技术

Nacos Naming Server源码分析-服务订阅

服务订阅的目的是为了，能够第一时间感知到被调用服务的实例变化，从而能够及时更新本地缓存，避免调用失败。

subscribe和unsubscribe的逻辑，只有临时实例才有，永久实例是不存在订阅逻辑的。所以还是比较推荐用临时类型的服务注册，这也是官方客户端默认的类型

看一下服务调用的逻辑如何实现的：

SubscribeServiceRequestHandler#handle(*)

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public SubscribeServiceResponse handle(SubscribeServiceRequest request, RequestMeta meta) throws NacosException {
    String namespaceId = request.getNamespace();
    String serviceName = request.getServiceName();
    String groupName = request.getGroupName();
    String app = request.getHeader("app", "unknown");
    String groupedServiceName = NamingUtils.getGroupedName(serviceName, groupName);
    Service service = Service.newService(namespaceId, groupName, serviceName, true);
    Subscriber subscriber = new Subscriber(meta.getClientIp(), meta.getClientVersion(), app, meta.getClientIp(),
            namespaceId, groupedServiceName, 0, request.getClusters());
    ServiceInfo serviceInfo = ServiceUtil.selectInstancesWithHealthyProtection(serviceStorage.getData(service), 
            metadataManager.getServiceMetadata(service).orElse(null), subscriber.getCluster(), false,
            true, subscriber.getIp());//做了按照cluster过滤。并且还存在防止服务实例过少，被调用方打爆的保护措施
    if (request.isSubscribe()) {
        clientOperationService.subscribeService(service, subscriber, meta.getConnectionId()); //继续往下看
        NotifyCenter.publishEvent(new SubscribeServiceTraceEvent(System.currentTimeMillis(),
                meta.getClientIp(), service.getNamespace(), service.getGroup(), service.getName()));
    } else {
        clientOperationService.unsubscribeService(service, subscriber, meta.getConnectionId());
        NotifyCenter.publishEvent(new UnsubscribeServiceTraceEvent(System.currentTimeMillis(),
                meta.getClientIp(), service.getNamespace(), service.getGroup(), service.getName()));
    }
    return new SubscribeServiceResponse(ResponseCode.SUCCESS.getCode(), "success", serviceInfo);
}

-> EphemeralClientOperationServiceImpl#subscribe(*)

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public void subscribeService(Service service, Subscriber subscriber, String clientId) {
    Service singleton = ServiceManager.getInstance().getSingletonIfExist(service).orElse(service);
    Client client = clientManager.getClient(clientId);
    checkClientIsLegal(client, clientId);
    client.addServiceSubscriber(singleton, subscriber); //继续往下看
    client.setLastUpdatedTime();
    NotifyCenter.publishEvent(new ClientOperationEvent.ClientSubscribeServiceEvent(singleton, clientId));
}

和上一篇一样，大部头的逻辑都是通过事件机制驱动的：

  1. SubscribeServiceTraceEvent //处理逻辑留给用户进行扩展，所以没啥好讲的
  2. ClientOperationEvent.ClientSubscribeServiceEvent 处理函数如下
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    //ClientServiceIndexesManager.class
     private void addSubscriberIndexes(Service service, String clientId) {
         Set<String> clientIds = subscriberIndexes.computeIfAbsent(service, key -> new ConcurrentHashSet<>());
         // Fix #5404, Only first time add need notify event.
         if (clientIds.add(clientId)) {//客户端第一次订阅服务，才会触发事件
             NotifyCenter.publishEvent(new ServiceEvent.ServiceSubscribedEvent(service, clientId));
         }
     }
  3. ServiceEvent.ServiceSubscribedEvent
    这里也就是客户端第一次订阅该服务，才会触发该事件，做一次服务实例信息推送。
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    //NamingSubscriberServiceV2Impl.class
     public void onEvent(Event event) {
         if (event instanceof ServiceEvent.ServiceChangedEvent) {
             // If service changed, push to all subscribers.
             ServiceEvent.ServiceChangedEvent serviceChangedEvent = (ServiceEvent.ServiceChangedEvent) event;
             Service service = serviceChangedEvent.getService();
             delayTaskEngine.addTask(service, new PushDelayTask(service, PushConfig.getInstance().getPushTaskDelay()));
             MetricsMonitor.incrementServiceChangeCount(service);
         } else if (event instanceof ServiceEvent.ServiceSubscribedEvent) {
             // If service is subscribed by one client, only push this client.
             ServiceEvent.ServiceSubscribedEvent subscribedEvent = (ServiceEvent.ServiceSubscribedEvent) event;
             Service service = subscribedEvent.getService();
             delayTaskEngine.addTask(service, new PushDelayTask(service, PushConfig.getInstance().getPushTaskDelay(),
                     subscribedEvent.getClientId()));
         }
     }

总结：

  1. 服务订阅功能的目的是避免服务调用失败
  2. 服务订阅第一次发起，服务端会全量推一次订阅服务所有实例信息