/application-logging

name: application-logging
description: >
  Implement structured logging across applications with log aggregation and
  centralized analysis. Use when setting up application logging, implementing
  ELK stack, or analyzing application behavior.

Application Logging

Table of Contents

• [Overview](#overview)
• [When to Use](#when-to-use)
• [Quick Start](#quick-start)
• [Reference Guides](#reference-guides)
• [Best Practices](#best-practices)

Overview

Implement comprehensive structured logging with proper levels, context, and centralized aggregation for effective debugging and monitoring.

When to Use

• Application debugging
• Audit trail creation
• Performance analysis
• Compliance requirements
• Centralized log aggregation

Quick Start

Minimal working example:

// logger.js
const winston = require("winston");

const logFormat = winston.format.combine(
  winston.format.timestamp({ format: "YYYY-MM-DD HH:mm:ss" }),
  winston.format.errors({ stack: true }),
  winston.format.json(),
);

const logger = winston.createLogger({
  level: process.env.LOG_LEVEL || "info",
  format: logFormat,
  defaultMeta: {
    service: "api-service",
    environment: process.env.NODE_ENV || "development",
  },
  transports: [
    new winston.transports.Console({
      format: winston.format.combine(
        winston.format.colorize(),
        winston.format.simple(),
      ),
    }),
    new winston.transports.File({
      filename: "logs/error.log",
// ... (see reference guides for full implementation)

Reference Guides

Detailed implementations in the `references/` directory:

| Guide | Contents | |---|---| | [Node.js Structured Logging with Winston](references/nodejs-structured-logging-with-winston.md) | Node.js Structured Logging with Winston | | [Express HTTP Request Logging](references/express-http-request-logging.md) | Express HTTP Request Logging | | [Python Structured Logging](references/python-structured-logging.md) | Python Structured Logging | | [Flask Integration](references/flask-integration.md) | Flask Integration | | [ELK Stack Setup](references/elk-stack-setup.md) | ELK Stack Setup | | [Logstash Configuration](references/logstash-configuration.md) | Logstash Configuration |

Best Practices

✅ DO

• Use structured JSON logging
• Include request IDs for tracing
• Log at appropriate levels
• Add context to error logs
• Implement log rotation
• Use timestamps consistently
• Aggregate logs centrally
• Filter sensitive data

❌ DON'T

• Log passwords or secrets
• Log at INFO for every operation
• Use unstructured messages
• Ignore log storage limits
• Skip context information
• Log to stdout in production
• Create unbounded log files