Mapped Diagnostic Context (MDC)

Mapped Diagnostic Context (MDC) will help us add more context on every log output. For more information, refer to this web page: https://logback.qos.ch/manual/mdc.html. It is a kind of Map attached to the Thread context and maintained by the logging framework where you can put values and use them in your log layout.

As it is attached to a Thread, we can put values at the beginning of a request and print them in all the logs related to this request. That can give you a lot of information about the context of the request, without having to add them to all logs manually.

Implement MDC

A good sketch is better than a long speech. In the next section, we will use MDC to add the card number and the POS ID to all the logs related to a request.

📝 Go to the easypay-service/src/main/java/com/worldline/easypay/payment/boundary/PaymentResource class and modify the method processPayment() to instantiate the MDC:

public ResponseEntity<PaymentResponse> processPayment(PaymentRequest paymentRequest) {
    // Add cardNumber to SLF4J MDC 
    MDC.put("cardNumber",paymentRequest.cardNumber());
    // Add Point Of Sale identifier to SLF4J MDC
    MDC.put("pos",paymentRequest.posId());
            
    try { // Add a try-finally construct and wrap the initial code here 
        //...
        return httpResponse;
    catch (Exception e) { 
        // Catch any exception to log it with MDC value
        LOG.error(e.getMessage());
        throw e;
    } finally {
        // Clear MDC at the end
        MDC.clear();
    }
}

Caution

Don’t forget to clear the MDC at the end of the method to avoid any data leak.

Now, we want to print these values when a log line is printed in the console.

📝 Modify to the spring configuration file (easypay-service/src/main/resources/application.yaml) and add the logging.level.pattern property to add both the cardNumber & pos fields to all logs:

logging:
  pattern:
    level: "%5p [%mdc]"

Tip

%mdc prints the full content of the MDC Map attached to the current thread.

If you want to print a single value, you can use %X{key} where key is the key of the value you want to print.

# Alternative to print specific fields instead
logging:
  pattern:
    level: "%5p [%X{cardNumber} - %X{pos}]"

Info

Using the Spring Boot logging.pattern.level property is just a way to configure the logback pattern. You can also use a logback configuration file to do the same thing.
You can also use the logging.pattern.correlation property (used by tracing) or a logback configuration file to do the same thing.

🛠️ Rebuild and redeploy the easypay-service:

$ docker compose up -d --build easypay-service

Adding more content in our logs

🛠️ To have more logs, we will run several HTTP requests using K6. Run the following command:

$ k6 run -u 5 -d 5s k6/01-payment-only.js

👀 Check then the logs to pinpoint some exceptions.

Logs Correlation

Tip

You are probably wondering how to smartly debug in production when you have plenty of logs for several users and by the way different transactions?

One approach would be to correlate all of your logs using a correlation Id. If an incoming request has no correlation id header, the API creates it. If there is one, it uses it instead.

ℹ️ This is a topic we will address in the tracing section.