private Map<String, List<Service>> mapAllServices() throws Exception {
        final List<Service> services = getMetadataQueryDAO().listServices();
        return services.stream().peek(service -> {
            if (service.getGroup() == null) {
                service.setGroup(Const.EMPTY_STRING);
            }
        }).collect(Collectors.toMap(Service::getId, s -> new ArrayList<>(List.of(s)), (s1, s2) -> {
            s1.addAll(s2);
            return s1;
        }));
    }
@return all services, key is service id, value is services with different layers.
oap-server/server-core/src/main/java/org/apache/skywalking/oap/server/core/query/MetadataQueryService.java