Tip of the day #65: Use MDC for Contextualized Logging

Mapped Diagnostic Context (MDC) is a powerful tool for adding context to your log entries, making them more meaningful and easier to trace in distributed systems or complex applications. Here's how you can implement it in a Spring Boot application using SLF4J and Logback:

Example Implementation:

1. Adding MDC to a Web Request:

   Use an interceptor or filter to add contextual information (e.g., request ID, user ID) to the MDC:

import org.slf4j.MDC;
import org.springframework.stereotype.Component;
import javax.servlet.FilterChain;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.util.UUID;

@Component
public class MdcFilter extends OncePerRequestFilter {

    @Override
    protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response, FilterChain filterChain)
            throws ServletException, IOException {
        try {
            // Add request ID to MDC
            MDC.put("requestId", UUID.randomUUID().toString());

            // Optionally add user information or other context
            String userId = request.getHeader("X-User-Id");
            if (userId != null) {
                MDC.put("userId", userId);
            }

            filterChain.doFilter(request, response);
        } finally {
            MDC.clear(); // Clean up MDC after request is processed
        }
    }
}

2. Logback Configuration:

   Update your logback.xml to include MDC values in your log pattern:

<configuration>
    <appender name="CONSOLE" class="ch.qos.logback.core.ConsoleAppender">
        <encoder>
            <pattern>%d{yyyy-MM-dd HH:mm:ss} [%X{requestId}] [%X{userId}] %-5level %logger{36} - %msg%n</pattern>
        </encoder>
    </appender>
    <root level="INFO">
        <appender-ref ref="CONSOLE" />
    </root>
</configuration>

3. Result: Your logs will now contain contextual information, such as:

2024-12-03 14:23:45 [9f3e2a7c-f4d6-4c69-a4f2-4d8e2e18b1a2] [user123] INFO  com.example.Service - Processing request

With this you:

• Improved Traceability: Easily correlate logs across microservices or threads.

• Better Debugging: Add critical information like user IDs, transaction IDs, or other metadata.

• Readability: Simplifies log analysis by adding meaningful context.

Do you use MDC in your project? Let me know!

Happy coding!