Dubbo3 服务发现的推空保护

推空保护是Dubbo服务发现新增的一个提升稳定性的功能，目的是当注册中心出现抖动，consumer接收到注册中心推送了空provider urls过来时，直接忽略掉，从而保护consumer维护的服务发现信息不受影响。

但是在某些场景下却是会出现异常，我们做个实验：
1､启动Provider A， ip为192.168.1.10
2､启动Provider B, ip为192.168.1.11
3､启动Consumer, 调用Provider A和Provider B， 此时正常调用
4､停掉Provider A 和Provider B
5､在ip为192.168.1.10的机器上重新启动Provider B

这时再在Consumer 调用Provider A和Provider B， 调用Provider B正常，但是调用Provider A 却会报错：

org.apache.dubbo.rpc.RpcException: Failed to invoke the method sayHello in the service org.apache.dubbo.springboot.demo.ProviderAService. Tried 3 times of the providers [192.168.1.9:20880] (1/1) from the registry 127.0.0.1:2181 on the consumer 192.168.1.9 using the dubbo version 3.0.8.1-SNAPSHOT. Last error is: Failed to invoke remote method: sayHello, provider: dubbo://192.168.1.9:20880/org.apache.dubbo.springboot.demo.ProviderAService?anyhost=true&application=dubbo-springboot-demo-provider&background=false&category=providers,configurators,routers&check=false&deprecated=false&dubbo=2.0.2&dynamic=true&generic=false&interface=org.apache.dubbo.springboot.demo.DService&methods=sayHello,sayHelloAsync&payload=10004&pid=19514&qos.enable=false&register-mode=interface&release=3.0.8.1-SNAPSHOT&side=provider&sticky=false, cause: org.apache.dubbo.remoting.RemotingException: Fail to decode request due to: java.io.IOException: Service org.apache.dubbo.springboot.demo.DemoService with version 0.0.0 not found, invocation rejected.
java.io.IOException: Service org.apache.dubbo.springboot.demo.DemoService with version 0.0.0 not found, invocation rejected.
	at org.apache.dubbo.remoting.transport.CodecSupport.checkSerialization(CodecSupport.java:166)
	at org.apache.dubbo.rpc.protocol.dubbo.DecodeableRpcInvocation.decode(DecodeableRpcInvocation.java:138)
	at org.apache.dubbo.rpc.protocol.dubbo.DecodeableRpcInvocation.decode(DecodeableRpcInvocation.java:93)
	at org.apache.dubbo.remoting.transport.DecodeHandler.decode(DecodeHandler.java:57)
	at org.apache.dubbo.remoting.transport.DecodeHandler.received(DecodeHandler.java:44)
	at org.apache.dubbo.remoting.transport.dispatcher.ChannelEventRunnable.run(ChannelEventRunnable.java:57)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	at org.apache.dubbo.common.threadlocal.InternalRunnable.run(InternalRunnable.java:41)
	at java.lang.Thread.run(Thread.java:750)

	at org.apache.dubbo.rpc.cluster.support.FailoverClusterInvoker.doInvoke(FailoverClusterInvoker.java:114)
	at org.apache.dubbo.rpc.cluster.support.AbstractClusterInvoker.invoke(AbstractClusterInvoker.java:340)
	at org.apache.dubbo.rpc.cluster.router.RouterSnapshotFilter.invoke(RouterSnapshotFilter.java:46)
	at org.apache.dubbo.rpc.cluster.filter.FilterChainBuilder$CopyOfFilterChainNode.invoke(FilterChainBuilder.java:321)
	at org.apache.dubbo.monitor.support.MonitorFilter.invoke(MonitorFilter.java:99)
	at org.apache.dubbo.rpc.cluster.filter.FilterChainBuilder$CopyOfFilterChainNode.invoke(FilterChainBuilder.java:321)
	at org.apache.dubbo.rpc.protocol.dubbo.filter.FutureFilter.invoke(FutureFilter.java:51)
	at org.apache.dubbo.rpc.cluster.filter.FilterChainBuilder$CopyOfFilterChainNode.invoke(FilterChainBuilder.java:321)
	at org.apache.dubbo.rpc.cluster.filter.support.ConsumerContextFilter.invoke(ConsumerContextFilter.java:110)
	at org.apache.dubbo.rpc.cluster.filter.FilterChainBuilder$CopyOfFilterChainNode.invoke(FilterChainBuilder.java:321)
	at org.apache.dubbo.rpc.cluster.filter.FilterChainBuilder$CallbackRegistrationInvoker.invoke(FilterChainBuilder.java:193)
	at org.apache.dubbo.rpc.cluster.support.wrapper.AbstractCluster$ClusterFilterInvoker.invoke(AbstractCluster.java:92)
	at org.apache.dubbo.rpc.cluster.support.wrapper.MockClusterInvoker.invoke(MockClusterInvoker.java:97)
	at org.apache.dubbo.registry.client.migration.MigrationInvoker.invoke(MigrationInvoker.java:283)
	at org.apache.dubbo.rpc.proxy.InvocationUtil.invoke(InvocationUtil.java:57)
	at org.apache.dubbo.rpc.proxy.InvokerInvocationHandler.invoke(InvokerInvocationHandler.java:73)
	at org.apache.dubbo.springboot.demo.DemoServiceDubboProxy1.sayHello(DemoServiceDubboProxy1.java)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at org.springframework.aop.support.AopUtils.invokeJoinpointUsingReflection(AopUtils.java:344)
	at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:205)
	at com.sun.proxy.$Proxy54.sayHello(Unknown Source)
	at org.apache.dubbo.springboot.demo.consumer.ConsumerApplication.doSayHello(ConsumerApplication.java:61)
	at org.apache.dubbo.springboot.demo.consumer.ConsumerApplication.main(ConsumerApplication.java:49)

观察日志发现，consumer并没有识别到Provider A目前并没有实例，仍然认为192.168.1.10也是Provider A的实例，将调用Provider A的请求发送过去，此时192.168.1.10服务器上运行的却是Provider B应用，无法处理该请求，就抛出异常。

如图所示，Consumer 第一次访问Provider A和Provider B时，consumer维护的信息和实际的部署信息是一致的

但是在Csonumser第二次访问Provider A和Provider B时，consumer维护的信息和实际的部署信息已经不一致了

出现这个情况的原因是dubbo3默认开启的推空保护（ZookeeperRegistry$RegistryChildListenerImpl.doNotify()方法会ZookeeperRegistry.toUrlsWithEmpty()方法获取Provider Urls，在该方法中会判断推空保护的开关），
本意是防止注册中心抖动，可是在某些场景下会引发问题，例如：
应用最后一台实例下线后，ip被回收，其它应用发布时从ip池获取到该ip并使用
这种情况在测试环境更容易出现，因为测试环境普遍都是一个实例。

如果要在测试环境避免这种情况，目前最好的办法是关闭推空保护：

• RegistryConfig.enableEmptyProtection 设置为false
• springboot 应用中配置参数 dubbo.registry.enable-empty-protection=false

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