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Log Level Reference Guide

Comprehensive guide for logging best practices

DEBUG

Detailed diagnostic information for development and troubleshooting

When to Use
  • Variable values and state information
  • Function entry/exit with parameters
  • Detailed flow of execution
  • Cache hits/misses
  • Query details and results
Characteristics
  • Volume: Very high
  • Production: Usually disabled
  • Audience: Developers
  • Performance: Can impact performance
Example Messages
DEBUG: Processing user request for ID 12345
DEBUG: Cache miss for key 'user:session:abc123'
DEBUG: Executing SQL: SELECT * FROM users WHERE id = ?
DEBUG: Function calculateTotal() returned 156.78
Warning: Never log sensitive data (passwords, tokens, PII) even at DEBUG level

INFO

General informational messages about system operation

When to Use
  • Application startup and shutdown
  • Configuration values loaded
  • Significant business events
  • Successful completion of major operations
  • User actions (login, logout, purchase)
  • API calls to external services
Characteristics
  • Volume: Moderate
  • Production: Enabled
  • Audience: Operations, support
  • Performance: Minimal impact
Example Messages
INFO: Application started on port 8080
INFO: User 'john@example.com' logged in successfully
INFO: Payment processed for order #ORD-12345
INFO: Daily backup completed successfully
INFO: Connected to database: prod-db-01
Best Practice: INFO should tell the story of what your application is doing

WARN / WARNING

Potentially harmful situations that don't prevent operation

When to Use
  • Deprecated API usage
  • Suboptimal configuration
  • Retry attempts (before failure)
  • Resource usage approaching limits
  • Unexpected but recoverable conditions
  • Fallback behavior triggered
Characteristics
  • Volume: Low to moderate
  • Production: Enabled
  • Audience: Operations, developers
  • Action Required: Review and investigate
Example Messages
WARN: API endpoint /old-api is deprecated, use /api/v2
WARN: Retrying failed request to payment service (attempt 2/3)
WARN: Connection pool at 80% capacity
WARN: Cache server unavailable, using local cache
WARN: Response time exceeded 500ms: 1,245ms
Rule of Thumb: WARN = "This worked, but you should look at this"

ERROR

Error events that still allow the application to continue

When to Use
  • Failed operations (request failed, save failed)
  • Unhandled exceptions in request handlers
  • Database connection failures
  • Failed external service calls (after retries)
  • Data validation failures
  • File I/O errors
Characteristics
  • Volume: Low (should be rare)
  • Production: Enabled
  • Audience: Operations, on-call engineers
  • Action Required: Immediate investigation
Example Messages
ERROR: Failed to process payment for order #ORD-12345: TimeoutException
ERROR: Database connection lost: Could not connect to prod-db-01
ERROR: Unable to send email notification: SMTP server unreachable
ERROR: Invalid data in request: User ID cannot be null
ERROR: Failed to write to file /var/log/app.log: Disk full
Important: ERROR should always include context (stack trace, request ID, user ID)

FATAL / CRITICAL

Severe errors that cause application shutdown or inability to function

When to Use
  • Application cannot start (missing critical config)
  • Out of memory errors
  • Database completely unavailable
  • Critical system resource unavailable
  • Security violations requiring shutdown
  • Data corruption detected
Characteristics
  • Volume: Very low (rarely seen)
  • Production: Enabled
  • Audience: On-call engineers, management
  • Action Required: Emergency response
Example Messages
FATAL: Cannot connect to database, application shutting down
FATAL: Out of memory, cannot allocate heap space
FATAL: Configuration file missing: /etc/app/config.yaml
FATAL: SSL certificate expired, cannot start HTTPS server
FATAL: Critical dependency 'payment-service' unreachable
Critical: FATAL logs should trigger immediate alerts (PagerDuty, phone calls)

Best Practices

General Logging Guidelines

  • Be consistent: Use the same format and structure across your application
  • Include context: Request IDs, user IDs, timestamps, correlation IDs
  • Be concise: Log messages should be clear and to the point
  • Use structured logging: JSON format for machine parsing
  • Don't log sensitive data: Passwords, tokens, credit cards, PII
  • Think about volume: Excessive logging impacts performance and costs

What to Log

DO Log:
  • Request/response metadata
  • Errors with stack traces
  • Performance metrics
  • Business events
  • Security events
  • State changes
  • External API calls
DON'T Log:
  • Passwords or secrets
  • Credit card numbers
  • Social security numbers
  • Authentication tokens
  • Complete request bodies
  • Personal health information
  • Encryption keys

Log Levels in Different Environments

Environment Recommended Level Reasoning
Development DEBUG Maximum visibility for debugging
Testing/Staging INFO Similar to production
Production INFO or WARN Balance visibility and performance
High-traffic Prod WARN Reduce volume and costs

Structured Logging Example

{
  "timestamp": "2024-01-15T14:30:45.123Z",
  "level": "ERROR",
  "service": "payment-api",
  "requestId": "req-abc-123",
  "userId": "user-456",
  "message": "Payment processing failed",
  "error": {
    "type": "TimeoutException",
    "message": "Connection timeout after 5000ms",
    "stack": "..."
  },
  "metadata": {
    "orderId": "ORD-789",
    "amount": 99.99,
    "currency": "USD",
    "gateway": "stripe"
  }
}
Quick Reference
DEBUG Diagnostic info
INFO General events
WARN Potential issues
ERROR Operation failed
FATAL App shutting down
Decision Tree

Will the app crash?

Yes → FATAL

Did an operation fail?

Yes → ERROR

Is this unexpected but handled?

Yes → WARN

Is this a significant event?

Yes → INFO

Is this for debugging?

Yes → DEBUG

Related Topics
  • Log aggregation (ELK, Splunk)
  • Distributed tracing
  • Metrics vs Logs
  • Log retention policies
  • GDPR compliance
  • Log analysis tools
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