Logging Guide

Language: English | 繁體中文

Version: 1.0.0 Last Updated: 2025-12-30 Applicability: Claude Code Skills

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Purpose

This skill helps implement consistent, structured, and actionable application logs across all environments.

Quick Reference

Log Levels

Level	Code	When to Use	Production
TRACE	10	Very detailed debugging info	Off
DEBUG	20	Detailed debugging info	Off
INFO	30	Normal operation events	On
WARN	40	Potential issues, recoverable	On
ERROR	50	Errors that need attention	On
FATAL	60	Critical failures	On

Level Selection Decision Tree

Is it debugging only?        → DEBUG (off in prod)
Normal operation completed?  → INFO
Something unexpected but OK? → WARN
Operation failed?            → ERROR
App cannot continue?         → FATAL

When to Use Each Level

Level	Examples
TRACE	Function entry/exit, loop iterations, variable values
DEBUG	State changes, configuration values, query parameters
INFO	App startup/shutdown, user actions, scheduled tasks
WARN	Deprecated API, retry attempts, resource approaching limits
ERROR	Failed operations, caught exceptions, integration failures
FATAL	Unrecoverable errors, startup failures, lost critical resources

Structured Logging

Required Fields

{
  "timestamp": "2025-01-15T10:30:00.123Z",
  "level": "INFO",
  "message": "User login successful",
  "service": "auth-service",
  "environment": "production"
}

Recommended Fields

{
  "timestamp": "2025-01-15T10:30:00.123Z",
  "level": "INFO",
  "message": "User login successful",
  "service": "auth-service",
  "environment": "production",
  "trace_id": "abc123",
  "span_id": "def456",
  "user_id": "usr_12345",
  "request_id": "req_67890",
  "duration_ms": 150,
  "http_method": "POST",
  "http_path": "/api/v1/login",
  "http_status": 200
}

Field Naming Conventions

Use snake_case and prefix with domain:

Domain	Common Fields
HTTP	http_method, http_path, http_status, http_duration_ms
Database	db_query_type, db_table, db_duration_ms, db_rows_affected
Queue	queue_name, queue_message_id, queue_delay_ms
User	user_id, user_role, user_action
Request	request_id, trace_id, span_id

Detailed Guidelines

For complete standards, see:

• Logging Standards

AI-Optimized Format (Token-Efficient)

For AI assistants, use the YAML format files for reduced token usage:

• Base standard: ai/standards/logging.ai.yaml

Sensitive Data Handling

Never Log

• Passwords or secrets
• API keys or tokens
• Credit card numbers
• Social security numbers
• Full authentication tokens

Mask or Redact

// Bad
logger.info('Login attempt', { password: userPassword });

// Good
logger.info('Login attempt', { password: '***REDACTED***' });

// Good - mask partial
logger.info('Card processed', { last_four: '4242' });

PII Handling

• Log user IDs, not email addresses when possible
• Use hashed identifiers for sensitive lookups
• Configure data retention policies

Error Logging

Required Error Fields

{
  "level": "ERROR",
  "message": "Database connection failed",
  "error_type": "ConnectionError",
  "error_message": "Connection refused",
  "error_code": "ECONNREFUSED",
  "stack": "Error: Connection refused\n    at connect (/app/db.js:45:11)..."
}

Error Context

Always include:

• What operation was attempted
• Relevant identifiers (user_id, request_id)
• Input parameters (sanitized)
• Retry count if applicable

logger.error('Failed to process order', {
  error_type: err.name,
  error_message: err.message,
  order_id: orderId,
  user_id: userId,
  retry_count: 2,
  stack: err.stack
});

Log Format

JSON Format (Production)

{"timestamp":"2025-01-15T10:30:00.123Z","level":"INFO","message":"Request completed","request_id":"req_123","duration_ms":45}

Human-Readable (Development)

2025-01-15T10:30:00.123Z [INFO] Request completed request_id=req_123 duration_ms=45

Performance Considerations

Log Volume by Environment

Environment	Level	Strategy
Development	DEBUG	All logs
Staging	INFO	Most logs
Production	INFO	Sampling for high-volume

High-Volume Endpoints

• Use sampling (log 1 in 100)
• Aggregate metrics instead of individual logs
• Use separate log streams

Checklist

Required Fields

• timestamp (ISO 8601)
• level
• message
• service name
• request_id or trace_id

Security

• No passwords or secrets
• No full tokens
• PII masked or hashed
• Credit cards never logged
• Retention policies configured

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Configuration Detection

This skill supports project-specific configuration.

Detection Order

1. Check for existing logging library configuration
2. Check CONTRIBUTING.md for logging guidelines
3. If not found, default to structured JSON logging

First-Time Setup

If no logging standard found:

1. Suggest: "This project hasn't configured logging standards. Would you like to set up structured logging?"
2. Suggest documenting in CONTRIBUTING.md:

## Logging Standards

### Log Levels
- DEBUG: Development only, detailed diagnostic info
- INFO: Normal operations (startup, user actions, tasks)
- WARN: Unexpected but recoverable situations
- ERROR: Failures that need investigation

### Required Fields
All logs must include: timestamp, level, message, service, request_id

### Sensitive Data
Never log: passwords, tokens, credit cards, SSN

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Related Standards

• Logging Standards
• Error Code Standards

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Version History

Version	Date	Changes
1.0.0	2025-12-30	Initial release

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License

This skill is released under CC BY 4.0.

Source: universal-dev-standards