一、客户端获取服务实例流程(以dubbo为例)

1.dubbo元数据服务初始化需要订阅的服务列表

1.1.获取与当前服务相同分组和集群的NACOS的注册服务列表。

1.2 首先是从spring-cloud-common的通用注册中心中，使用组合注册客户端类获取服务，此组合会逐个调用注册客户端进行获取。

1.3 接下来向NACOS服务端获取服务列表。看到只返回了loveday这个服务提供者。

2.接下来获取服务的实例列表。

2.1 获取NacosDiscoverClient的实例接口。

2.2.注意，这里有一个hostreactor类，会定时从服务器更新最新的服务实例列表。

2.3 最终的获取服务实例列表，也是调用 NACOS服务端的HTTP接口

2.4 在获取实例时也会同时添加一个定时更新任务。定期从服务端拉取最新的服务列表到本地内存。此类的serviceInfoMap就是存服务对应的内存缓存实例列表。

public class HostReactor implements Closeable {
private final Map<String, ServiceInfo> serviceInfoMap;

    public synchronized ScheduledFuture<?> addTask(UpdateTask task) {
        return executor.schedule(task, DEFAULT_DELAY, TimeUnit.MILLISECONDS);
    }

public ServiceInfo getServiceInfo(final String serviceName, final String clusters) {
        
        NAMING_LOGGER.debug("failover-mode: " + failoverReactor.isFailoverSwitch());
        String key = ServiceInfo.getKey(serviceName, clusters);
        if (failoverReactor.isFailoverSwitch()) {
            return failoverReactor.getService(key);
        }
        
        ServiceInfo serviceObj = getServiceInfo0(serviceName, clusters);
        
        if (null == serviceObj) {
            serviceObj = new ServiceInfo(serviceName, clusters);
            
            serviceInfoMap.put(serviceObj.getKey(), serviceObj);
            
            updatingMap.put(serviceName, new Object());
            updateServiceNow(serviceName, clusters);
            updatingMap.remove(serviceName);
            
        } else if (updatingMap.containsKey(serviceName)) {
            
            if (UPDATE_HOLD_INTERVAL > 0) {
                // hold a moment waiting for update finish
                synchronized (serviceObj) {
                    try {
                        serviceObj.wait(UPDATE_HOLD_INTERVAL);
                    } catch (InterruptedException e) {
                        NAMING_LOGGER
                                .error("[getServiceInfo] serviceName:" + serviceName + ", clusters:" + clusters, e);
                    }
                }
            }
        }
        
        scheduleUpdateIfAbsent(serviceName, clusters);
        
        return serviceInfoMap.get(serviceObj.getKey());
    }

    public void scheduleUpdateIfAbsent(String serviceName, String clusters) {
        if (futureMap.get(ServiceInfo.getKey(serviceName, clusters)) != null) {
            return;
        }
        
        synchronized (futureMap) {
            if (futureMap.get(ServiceInfo.getKey(serviceName, clusters)) != null) {
                return;
            }
            
            ScheduledFuture<?> future = addTask(new UpdateTask(serviceName, clusters));
            futureMap.put(ServiceInfo.getKey(serviceName, clusters), future);
        }
    }


}

2.5 最核心的更新实例任务类

public class UpdateTask implements Runnable {
        
        long lastRefTime = Long.MAX_VALUE;
        
        private final String clusters;
        
        private final String serviceName;
        
        /**
         * the fail situation. 1:can't connect to server 2:serviceInfo's hosts is empty
         */
        private int failCount = 0;
        
        public UpdateTask(String serviceName, String clusters) {
            this.serviceName = serviceName;
            this.clusters = clusters;
        }
        
        private void incFailCount() {
            int limit = 6;
            if (failCount == limit) {
                return;
            }
            failCount++;
        }
        
        private void resetFailCount() {
            failCount = 0;
        }
        
        @Override
        public void run() {
            long delayTime = DEFAULT_DELAY;
            
            try {
                ServiceInfo serviceObj = serviceInfoMap.get(ServiceInfo.getKey(serviceName, clusters));
                
                if (serviceObj == null) {
                    updateService(serviceName, clusters);
                    return;
                }
                
                if (serviceObj.getLastRefTime() <= lastRefTime) {
                    updateService(serviceName, clusters);
                    serviceObj = serviceInfoMap.get(ServiceInfo.getKey(serviceName, clusters));
                } else {
                    // if serviceName already updated by push, we should not override it
                    // since the push data may be different from pull through force push
                    refreshOnly(serviceName, clusters);
                }
                
                lastRefTime = serviceObj.getLastRefTime();
                
                if (!notifier.isSubscribed(serviceName, clusters) && !futureMap
                        .containsKey(ServiceInfo.getKey(serviceName, clusters))) {
                    // abort the update task
                    NAMING_LOGGER.info("update task is stopped, service:" + serviceName + ", clusters:" + clusters);
                    return;
                }
                if (CollectionUtils.isEmpty(serviceObj.getHosts())) {
                    incFailCount();
                    return;
                }
                delayTime = serviceObj.getCacheMillis();
                resetFailCount();
            } catch (Throwable e) {
                incFailCount();
                NAMING_LOGGER.warn("[NA] failed to update serviceName: " + serviceName, e);
            } finally {
                executor.schedule(this, Math.min(delayTime << failCount, DEFAULT_DELAY * 60), TimeUnit.MILLISECONDS);
            }
        }
    }

2.6 定时任务更新实例列表到内存完成。