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操作步骤如下：
1. k8s环境下，consumer端开启mesh模式
2. consumer端需要调用多个provider
3. provider有应用级注册和接口级注册，并且需要的provider不再同一个namespace下
4. 原有项目存在dubbo2.6与3.1多版本间的相互调用，在开启消费端mesh（dubbo3.1）后，无法调用到2.6的服务提供者

**问题：**

1. 如何实现在一个consumer下，调用多个namespace下的provider，并且provider有应用级注册和接口级注册
2. dubbo3.1消费端在开启mesh后，如何调用版本是2.6的服务提供者
invoker = Cluster.getCluster(scopeModel, cluster).join(new StaticDirectory(curUrl, invokers), true);
            }
        }
    }
    private void checkInvokerAvailable() throws IllegalStateException {
        if (shouldCheck() && !invoker.isAvailable()) {
            // 
            IllegalStateException illegalStateException = new IllegalStateException("Failed to check the status of the service "
                    + interfaceName
                    + ". No provider available for the service "
                    + (group == null ? "" : group + "/")
                    + interfaceName +
                    (version == null ? "" : ":" + version)
                    + " from the url "
                    + invoker.getUrl()
                    + " to the consumer "
                    + NetUtils.getLocalHost() + " use dubbo version " + Version.getVersion());
            logger.error(CLUSTER_NO_VALID_PROVIDER, "provider not started", "", "No provider available.", illegalStateException);
            throw illegalStateException;
        }
    }
    /**
     * This method should be called right after the creation of this class's instance, before any property in other config modules is used.
     * Check each config modules are created properly and override their properties if necessary.
     */
    protected void checkAndUpdateSubConfigs() {
        if (StringUtils.isEmpty(interfaceName)) {
            throw new IllegalStateException("<dubbo:reference interface=\"\" /> interface not allow null!");
        }
        //