@Override
        public void onNext(O outMesg) {
            try (TaskCloseable ignored = PerfMark.traceTask(filter, f -> f.filterName() + ".onNextAsync")) {
                PerfMark.linkIn(onNextLinkOut.get());
                addPerfMarkTags(inMesg);
                this.outMesg = outMesg;
            } catch (Exception e) {
                decrementConcurrency();
                handleException(inMesg, filter.filterName(), e);
            }
        }
This is typically set by a filter when wanting to reject a request and also reduce load on the server by
 not processing anymore filterChain
zuul-core/src/main/java/com/netflix/zuul/netty/filter/BaseZuulFilterRunner.java