private MetadataQueryService getMetadataQueryService() {
        if (metadataQueryService == null) {
            this.metadataQueryService = manager.find(CoreModule.NAME)
                                                     .provider()
                                                     .getService(MetadataQueryService.class);
        }
        return metadataQueryService;
    }
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