstream
- [ ] `redact` config covers at minimum: authorization header, cookie, password, token, email
- [ ] Log at request boundary (in + out), DB call, and external API call
- [ ] Slow query threshold defined and logs a `warn`
- [ ] Production log level is `info` (not `debug` or `trace`)
- [ ] 5xx responses produce an `error` log; 4xx produce a `warn`
- [ ] No PII fields passed as log data — use IDs, not names/emails/phones
- [ ] Log format is JSON in production, pretty-print allowed in development only