private String getLayer(String serviceId) throws IOException {
        Service service = getMetadataQueryService().getService(serviceId);
        if (service != null) {
            return service.getLayers().iterator().next();
        } else {
            return Layer.UNDEFINED.name();
        }
    }
EventCallBack: When an alert is present, an event is generated for each alert message. These events are then sent to the internal event analyzer.
oap-server/server-alarm-plugin/src/main/java/org/apache/skywalking/oap/server/core/alarm/provider/EventHookCallback.java