Headers not always propagated to MDC (with sample)

I want to read header values, and have them available on MDC. One single request works, but not when a test does many requests after each other.

Complete project that reproduces the error every time (on my computer, Java 17). Most often after only 6-7 iterations:

https://github.com/anderius/spring-sleuth-problem-demo

Excerpt from the sample:

@RestController
@SpringBootApplication
class DemoApplication {

    @Bean
    fun sleuthHttpServerRequestParser() = HttpRequestParser { request, context, _ ->
        request.header("customHeader")
            ?.let { BaggageField.create("customField").updateValue(context, it) }
    }

    @Bean
    fun spanHandler(): SpanHandler = SpanHandler.NOOP

    @GetMapping("/mdc")
    fun getMdc(): Map<String, String> = MDC.getCopyOfContextMap()
}

This issue might be related to https://github.com/spring-cloud/spring-cloud-sleuth/issues/2064.

[anderius]

@marcingrzejszczak Thank you for looking into this!

That means Spring Cloud Sleuth should not be used for any headers other than the ones used for sampling/tracing, unless we sample everything.

I had the idea the sampling was only how much was sent to zipkin (or similar), but it is also how much is instrumentated.

[anderius]

@marcingrzejszczak Maven and Java is ready! 😃

Project is same place: https://github.com/anderius/spring-sleuth-problem-demo

The setup in java:

@RestController
@SpringBootApplication
public class DemoApplication {

    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);
    }

    @Bean
    HttpRequestParser sleuthHttpServerRequestParser() {
        return (request, context, span) -> {
            String headerValue = request.header("customHeader");
            if (headerValue != null) {
                BaggageField.create("customField").updateValue(context, headerValue);
            }
        };
    }

    @Bean
    SpanHandler spanHandler() {
        return SpanHandler.NOOP;
    }

    @GetMapping("/mdc")
    Map<String, String> getMdc() {
        return MDC.getCopyOfContextMap();
    }
}