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Logs are timestamped, immutable records of discrete events in your system. They provide the detailed context that metrics cannot — the exact error message, the stack trace, the user ID, the request payload. Effective logging is essential for debugging, auditing, and understanding system behaviour.
Traditional plain-text logs are difficult to parse and search:
2025-03-15 10:23:45 ERROR PaymentService - Failed to charge card for user 12345: insufficient funds
Structured logs use a consistent format (typically JSON) with key-value pairs:
{
"timestamp": "2025-03-15T10:23:45Z",
"level": "ERROR",
"service": "payment-service",
"message": "Failed to charge card",
"user_id": "12345",
"error": "insufficient funds",
"trace_id": "abc123def456",
"amount": 49.99,
"currency": "GBP"
}
Always use structured logging. It enables:
Standard log levels provide severity classification:
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