Logging

Logging is essential for debugging, monitoring, and auditing backend applications.

SLF4J + Logback (Spring Boot Default)

Spring Boot uses SLF4J as the logging facade and Logback as the implementation:

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

@Service
public class ProductService {

    private static final Logger log = LoggerFactory.getLogger(ProductService.class);

    public ProductDto getProduct(Long id) {
        log.debug("Fetching product with id: {}", id);

        Product product = productRepository.findById(id)
            .orElseThrow(() -> {
                log.warn("Product not found with id: {}", id);
                return new ResourceNotFoundException("Product not found");
            });

        log.info("Successfully retrieved product: {}", product.getName());
        return productMapper.toDto(product);
    }
}

Log Levels

Level	When to Use
ERROR	Something failed — needs immediate attention
WARN	Something unexpected — not critical
INFO	Key business events — order placed, user logged in
DEBUG	Detailed info for debugging — method entry/exit
TRACE	Very detailed — variable values, flow control

Configuration (application.yml)

logging:
  level:
    root: INFO
    com.example: DEBUG
    com.example.repository: WARN

Levels

TRACE < DEBUG < INFO < WARN < ERROR < FATAL

Structured Logging

{
  "timestamp": "...",
  "level": "INFO",
  "message": "...",
  "requestId": "..."
}

Best Practices

• Use SLF4J facade
• Include correlation IDs
• Don't log sensitive data
• Use appropriate levels

Key Points

• Understanding Logging is essential for production systems
• Always consider scalability and maintainability
• Test thoroughly before deploying to production
• Monitor performance and set up alerting

Common Patterns

1. Validation: Always validate input at the boundary
2. Error Handling: Use structured error responses
3. Logging: Log key events for debugging
4. Testing: Unit, integration, and load tests
5. Documentation: Keep docs updated with code changes