@Override
    public Class[] services() {
        List<Class> classes = new ArrayList<>();
        classes.add(ConfigService.class);
        classes.add(ServerStatusService.class);
        classes.add(DownSamplingConfigService.class);
        classes.add(NamingControl.class);
        classes.add(IComponentLibraryCatalogService.class);
        classes.add(HierarchyDefinitionService.class);

        classes.add(IWorkerInstanceGetter.class);
        classes.add(IWorkerInstanceSetter.class);

        classes.add(MeterSystem.class);

        addServerInterface(classes);
        addReceiverInterface(classes);
        addInternalServices(classes);
        addCacheService(classes);
        addQueryService(classes);
        addProfileService(classes);
        addOALService(classes);
        addManagementService(classes);
        addEBPFProfilingService(classes);
        addAsyncProfilerService(classes);

        classes.add(CommandService.class);
        classes.add(HierarchyService.class);
        classes.add(EndpointNameGroupService.class);
        return classes.toArray(new Class[]{});
    }
Core module definition. Define all open services to other modules.
oap-server/server-core/src/main/java/org/apache/skywalking/oap/server/core/CoreModule.java