4.2Unstructured vs. Structured Logging

# BAD: Unstructured -- human-readable, machine-hostile
2026-01-15 14:23:45 ERROR Failed to process order for user john@example.com: timeout after 30s

# GOOD: Structured JSON -- machine-parseable, human-readable with tooling
{
  "timestamp": "2026-01-15T14:23:45.123Z",
  "level": "error",
  "service": "order-service",
  "event": "order_processing_failed",
  "user_id": "usr_42",
  "order_id": "ord_789",
  "error_type": "timeout",
  "timeout_seconds": 30,
  "downstream_service": "payment-service",
  "trace_id": "abc123def456",
  "span_id": "span_789",
  "environment": "production",
  "version": "2.4.1"
}

The structured version enables queries like:

• "Show all timeout errors in the last hour for the payment-service dependency"
• "Count errors by type for user usr_42"
• "Correlate this error with the trace abc123def456 to see the full request flow"