Propagation of MDC to a Feign Thread: A Comprehensive Guide

Are you tired of dealing with the complexities of Microservices architecture? Do you struggle to maintain context between services? Look no further! In this article, we’ll delve into the world of MDC (Mapped Diagnostic Context) and Feign threads, and explore how to propagate MDC to a Feign thread. Buckle up, folks, and let’s dive in!

What is MDC?

MDC, or Mapped Diagnostic Context, is a mechanism for passing diagnostic information, such as trace IDs, correlation IDs, and other context data, between different components of an application. It’s a crucial component of distributed tracing and logging, allowing developers to maintain context and visibility across services. In essence, MDC is like a thread-local storage for diagnostic data.

Why do we need MDC propagation?

In a Microservices architecture, each service is a separate entity, and maintaining context between them can be a daunting task. Without MDC propagation, each service would need to replicate the same diagnostic data, leading to redundancy and potential errors. By propagating MDC, we ensure that diagnostic data is shared seamlessly between services, enabling better logging, tracing, and debugging.

What is Feign?

Feign is a popular Java library for building declarative web services. It provides a simple, annotation-based way to create web services that can be easily consumed by clients. Feign uses a dynamic proxy to intercept and handle HTTP requests, allowing developers to focus on the business logic of their services.

Why do we need to propagate MDC to a Feign thread?

When using Feign to make HTTP requests to other services, the MDC context is lost by default. This is because Feign creates a new thread for each request, which doesn’t inherit the MDC context from the original thread. Without propagating MDC to the Feign thread, we’d lose valuable diagnostic information, making it difficult to troubleshoot and debug our services.

How to Propagate MDC to a Feign Thread

Now that we’ve established the importance of MDC propagation, let’s dive into the nitty-gritty of how to do it. We’ll explore two approaches: using Feign’s built-in support and implementing a custom solution using MDC-enabled interceptors.

Approach 1: Using Feign’s built-in support

Feign provides a built-in mechanism for propagating MDC context to the Feign thread. We can achieve this by enabling the ` propagation` property in the Feign client configuration.

@FeignClient(name = "my-service", url = "https://my-service.com")
public interface MyServiceClient {
  
  @GetMapping("/api/data")
  List<Data> getData();
  
  @Configuration
  @Bean
  public Feign.Builder feignBuilder() {
    return Feign.builder()
        . propulsion(new ThreadLocalPropagation())
        .target(MyServiceClient.class);
  }
}

In this example, we’ve enabled the `propulsion` property and specified the `ThreadLocalPropagation` strategy. This tells Feign to propagate the MDC context from the original thread to the Feign thread.

Approach 2: Implementing a custom solution using MDC-enabled interceptors

While Feign’s built-in support is convenient, it may not be sufficient for all scenarios. In this approach, we’ll implement a custom MDC-enabled interceptor to propagate the MDC context to the Feign thread.

public class MdcPropagatingInterceptor implements RequestInterceptor {
  
  @Override
  public void apply(RequestTemplate template) {
    MDCContext context = MDC.getCopyOfContextMap();
    if (context != null) {
      template.header("X-MDC-Context", context);
    }
  }
}

In this example, we’ve created an `MdcPropagatingInterceptor` that captures the current MDC context and adds it as a header to the Feign request. We can then configure Feign to use this interceptor.

@FeignClient(name = "my-service", url = "https://my-service.com")
public interface MyServiceClient {
  
  @GetMapping("/api/data")
  List<Data> getData();
  
  @Configuration
  @Bean
  public Feign.Builder feignBuilder() {
    return Feign.builder()
        .requestInterceptor(new MdcPropagatingInterceptor())
        .target(MyServiceClient.class);
  }
}

Best Practices for MDC Propagation

Now that we’ve explored the different approaches for propagating MDC to a Feign thread, let’s highlight some best practices to keep in mind:

• Always enable MDC propagation for all Feign clients to maintain consistent logging and tracing across services.

• Use a unique MDC key for each service to avoid conflicts and ensure clear visibility.

• Log the MDC context at the start and end of each service to facilitate debugging and troubleshooting.

• Consider using a centralized MDC management system to simplify configuration and management across services.

Conclusion

In this article, we’ve delved into the world of MDC and Feign threads, exploring the importance of propagating MDC context and how to achieve it using Feign’s built-in support and custom MDC-enabled interceptors. By following the best practices outlined above, you’ll be well on your way to maintaining a robust and context-rich Microservices architecture. Remember, MDC propagation is not just a nice-to-have; it’s a must-have for ensuring the reliability and scalability of your distributed systems.

MDC Propagation Approach	Description
Using Feign’s built-in support	Enable the `propulsion` property in the Feign client configuration.
Implementing a custom solution using MDC-enabled interceptors	Create a custom MDC-enabled interceptor to propagate the MDC context to the Feign thread.

By propagating MDC to a Feign thread, you’ll unlock the full potential of distributed tracing and logging, allowing you to build more resilient and maintainable Microservices architecture. Happy coding!

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