A simple WebFilter (from Spring WebFlux framework) for correct handling of 'rid' URL query parameter as a similarly-named MDC entry (even in threads spawned by Reactor's Schedulers)

ReactiveMdcFilter.java

package pro.toparvion.sample.gateway.gatewaydemo;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.slf4j.MDC;
import org.springframework.boot.autoconfigure.condition.ConditionalOnWebApplication;
import org.springframework.stereotype.Component;
import org.springframework.web.server.ServerWebExchange;
import org.springframework.web.server.WebFilter;
import org.springframework.web.server.WebFilterChain;
import reactor.core.publisher.Mono;
import reactor.core.scheduler.Schedulers;

import javax.annotation.PostConstruct;
import javax.annotation.PreDestroy;

import static org.springframework.boot.autoconfigure.condition.ConditionalOnWebApplication.Type.REACTIVE;

@Component
@ConditionalOnWebApplication(type = REACTIVE)
public class ReactiveMdcFilter implements WebFilter {
  private static final Logger log = LoggerFactory.getLogger(ReactiveMdcFilter.class);
  private static final String RID = "rid";
  
  @Override
  public Mono<Void> filter(ServerWebExchange exchange, WebFilterChain chain) {
    String rid = exchange.getRequest().getQueryParams().getFirst(RID);
    return chain.filter(exchange)
      .doOnSubscribe(subscription -> {
        MDC.put(RID, rid);
        log.trace("rid={} has been set in filter", rid);
      })
      .doFinally(whatever -> {
        MDC.remove(RID);
        log.trace("rid={} has been removed in filter", rid);
      });
  }

  @PostConstruct
  void setupReactorThreadsDecorator() {
    Schedulers.onScheduleHook("mdc", runnable -> {
      String rid = MDC.get(RID);        
      return () -> {
        MDC.put(RID, rid);     
        log.trace("'rid' mark has been assigned to the thread");
        try {
          runnable.run();
        } finally {
          MDC.remove(RID);
          log.trace("'rid' mark removed from the thread");
        }
      };
    });
    log.info("Reactive MDC hook has been set");
  }

  @PreDestroy
  void shutdownThreadsDecorator() {
    Schedulers.resetOnScheduleHook("mdc");
    log.info("Reactive MDC hook has been removed");
  }
}