上一篇中進行負載均衡選擇具體服務時，是使用的Ribbon 的 LoadBalancer 的 chooseServer 方法，方法中是使用的 Ribbon的 Rule 的 choose 方法。

看一下Ribbon 的 LoadBalancer

1、LoadBalancer 接口

//com.netflix.loadbalancer.ILoadBalancer
public interface ILoadBalancer {
   //添加server
   public void addServers(List<Server> newServers);
	//選擇server
   public Server chooseServer(Object key);
	//標記server 掛掉
   public void markServerDown(Server server);
	//丟棄的方法，獲取serverList，根據是否可用
   @Deprecated
   public List<Server> getServerList(boolean availableOnly);
	//獲取可用的servers
    public List<Server> getReachableServers();
	//獲取所有的servers
   public List<Server> getAllServers();
}

2、Loadbalancer的抽象實現類

對比接口，添加了一個枚舉，添加了三個方法

public abstract class AbstractLoadBalancer implements ILoadBalancer {
    public enum ServerGroup{
        ALL,
        STATUS_UP,
        STATUS_NOT_UP        
    }
	//選擇server
    public Server chooseServer() {
       return chooseServer(null);
    }
	//根據分組獲取serverList
    public abstract List<Server> getServerList(ServerGroup serverGroup);
	//獲取負載均衡狀態
    public abstract LoadBalancerStats getLoadBalancerStats();    
}

AbstractLoadBalancer有兩個實現類BaseLoadBalancer，NoOpLoadBalancer

3、BaseLoadBalancer

public class BaseLoadBalancer extends AbstractLoadBalancer implements
        PrimeConnections.PrimeConnectionListener, IClientConfigAware {
	//默認的rule是輪詢	在這個類中才 組合了 IRule
    private final static IRule DEFAULT_RULE = new RoundRobinRule();
    //實現接口的方法  添加servers
    public void addServers(List<Server> newServers) {
        if (newServers != null && newServers.size() > 0) {
            try {
                ArrayList<Server> newList = new ArrayList<Server>();
                newList.addAll(allServerList);
                newList.addAll(newServers);
                //這個方法里面使用了一個讀寫鎖
                setServersList(newList);
            } catch (Exception e) {
                logger.error("LoadBalancer [{}]: Exception while adding Servers", name, e);
            }
        }
    }
    //實現接口的方法，選擇server	調用rule的choose方法
    public Server chooseServer(Object key) {
        if (counter == null) {
            counter = createCounter();
        }
        counter.increment();
        if (rule == null) {
            return null;
        } else {
            try {
                return rule.choose(key);
            } catch (Exception e) {
                logger.warn("LoadBalancer [{}]:  Error choosing server for key {}", name, key, e);
                return null;
            }
        }
    }
    //實現接口的方法，標記server下線
    public void markServerDown(Server server) {
        if (server == null || !server.isAlive()) {
            return;
        }

        logger.error("LoadBalancer [{}]:  markServerDown called on [{}]", name, server.getId());
        server.setAlive(false);
        // forceQuickPing();
		//通知server狀態的變化
        notifyServerStatusChangeListener(singleton(server));
    }
    //實現接口的方法，獲取可用servers
    public List<Server> getReachableServers() {
        return Collections.unmodifiableList(upServerList);
    }
    //實現接口的方法，獲取所有的server
    public List<Server> getAllServers() {
        return Collections.unmodifiableList(allServerList);
    }
    //實現抽象類的方法，獲取serverList
    public List<Server> getServerList(ServerGroup serverGroup) {
        switch (serverGroup) {
        case ALL:
            return allServerList;
        case STATUS_UP:
            return upServerList;
        case STATUS_NOT_UP:
            ArrayList<Server> notAvailableServers = new ArrayList<Server>(
                    allServerList);
            ArrayList<Server> upServers = new ArrayList<Server>(upServerList);
            notAvailableServers.removeAll(upServers);
            return notAvailableServers;
        }
        return new ArrayList<Server>();
    }
    //抽象類的實現方法，獲取負載均衡狀態
    public LoadBalancerStats getLoadBalancerStats() {
        return lbStats;
    }
}

4、NoOpLoadBalancer 就是什么都不做

public class NoOpLoadBalancer extends AbstractLoadBalancer {
    @Override
    public void addServers(List<Server> newServers) {
        logger.info("addServers to NoOpLoadBalancer ignored");
    }
    @Override
    public Server chooseServer(Object key) {       
        return null;
    }
    @Override
    public LoadBalancerStats getLoadBalancerStats() {        
        return null;
    }
    @Override
    public List<Server> getServerList(ServerGroup serverGroup) {     
        return Collections.emptyList();
    }
    @Override
    public void markServerDown(Server server) {
        logger.info("markServerDown to NoOpLoadBalancer ignored");
    }
   @Override
   public List<Server> getServerList(boolean availableOnly) {
      // TODO Auto-generated method stub
      return null;
   }
    @Override
    public List<Server> getReachableServers() {
        return null;
    }
    @Override
    public List<Server> getAllServers() {
        return null;
    }
}

BaseLoadBalancer有個實現類DynamicServerListLoadBalancer

5、DynamicServerListLoadBalancer

public class DynamicServerListLoadBalancer<T extends Server> extends BaseLoadBalancer {
	//是否https
    boolean isSecure = false;
    boolean useTunnel = false;
    // server狀態是否正在更新
    protected AtomicBoolean serverListUpdateInProgress = new AtomicBoolean(false);
    volatile ServerList<T> serverListImpl;
    volatile ServerListFilter<T> filter;
    //這里實現動態刷新服務列表	會有一個定時器來執行
    protected final ServerListUpdater.UpdateAction updateAction = new ServerListUpdater.UpdateAction() {
        @Override
        public void doUpdate() {
            updateListOfServers();
        }
    };
    protected volatile ServerListUpdater serverListUpdater;
	...
    public DynamicServerListLoadBalancer(IClientConfig clientConfig, IRule rule, IPing ping,
                                         ServerList<T> serverList, ServerListFilter<T> filter,
                                         ServerListUpdater serverListUpdater) {
        super(clientConfig, rule, ping);
        this.serverListImpl = serverList;
        this.filter = filter;
        //傳入update定時器
        this.serverListUpdater = serverListUpdater;
        if (filter instanceof AbstractServerListFilter) {
            ((AbstractServerListFilter) filter).setLoadBalancerStats(getLoadBalancerStats());
        }
        restOfInit(clientConfig);
    }
    ...
    void restOfInit(IClientConfig clientConfig) {
        boolean primeConnection = this.isEnablePrimingConnections();
        // turn this off to avoid duplicated asynchronous priming done in BaseLoadBalancer.setServerList()
        this.setEnablePrimingConnections(false);
        //啟動update的定時器
        enableAndInitLearnNewServersFeature();

        updateListOfServers();
        if (primeConnection && this.getPrimeConnections() != null) {
            this.getPrimeConnections()
                    .primeConnections(getReachableServers());
        }
        this.setEnablePrimingConnections(primeConnection);
        LOGGER.info("DynamicServerListLoadBalancer for client {} initialized: {}", clientConfig.getClientName(), this.toString());
    }
	//更新servers
    @VisibleForTesting
    public void updateListOfServers() {
        List<T> servers = new ArrayList<T>();
        if (serverListImpl != null) {
            servers = serverListImpl.getUpdatedListOfServers();
            LOGGER.debug("List of Servers for {} obtained from Discovery client: {}",
                    getIdentifier(), servers);

            if (filter != null) {
                servers = filter.getFilteredListOfServers(servers);
                LOGGER.debug("Filtered List of Servers for {} obtained from Discovery client: {}",
                        getIdentifier(), servers);
            }
        }
        updateAllServerList(servers);
    }
	//更新所有servers
    protected void updateAllServerList(List<T> ls) {
        // other threads might be doing this - in which case, we pass
        if (serverListUpdateInProgress.compareAndSet(false, true)) {
            try {
                for (T s : ls) {
                    s.setAlive(true); // set so that clients can start using these
                                      // servers right away instead
                                      // of having to wait out the ping cycle.
                }
                setServersList(ls);
                super.forceQuickPing();
            } finally {
                serverListUpdateInProgress.set(false);
            }
        }
    }
    ...
    public void setServersList(List lsrv) {
        //調用BaseLoadbalancer的setServersList，更新服務列表 
        //allServerList upServerList
        super.setServersList(lsrv);
        //更新zone 服務列表
        List<T> serverList = (List<T>) lsrv;
        Map<String, List<Server>> serversInZones = new HashMap<String, List<Server>>();
        for (Server server : serverList) {
            // make sure ServerStats is created to avoid creating them on hot
            // path
            getLoadBalancerStats().getSingleServerStat(server);
            String zone = server.getZone();
            if (zone != null) {
                zone = zone.toLowerCase();
                List<Server> servers = serversInZones.get(zone);
                if (servers == null) {
                    servers = new ArrayList<Server>();
                    serversInZones.put(zone, servers);
                }
                servers.add(server);
            }
        }
        setServerListForZones(serversInZones);
    }
    protected void setServerListForZones(
            Map<String, List<Server>> zoneServersMap) {
        LOGGER.debug("Setting server list for zones: {}", zoneServersMap);
        getLoadBalancerStats().updateZoneServerMapping(zoneServersMap);
    }
    ...
}

DynamicServerListLoadBalancer中添加了一個屬性ServerListUpdater.UpdateAction，通過匿名內部類實現。

public void enableAndInitLearnNewServersFeature() {
    LOGGER.info("Using serverListUpdater {}", serverListUpdater.getClass().getSimpleName());
    serverListUpdater.start(updateAction);
}

6、ZoneAwareLoadBalancer

DynamicServerListLoadBalancer的唯一實現類，也是Ribbon負載均衡默認的負載具衡器

public class ZoneAwareLoadBalancer<T extends Server> extends DynamicServerListLoadBalancer<T> {
	//key:zone, value:LoadBalancer
    private ConcurrentHashMap<String, BaseLoadBalancer> balancers = new ConcurrentHashMap<String, BaseLoadBalancer>();
    private static final Logger logger = LoggerFactory.getLogger(ZoneAwareLoadBalancer.class);
    private volatile DynamicDoubleProperty triggeringLoad;
    private volatile DynamicDoubleProperty triggeringBlackoutPercentage; 
    private static final DynamicBooleanProperty ENABLED = DynamicPropertyFactory.getInstance().getBooleanProperty("ZoneAwareNIWSDiscoveryLoadBalancer.enabled", true);
	...
	//設置區域和服務
    @Override
    protected void setServerListForZones(Map<String, List<Server>> zoneServersMap) {
        super.setServerListForZones(zoneServersMap);
        if (balancers == null) {
            balancers = new ConcurrentHashMap<String, BaseLoadBalancer>();
        }
        for (Map.Entry<String, List<Server>> entry: zoneServersMap.entrySet()) {
           String zone = entry.getKey().toLowerCase();
            getLoadBalancer(zone).setServersList(entry.getValue());
        }
        // check if there is any zone that no longer has a server
        // and set the list to empty so that the zone related metrics does not
        // contain stale data
        for (Map.Entry<String, BaseLoadBalancer> existingLBEntry: balancers.entrySet()) {
            if (!zoneServersMap.keySet().contains(existingLBEntry.getKey())) {
                existingLBEntry.getValue().setServersList(Collections.emptyList());
            }
        }
    }    
    //選擇server，上一篇講過    
    @Override
    public Server chooseServer(Object key) {
        if (!ENABLED.get() || getLoadBalancerStats().getAvailableZones().size() <= 1) {
            logger.debug("Zone aware logic disabled or there is only one zone");
            return super.chooseServer(key);
        }
        Server server = null;
        try {
            LoadBalancerStats lbStats = getLoadBalancerStats();
            Map<String, ZoneSnapshot> zoneSnapshot = ZoneAvoidanceRule.createSnapshot(lbStats);
            logger.debug("Zone snapshots: {}", zoneSnapshot);
            if (triggeringLoad == null) {
                triggeringLoad = DynamicPropertyFactory.getInstance().getDoubleProperty(
                        "ZoneAwareNIWSDiscoveryLoadBalancer." + this.getName() + ".triggeringLoadPerServerThreshold", 0.2d);
            }

            if (triggeringBlackoutPercentage == null) {
                triggeringBlackoutPercentage = DynamicPropertyFactory.getInstance().getDoubleProperty(
                        "ZoneAwareNIWSDiscoveryLoadBalancer." + this.getName() + ".avoidZoneWithBlackoutPercetage", 0.99999d);
            }
            Set<String> availableZones = ZoneAvoidanceRule.getAvailableZones(zoneSnapshot, triggeringLoad.get(), triggeringBlackoutPercentage.get());
            logger.debug("Available zones: {}", availableZones);
            if (availableZones != null &&  availableZones.size() < zoneSnapshot.keySet().size()) {
                String zone = ZoneAvoidanceRule.randomChooseZone(zoneSnapshot, availableZones);
                logger.debug("Zone chosen: {}", zone);
                if (zone != null) {
                    BaseLoadBalancer zoneLoadBalancer = getLoadBalancer(zone);
                    server = zoneLoadBalancer.chooseServer(key);
                }
            }
        } catch (Exception e) {
            logger.error("Error choosing server using zone aware logic for load balancer={}", name, e);
        }
        if (server != null) {
            return server;
        } else {
            logger.debug("Zone avoidance logic is not invoked.");
            return super.chooseServer(key);
        }
    }
     //根據zone獲取負載均衡器
    @VisibleForTesting
    BaseLoadBalancer getLoadBalancer(String zone) {
        zone = zone.toLowerCase();
        BaseLoadBalancer loadBalancer = balancers.get(zone);
        if (loadBalancer == null) {
           // We need to create rule object for load balancer for each zone
           IRule rule = cloneRule(this.getRule());
            loadBalancer = new BaseLoadBalancer(this.getName() + "_" + zone, rule, this.getLoadBalancerStats());
            BaseLoadBalancer prev = balancers.putIfAbsent(zone, loadBalancer);
            if (prev != null) {
               loadBalancer = prev;
            }
        } 
        return loadBalancer;        
    }
	//克隆rule
    private IRule cloneRule(IRule toClone) {
       IRule rule;
       if (toClone == null) {
          rule = new AvailabilityFilteringRule();
       } else {
          String ruleClass = toClone.getClass().getName();              
          try {
            rule = (IRule) ClientFactory.instantiateInstanceWithClientConfig(ruleClass, this.getClientConfig());
         } catch (Exception e) {
            throw new RuntimeException("Unexpected exception creating rule for ZoneAwareLoadBalancer", e);
         }
       }
       return rule;
    }
    //設置rule  
    @Override
    public void setRule(IRule rule) {
        super.setRule(rule);
        if (balancers != null) {
            for (String zone: balancers.keySet()) {
                balancers.get(zone).setRule(cloneRule(rule));
            }
        }
    }
}

在RibbonClientConfiguration注冊ZoneAwareLoadBalancer時，同時注冊了serverList和serverListUpdater。

@Bean
@ConditionalOnMissingBean
public IRule ribbonRule(IClientConfig config) {
   if (this.propertiesFactory.isSet(IRule.class, name)) {
      return this.propertiesFactory.get(IRule.class, config, name);
   }
   ZoneAvoidanceRule rule = new ZoneAvoidanceRule();
   rule.initWithNiwsConfig(config);
   return rule;
}

@Bean
@ConditionalOnMissingBean
public IPing ribbonPing(IClientConfig config) {
   if (this.propertiesFactory.isSet(IPing.class, name)) {
      return this.propertiesFactory.get(IPing.class, config, name);
   }
   return new DummyPing();
}

@Bean
@ConditionalOnMissingBean
@SuppressWarnings("unchecked")
public ServerList<Server> ribbonServerList(IClientConfig config) {
   if (this.propertiesFactory.isSet(ServerList.class, name)) {
      return this.propertiesFactory.get(ServerList.class, config, name);
   }
   ConfigurationBasedServerList serverList = new ConfigurationBasedServerList();
   serverList.initWithNiwsConfig(config);
   return serverList;
}

@Bean
@ConditionalOnMissingBean
public ServerListUpdater ribbonServerListUpdater(IClientConfig config) {
   return new PollingServerListUpdater(config);
}

@Bean
@ConditionalOnMissingBean
public ILoadBalancer ribbonLoadBalancer(IClientConfig config,
      ServerList<Server> serverList, ServerListFilter<Server> serverListFilter,
      IRule rule, IPing ping, ServerListUpdater serverListUpdater) {
   if (this.propertiesFactory.isSet(ILoadBalancer.class, name)) {
      return this.propertiesFactory.get(ILoadBalancer.class, config, name);
   }
   return new ZoneAwareLoadBalancer<>(config, rule, ping, serverList,
         serverListFilter, serverListUpdater);
}

@Bean
@ConditionalOnMissingBean
@SuppressWarnings("unchecked")
public ServerListFilter<Server> ribbonServerListFilter(IClientConfig config) {
   if (this.propertiesFactory.isSet(ServerListFilter.class, name)) {
      return this.propertiesFactory.get(ServerListFilter.class, config, name);
   }
   ZonePreferenceServerListFilter filter = new ZonePreferenceServerListFilter();
   filter.initWithNiwsConfig(config);
   return filter;
}

由此可見，ribbonServerListUpdater是注入的PollingServerListUpdater。

在上面DynamicServerListLoadBalancer的enableAndInitLearnNewServersFeature方法中啟動了ServerListUpdater的start方法，就是調用了PollingServerListUpdater的start方法。

//com.netflix.loadbalancer.PollingServerListUpdater#start
public synchronized void start(final UpdateAction updateAction) {
    if (isActive.compareAndSet(false, true)) {
        final Runnable wrapperRunnable = new Runnable() {
            @Override
            public void run() {
                if (!isActive.get()) {
                    if (scheduledFuture != null) {
                        scheduledFuture.cancel(true);
                    }
                    return;
                }
                try {
                    //調用update方法，在DynamicServerListLoadBalancer
                    updateAction.doUpdate();
                    lastUpdated = System.currentTimeMillis();
                } catch (Exception e) {
                    logger.warn("Failed one update cycle", e);
                }
            }
        };
		//定時器延遲1s，定時30s執行一次
        scheduledFuture = getRefreshExecutor().scheduleWithFixedDelay(
                wrapperRunnable,
                initialDelayMs,
                refreshIntervalMs,
                TimeUnit.MILLISECONDS
        );
    } else {
        logger.info("Already active, no-op");
    }
}

定時器會刷新服務列表