@Override
            public void onMessage(final REQ message) {
                if (isWatermarkExceeded()) {
                    call.close(Status.RESOURCE_EXHAUSTED.withDescription("Watermark exceeded"), new Metadata());
                    logWarning("Watermark exceeded, reject the gRPC request by Circuit Breaking mechanism.");
                    return;
                }

                super.onMessage(message);
            }
gRPCWatermarkInterceptor is a gRPC interceptor that checks if the watermark is exceeded before processing the request.
oap-server/server-core/src/main/java/org/apache/skywalking/oap/server/core/watermark/WatermarkGRPCInterceptor.java