dubbo与zookeeper的交互

读了几次源码之后，感觉对dubbo很是熟练了，结果当别人一问，dubbo与zookeeper是怎么交互的，就蒙了，带着问题去看源码或许是最好的学习方式了吧。
众所周知provider端在发布服务的时候，会在注册中心注册服务，主要流程体现在RegistryProtocol#export方法

public <T> Exporter<T> export(final Invoker<T> originInvoker) throws RpcException {
        URL registryUrl = getRegistryUrl(originInvoker);
        // url to export locally
        URL providerUrl = getProviderUrl(originInvoker);

        // Subscribe the override data
        // FIXME When the provider subscribes, it will affect the scene : a certain JVM exposes the service and call
        //  the same service. Because the subscribed is cached key with the name of the service, it causes the
        //  subscription information to cover.
        final URL overrideSubscribeUrl = getSubscribedOverrideUrl(providerUrl);
        final OverrideListener overrideSubscribeListener = new OverrideListener(overrideSubscribeUrl, originInvoker);
        overrideListeners.put(overrideSubscribeUrl, overrideSubscribeListener);

        providerUrl = overrideUrlWithConfig(providerUrl, overrideSubscribeListener);
        //export invoker
        final ExporterChangeableWrapper<T> exporter = doLocalExport(originInvoker, providerUrl);

        // url to registry
        final Registry registry = getRegistry(originInvoker);
        final URL registeredProviderUrl = getRegisteredProviderUrl(providerUrl, registryUrl);
        ProviderInvokerWrapper<T> providerInvokerWrapper = ProviderConsumerRegTable.registerProvider(originInvoker,
                registryUrl, registeredProviderUrl);
        //to judge if we need to delay publish
        boolean register = registeredProviderUrl.getParameter("register", true);
        if (register) {
        	//进行服务注册
            register(registryUrl, registeredProviderUrl);
            providerInvokerWrapper.setReg(true);
        }

        // Deprecated! Subscribe to override rules in 2.6.x or before.
        registry.subscribe(overrideSubscribeUrl, overrideSubscribeListener);

        exporter.setRegisterUrl(registeredProviderUrl);
        exporter.setSubscribeUrl(overrideSubscribeUrl);
        //Ensure that a new exporter instance is returned every time export
        return new DestroyableExporter<>(exporter);
    }

同样在这个类里面的doRefer方法，消费端也会注册一个consumer节点，并且订阅我们的服务端信息，关于订阅服务信息，之前的文章有提到过，这里不再概述。而关于具体的注册逻辑，在ZookeeperRegistry#doRegister方法里，该类的doSubscribe也是基于zk的watcher实现了订阅。

private <T> Invoker<T> doRefer(Cluster cluster, Registry registry, Class<T> type, URL url) {
        RegistryDirectory<T> directory = new RegistryDirectory<T>(type, url);
        directory.setRegistry(registry);
        directory.setProtocol(protocol);
        // all attributes of REFER_KEY
        Map<String, String> parameters = new HashMap<String, String>(directory.getUrl().getParameters());
        URL subscribeUrl = new URL(CONSUMER_PROTOCOL, parameters.remove(REGISTER_IP_KEY), 0, type.getName(), parameters);
        if (!ANY_VALUE.equals(url.getServiceInterface()) && url.getParameter(REGISTER_KEY, true)) {
            directory.setRegisteredConsumerUrl(getRegisteredConsumerUrl(subscribeUrl, url));
            //向注册中心注册消费端信息
            registry.register(directory.getRegisteredConsumerUrl());
        }
        directory.buildRouterChain(subscribeUrl);
        //订阅服务端信息
        directory.subscribe(subscribeUrl.addParameter(CATEGORY_KEY,
                PROVIDERS_CATEGORY + "," + CONFIGURATORS_CATEGORY + "," + ROUTERS_CATEGORY));

        Invoker invoker = cluster.join(directory);
        ProviderConsumerRegTable.registerConsumer(invoker, url, subscribeUrl, directory);
        return invoker;
    }

最后借用官网的图片

总结

dubbo服务端会在注册中心注册服务信息，provider挂掉或者异常时，注册中心会剔除这些服务。消费端也会在注册中心的某个服务节点**册消费端信息，并且还会订阅服务端信息，服务发生变化时，也能及时获取服务列表。