Support service change event

[maketubo]

#2133 What is the purpose of the change

to give a api to listen special event like (service remove, service register or just service info change).

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[maketubo]

if the PR be accept, for Nacos Spring Cloud Projects, we can add a eventpublisher and a evenlister as this simple spring event listener example, which u can very free to design for your client project story.

public class NacosInstanceCanceledEvent extends ApplicationEvent {

    GlobalEventType type;

    ServiceInfo oldInfo;

    ServiceInfo newInfo;

    public NacosInstanceCanceledEvent(Object source, GlobalEventType type, ServiceInfo oldInfo, ServiceInfo newInfo) {
        super(source);
        this.type = type;
        this.oldInfo = oldInfo;
        this.newInfo = newInfo;
    }

    //getter and setter
}

@Component
public class SpringEventPublisher extends EventPublisher implements ApplicationContextAware {


    ApplicationContext applicationContext;

    @Override
    public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
        this.applicationContext = applicationContext;
    }

    @Override
    public void publish(GlobalEventType type, ServiceInfo oldInfo, ServiceInfo newInfo) {
        applicationContext.publishEvent(new NacosInstanceCanceledEvent(this, type, oldInfo, newInfo));
    }
}

@Component
public class NacosStateChangeListener {

    @EventListener
    public void listen(NacosInstanceCanceledEvent event) {
        System.out.println(event.getType() + "\t" + event.getOldInfo() + " 服务下线");
    }
}

what we need is just to find a agent way to set your EventPublisher to HostReactor.
may override NacosServiceRegistry constructor like this

public NacosServiceRegistry(NacosDiscoveryProperties nacosDiscoveryProperties,
								EventPublisher springEventPublisher) {
		this.nacosDiscoveryProperties = nacosDiscoveryProperties;
		this.namingService = nacosDiscoveryProperties.namingServiceInstance();
		((NacosNamingService)namingService).getHostReactor().setEventPublisher(springEventPublisher);
	}

[chuntaojun]

You just need to add an Event and reuse the EventDispatcher, it doesn't have to be that complicated

[maketubo]

Do you mean to listen to a namingEvent as this example?

NamingService namingService = NacosFactory.createNamingService("127.0.0.1:8848");
            namingService.subscribe("xxxServiceName", ... ,event->{
                System.out.println("i catch the event!");
            });

i know that EventDispatcher will pushlish namingEvent for changeService, but in issue 2133 what they need is a global event which not need to set a special service and other params.

nacos/client/src/main/java/com/alibaba/nacos/client/naming/core/EventDispatcher.java

Lines 129 to 134 in 4baaef4

	if (!CollectionUtils.isEmpty(listeners)) {
	for (EventListener listener : listeners) {
	List<Instance> hosts = Collections.unmodifiableList(serviceInfo.getHosts());
	listener.onEvent(new NamingEvent(serviceInfo.getName(), serviceInfo.getGroupName(), serviceInfo.getClusters(), hosts));
	}
	}

maybe something like zookeeper watcher. This is a zookeeper watcher example (随手百度第一个).

//连接启动k
        try {
            ZooKeeper zk = new ZooKeeper("10.218.137.73:2181", 500000,new Watcher() {
                   // 监控所有被触发的事件
                     public void process(WatchedEvent event) {
                     System.out.println("changing...");
                   }
              });
        //设置监听器 
            Watcher wc = new Watcher() {

                public void process(WatchedEvent event) {
                    // TODO Auto-generated method stub
                    if (event.getType() == EventType.NodeDataChanged) {  
                        System.out.println("change"); 
                    }  
                    if (event.getType() == EventType.NodeDeleted){  
                        System.out.println("dele");  
                    }  
                    if(event.getType()== EventType.NodeCreated){ 
                        System.out.println("create"); 
                }
                }

            };
            //进行轮询，其中exists方法用来询问状态，并且设置了监听器，如果发生变化，则会回调监听器里的方法。
            while(true)
            {
                zk.exists("/jianghuiwen", wc);
            }



        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (KeeperException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }



    }

This PR is a normal design like other Service Registry and their client.
https:/apache/zookeeper/blob/master/zookeeper-server/src/main/java/org/apache/zookeeper/Watcher.java
https:/apache/curator/blob/master/curator-framework/src/main/java/org/apache/curator/framework/api/CuratorEventType.java
(但是这个PR应该还是有点简单，没有考虑太多细节，期待您@chuntaojun 给出新的建议。也许直接改一下namingEvent就好了;;

[chuntaojun]

参考这个pr：https:/alibaba/nacos/pull/2391/files

[maketubo]

@chuntaojun 这个是issue作者的需求，其实我觉得这种业务不该由某个客户端来做，也许他们有自己的考虑吧。其实在zookeeper和nacos这样的CP>AP的注册中心才会考虑这样的客户端订阅事件。对于eureka这种AP>CP可能仅仅只是提供一个cacheRreshEvent 见此
你的意思是让他们还是指定一个servicename去调用还是提一个PR修改subscribe的入参提供一个全局的监听入口，实质上是修改pushlishEvent的逻辑，还是说建议他们放弃这个需求？

[maketubo]

他们现在应该是基于许长老的使用Nacos实现Spring Cloud Gateway的动态路由| 去做的

[chuntaojun]

可以说在NamingService添加一个方法，subscribeGlobalService，重新创建一个事件类型就好了，不必要添加一个EventPublisher，并且在nacos-client有一些东西不要暴露出去，比如HostReactor，

[maketubo]

我本来的想法是如此，但是后来想把接口暴露出去，让他们自己针对自己的业务实现一套发布和监听逻辑会不会更好。可以基于spring event去做（好管理，容易适配），又可以自己定义一个机制，基于队列和生成者消费者模型去做，publish的时候只是把事件放到队列中然后异步返回，有一个listenContainer来处理这个队列里的事件，这样的自由度就很高了，完全基于业务定制，而不是像现在直接去invoke onEvent，也许这个需求并没有复杂到这个程度。

[maketubo]

目前PR中的方式确实不够优雅，而且setEventPublisher是在构造函数即init逻辑设置之后进行的，严格来讲会有问题。

[chuntaojun]

这类就不要涉及任何spring的实现了，任何客户端的操作，都通过NamingService 2020年5月5日 下午6:15，maketubo <[email protected]<mailto:[email protected]>> 写道： 我本来的想法是如此，但是后来想把接口暴露出去，让他们自己针对自己的业务实现一套发布和监听逻辑会不会更好。可以基于spring event去做（好管理，容易适配），又可以自己定义一个机制，基于队列和生成者消费者模型去做，publish的时候只是把事件放到队列中然后异步返回，有一个listenContainer来处理这个队列里的事件，这样的自由度就很高了，完全基于业务定制，而不是像现在直接去invoke onEvent，也许这个需求并没有复杂到这个程度。 — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub<#2733 (comment)>, or unsubscribe<https:/notifications/unsubscribe-auth/AFS35NEQ5NZULSWSUO6OQNDRP7RMVANCNFSM4MY3533A>.

[maketubo]

@chuntaojun 很遗憾你没懂我的意思

[chuntaojun]

我觉得你把这个需求弄的过于复杂了，我的想法就只有一个，多一个Event类型和一个subscribe类型，没有其他额外的东西，因为我觉得你的设计中，外引入了一个publisher和subscriber，这对于我们后面的统一事件机制模式并不友好，而且我希望你在设计中，任何的操作，都是通过namingService去做，而不是在namingService提供的方法之外，就比如你说的接口暴露出去，这不是我所希望看到的 2020年5月5日 下午6:45，maketubo <[email protected]<mailto:[email protected]>> 写道： @chuntaojun<https:/chuntaojun> 很遗憾你没懂我的意思 — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub<#2733 (comment)>, or unsubscribe<https:/notifications/unsubscribe-auth/AFS35NBJ7EK4BJUDYM6UU7DRP7U3PANCNFSM4MY3533A>.

[chuntaojun]

至少对于目前的实现来说，我觉得并不好，期待你的重新提交

[maketubo]

我想我明白你的意思了，可能是表述和理解上的差异。我的事件切入点在PushReceiver收到消息的时候，

而你的事件切入点在监控changeService队列的EventDispatcher上。你需要的是一个是整合两者或者说把globalEvent就放在监控changeService队列的逻辑下面的实现。
目前需要找到一个方式拿到GlobalNamingEvent（假设有）的listener 然后针对不同的事件类型进行触发，修改的尴尬点在于目前listener列表是根据serviceInfo拿到的。你有什么建议么@chuntaojun

[maketubo]

你的意思应该是以下的伪代码，应该要在EventDispatcher维护一个以GlobalEventType为key的listener列表就可以。

namingService.subscribeGlobal(EventType.APP_UP, new EventListener() {
            @Override
            public void onEvent(Event event) {
                System.out.println(((NamingEvent)event).getServiceName() + "服务上线");
            }
        });

[chuntaojun]

你的意思应该是以下的伪代码，应该要在EventDispatcher维护一个以GlobalEventType为key的listener列表就可以。

namingService.subscribeGlobal(EventType.APP_UP, new EventListener() {
            @Override
            public void onEvent(Event event) {
                System.out.println(((NamingEvent)event).getServiceName() + "服务上线");
            }
        });

关于怎么触发各种类型的回调，可以看看这个

public abstract class TestEventListener implements EventListener {

	@Override
	public final void onEvent(Event event) {
		if (event instanceof GlobalEvent) {
			GlobalEvent globalEvent = (GlobalEvent) event;
			int type = globalEvent.getType();
			switch (type) {
			case 1:
				onServiceRegister(globalEvent);
				break;
			case 2:
				onServiceDown(globalEvent);
				break;
			case 3:
				onServiceModify(globalEvent);
				break;
			case 4:
				onServiceNormal(globalEvent);
				break;
			default:
				throw new IllegalArgumentException();
			}
		}
	}

	protected void onServiceRegister(GlobalEvent event) {

	}

	protected void onServiceDown(GlobalEvent event) {

	}

	protected void onServiceModify(GlobalEvent event) {

	}

	protected void onServiceNormal(GlobalEvent event) {

	}

}

[maketubo]

关于可能的改动点，可能如下

public class EventDispatcher {

    private ExecutorService executor = null;

    private BlockingQueue<ServiceInfo> changedServices = new LinkedBlockingQueue<ServiceInfo>();

    //private BlockingQueue<NamingEvent> globalEvents = new LinkedBlockingQueue<NamingEvent>();

    private ConcurrentMap<String, List<EventListener>> observerMap
        = new ConcurrentHashMap<String, List<EventListener>>();

    private ConcurrentMap<GlobalEventType, List<EventListener>> observerGlobalMap
        = new ConcurrentHashMap<GlobalEventType, List<EventListener>>();

   //  addGlobalListener() {
   //  to add a globalEventListener to observerGlobalMap
   //}
   ....

  NamingService增加一个接口
    void subscribeGlobal(GlobalEventType type, EventListener listener) throws NacosException;

实现类增加实现
@Override
    public void subscribeGlobal(String type, EventListener listener) throws NacosException {
        eventDispatcher.addGlobalListener(type, listener);
    }

接下来就是在 Notifier 遍历监听GlobalEvent的listener列表触发他们的onEvent了。

@chuntaojun 看到你提供的那个PR上的改动 觉得等那个PR的提交合到基线版本上再进行这个issue的处理也不迟。 其实我也想了一下，我的PR其实方向错了，其实我的意思，应该是给EventDispatcher抽象成一个接口，框架内部提供的只是默认的实现，外部可以重定义这个EventDispatcher，实现自己的发布逻辑，而无需再去定义一个eventPublisher。
但是毕竟我们在实际使用的过程中，自己去构建namingServie太少了，很多人都不知道springCloud项目中，EnableDiscoveryClient => NacosDiscoveryAutoConfiguration => 触发namingService的构建。而且本质上来说可能服务的上下线频率并不高，这并非一个需要特别设计和考虑性能的场景。我的PR显得鸡肋了，基于以上原因我关闭这个PR。