Logging Fundamentals

Establish proper logging practices including log levels, structured logging, context propagation, and comprehensive logging strategies for applications, services, and infrastructure.

When to use me

Use this skill when:

• Setting up logging for new applications or services
• Reviewing existing logging implementations
• Establishing logging standards for teams or projects
• Debugging issues with incomplete or poor logging
• Implementing structured logging for better observability
• Configuring log levels and verbosity for different environments
• Ensuring logs contain sufficient context for troubleshooting

What I do

1. Log Level Management

• Define appropriate log levels (DEBUG, INFO, WARN, ERROR, FATAL)
• Configure level filtering per environment (development, staging, production)
• Implement dynamic level adjustment without application restarts
• Establish level usage guidelines for different types of information
• Create level-based alerting thresholds for operational monitoring

2. Structured Logging Implementation

• Design log schema with consistent field naming and types
• Implement structured formats (JSON, key-value pairs, structured text)
• Include mandatory context fields (timestamp, service, trace_id, user_id, etc.)
• Handle nested structures and arrays appropriately in logs
• Ensure PII (Personally Identifiable Information) compliance in log content

3. Context Propagation

• Implement correlation IDs for request tracing across services
• Propagate context through async operations (queues, background jobs)
• Maintain user/session context throughout request lifecycle
• Include business context (order_id, transaction_id, etc.) in logs
• Handle context in distributed systems with proper header propagation

4. Log Content Best Practices

• Include sufficient context for debugging without external systems
• Balance detail with noise - log enough but not too much
• Use consistent message formats across the codebase
• Log before and after significant operations
• Include error details (stack traces, error codes, recovery suggestions)
• Avoid logging secrets, passwords, or sensitive data

5. Environment-Specific Configuration

• Development logging - Verbose, human-readable, local file output
• Staging logging - Balanced detail, structured format, aggregation
• Production logging - Minimal noise, structured only, remote aggregation
• Debug mode logging - Temporary increased verbosity for troubleshooting

Logging Principles

The 5 Ws of Logging

1. WHO - Which user/service/process generated the log?
2. WHAT - What action/event is being logged?
3. WHEN - Precise timestamp with timezone information
4. WHERE - Which component/function/file generated the log?
5. WHY - What is the significance/severity of the logged event?

Structured Logging Fields

{
  "timestamp": "2026-02-26T18:00:00Z",
  "level": "ERROR",
  "service": "payment-service",
  "component": "process_payment",
  "trace_id": "abc123-def456",
  "span_id": "def456",
  "user_id": "user-789",
  "session_id": "session-xyz",
  "request_id": "req-123456",
  "message": "Payment processing failed",
  "error_code": "PAYMENT_GATEWAY_TIMEOUT",
  "error_details": "Gateway response timeout after 5000ms",
  "stack_trace": "...",
  "context": {
    "order_id": "ord-987654",
    "amount": 99.99,
    "currency": "USD"
  },
  "duration_ms": 5123,
  "environment": "production",
  "hostname": "payment-host-01",
  "version": "1.2.3"
}

Log Level Guidelines

• DEBUG: Detailed information for debugging, typically disabled in production
• INFO: Routine information about normal operation
• WARN: Warning conditions that might require attention but don't indicate failure
• ERROR: Error conditions that indicate failure of a specific operation
• FATAL: Critical errors that cause application termination

Examples

# Configure logging levels by environment
npm run logging:configure -- --environment production --level WARN

# Analyze current logging implementation
npm run logging:analyze -- --path src/ --output logging-report.json

# Generate structured logging configuration
npm run logging:generate-config -- --format json --output logging-config.json

# Test logging output
npm run logging:test -- --scenario "payment-failure" --levels "ERROR,WARN"

# Validate logging best practices
npm run logging:validate -- --strict --check-pii --check-context

Output format

Logging Configuration Template:

logging:
  level: 
    root: INFO
    specific:
      "com.example.service": DEBUG
      "org.springframework": WARN
  
  format:
    type: json
    timestamp_format: "ISO8601"
    include_fields:
      - timestamp
      - level  
      - service
      - component
      - trace_id
      - message
    
  context:
    auto_included:
      - thread_id
      - hostname
      - service_version
    propagated:
      - trace_id
      - user_id
      - request_id
    
  appenders:
    - type: console
      level: INFO
    - type: file
      path: /var/log/app.log
      level: WARN
    - type: http
      endpoint: https://logs.example.com
      level: ERROR

Logging Best Practices Report:

Logging Fundamentals Assessment
───────────────────────────────
Application: payment-service
Assessment Date: 2026-02-26
Score: 78/100

Strengths:
✅ Structured logging implemented (JSON format)
✅ Correlation IDs propagated across services
✅ Log levels appropriately configured
✅ PII filtering in place

Areas for Improvement:
⚠️  Insufficient context in error logs (missing user_id in 45% of error logs)
⚠️  Debug logs enabled in production for some components
⚠️  Inconsistent timestamp formats across services
⚠️  Missing business context in 30% of transaction logs

Critical Issues:
❌ No log sampling for high-volume debug logs
❌ Secret leakage detected in 2 log patterns
❌ Incomplete error context for 15% of database errors

Recommendations:
1. Implement consistent context inclusion middleware
2. Configure log sampling for debug-level logs
3. Add business context to all transaction logs
4. Update secret detection patterns
5. Standardize timestamp format across services

Implementation Priority:
- High: Fix secret leakage immediately
- Medium: Add missing context fields
- Low: Standardize timestamp format

Notes

• Structured logging is essential for modern log analysis and observability
• Context is more important than message content - ensure logs can be correlated
• Log levels should reflect operational importance, not developer convenience
• Consider log volume - too many logs can overwhelm systems and teams
• Test logging in production-like environments - logging behavior can differ
• Monitor your logging - ensure logs are being captured, processed, and stored
• Regularly review and update logging practices as systems evolve
• Balance human readability with machine parsability in log formats
• Document logging standards and ensure team adherence
• Consider the cost of logging - storage, processing, and analysis have expenses