3.12 — Quick Revision: Logging and Monitoring

Your cheat sheet for log levels, Winston configuration, Morgan formats, error handling patterns, and monitoring tools. Bookmark this page for quick reference.

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< Interview Questions | README >

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Log Levels (Severity Order)

Level	Priority	When to Use	Example
error	0 (highest)	Something broke	DB connection lost
warn	1	Potential issue	Slow query, deprecation
info	2	Normal events	User login, order placed
http	3	HTTP requests	GET /api/users 200 45ms
debug	4	Dev debugging	Variable values
silly	5 (lowest)	Verbose tracing	Loop iterations

Setting level = "warn" logs: error + warn (suppresses info, debug, silly)

Recommended Levels by Environment

Environment	Level	Format
Development	debug	Colorized console
Testing	error	Minimal/silent
Staging	info	JSON files
Production	warn or info	JSON files + aggregator

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Winston Setup

const winston = require("winston");
require("winston-daily-rotate-file");

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()
  ),
  defaultMeta: { service: "my-api" },
  transports: [
    new winston.transports.DailyRotateFile({
      filename: "logs/combined-%DATE%.log",
      datePattern: "YYYY-MM-DD",
      maxFiles: "14d",
      zippedArchive: true,
    }),
    new winston.transports.DailyRotateFile({
      filename: "logs/error-%DATE%.log",
      level: "error",
      datePattern: "YYYY-MM-DD",
      maxFiles: "30d",
    }),
  ],
});

if (process.env.NODE_ENV !== "production") {
  logger.add(new winston.transports.Console({
    format: winston.format.combine(
      winston.format.colorize(),
      winston.format.simple()
    ),
  }));
}

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Winston Transports

Transport	Purpose	Key Options
Console	Terminal output	format, level
File	Write to file	filename, level, maxsize, maxFiles
DailyRotateFile	Daily file rotation	datePattern, maxSize, maxFiles, zippedArchive
Http	Send to endpoint	host, port, path

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Winston Formats

Format	Purpose	Environment
format.json()	Structured JSON output	Production
format.simple()	level: message	Basic/testing
format.colorize()	Color-coded levels	Development
format.timestamp()	Add timestamp field	All
format.errors({ stack: true })	Include stack traces	All
format.printf(fn)	Custom format function	Custom
format.combine(...)	Chain multiple formats	All

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Morgan Formats

Format	Output Example	Best For
dev	GET /api/users 200 45.123 ms (colorized)	Development
combined	Full Apache format with referrer + user-agent	Production
common	Apache common (no referrer/agent)	Production
short	Shorter than combined	General
tiny	GET /api/users 200 1234 - 45.123 ms	Simple

Morgan + Winston Integration

const morgan = require("morgan");
app.use(morgan("combined", {
  stream: { write: (msg) => logger.http(msg.trim()) },
  skip: (req) => req.url === "/health",
}));

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Request ID Middleware

const { v4: uuidv4 } = require("uuid");
app.use((req, res, next) => {
  req.id = req.headers["x-request-id"] || uuidv4();
  res.setHeader("x-request-id", req.id);
  next();
});

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Error Handling Middleware

app.use((err, req, res, next) => {
  logger.error("Request error", {
    error: err.message,
    stack: err.stack,
    method: req.method,
    url: req.originalUrl,
    requestId: req.id,
    userId: req.user?._id,
  });
  const status = err.statusCode || 500;
  res.status(status).json({
    status: "error",
    message: status === 500 ? "Internal server error" : err.message,
  });
});

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Process-Level Error Handlers

process.on("unhandledRejection", (reason) => {
  logger.error("Unhandled Rejection", {
    error: reason instanceof Error ? reason.message : reason,
    stack: reason instanceof Error ? reason.stack : undefined,
  });
});

process.on("uncaughtException", (err) => {
  logger.error("Uncaught Exception", { error: err.message, stack: err.stack });
  process.exit(1);  // MUST exit
});

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What to Log vs What NOT to Log

Always Log

Event	Level
Server start/stop	info
User auth (login/logout/failure)	info / warn
API errors (4xx, 5xx)	warn / error
Database errors	error
External API calls (time + status)	info
Business events (orders, payments)	info
Slow queries/responses	warn

Never Log

Data	Reason
Passwords	Security
API keys / tokens	Security
Credit card numbers	PCI compliance
SSN / personal health data	Privacy laws
Full request bodies with PII	Data protection

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Monitoring Tools Overview

Tool	Type	Purpose
Sentry	Error monitoring	Capture errors, stack traces, affected users
LogRocket	Session replay	See what the user saw (frontend)
ELK Stack	Log aggregation	Centralized search + dashboards
Datadog	Full observability	Metrics, logs, traces, dashboards
CloudWatch	AWS logging	AWS-native log management
Prometheus + Grafana	Metrics	Real-time metrics and alerting
Papertrail	Cloud logging	Simple log aggregation

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ELK Stack Components

Component	Role
Elasticsearch	Store + index + search logs
Logstash	Ingest + transform + route logs
Kibana	Visual dashboard + search UI
Filebeat	Lightweight log file shipper

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Quick Debugging Checklist

• Get the user ID or request ID from the error report
• Search error logs for that ID within the time window
• Find the request ID from the error log entry
• Search ALL logs for that request ID to see the full trace
• Read the stack trace to identify the failure point
• Check for related errors (DB down? External API failure?)
• Check recent deployments for the time window
• Reproduce in staging, fix, deploy, verify

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Library Comparison

Feature	Winston	Pino	Morgan
Type	General logger	General logger	HTTP logger
Speed	Moderate	Fastest	N/A
Formats	Multiple	JSON	Predefined strings
Transports	Built-in	Separate process	Stream
Express	Manual	pino-http	Built-in middleware
Best for	Most apps	High-throughput	HTTP access logs
Common combo	Winston + Morgan	Pino + pino-http	With Winston/Pino