SKILL.md
Logging Best Practices
Table of Contents
- [Overview](#overview)
- [When to Use](#when-to-use)
- [Quick Start](#quick-start)
- [Reference Guides](#reference-guides)
- [Best Practices](#best-practices)
Overview
Comprehensive guide to implementing structured, secure, and performant logging across applications. Covers log levels, structured logging formats, contextual information, PII protection, and centralized logging systems.
When to Use
- Setting up application logging infrastructure
- Implementing structured logging
- Configuring log levels for different environments
- Managing sensitive data in logs
- Setting up centralized logging
- Implementing distributed tracing
- Debugging production issues
- Compliance with logging regulations
Quick Start
Minimal working example:
// logger.ts
enum LogLevel {
DEBUG = 0, // Detailed information for debugging
INFO = 1, // General informational messages
WARN = 2, // Warning messages, potentially harmful
ERROR = 3, // Error messages, application can continue
FATAL = 4, // Critical errors, application must stop
}
class Logger {
constructor(private minLevel: LogLevel = LogLevel.INFO) {}
debug(message: string, context?: object) {
if (this.minLevel <= LogLevel.DEBUG) {
this.log(LogLevel.DEBUG, message, context);
}
}
info(message: string, context?: object) {
if (this.minLevel <= LogLevel.INFO) {
this.log(LogLevel.INFO, message, context);
}
}
warn(message: string, context?: object) {
// ... (see reference guides for full implementation)Reference Guides
Detailed implementations in the references/ directory:
Guide
Contents
Log Levels
Structured Logging (JSON)
Contextual Logging
PII and Sensitive Data Handling
PII and Sensitive Data Handling
Performance Logging
Centralized Logging
Distributed Tracing
Log Sampling (High-Volume Services)
Log Sampling (High-Volume Services)
Best Practices
✅ DO
- Use structured logging (JSON) in production
- Include correlation/request IDs in all logs
- Log at appropriate levels (don't overuse DEBUG)
- Redact sensitive data (PII, passwords, tokens)
- Include context (userId, requestId, etc.)
- Log errors with full stack traces
- Use centralized logging in distributed systems
- Set up log rotation to manage disk space
- Monitor log volume and costs
- Use async logging for performance
- Include timestamps in ISO 8601 format
- Log business events (user actions, transactions)
- Set up alerts for error patterns
❌ DON'T
- Log passwords, tokens, or sensitive data
- Use console.log in production
- Log at DEBUG level in production by default
- Log inside tight loops (use sampling)
- Include PII without anonymization
- Ignore log rotation (disk will fill up)
- Use synchronous logging in hot paths
- Log to multiple transports without need
- Forget to include error stack traces
- Log binary data or large objects
- Use string concatenation (use structured fields)
- Log every single request in high-volume APIs