Eureka Server Replicate

      为了方便说明，就把上篇博客的图再贴一遍了。

      上篇说道Application Service向Eureka Server注册服务的过程，在完成注册之后，由于Eureka Server是对等集群，其他Server也需要同步这一注册信息。与zookeeper相比，Eureka并不追求很强的一致性，而是认为A（可用性）和P（分区容错性）更重要。

      基于此，Eureka采用复制的方式进行注册信息的同步。

1、在完成注册方法之后，进行复制。

PeerAwareInstanceRegistryImpl类

 public void register(final InstanceInfo info, final boolean isReplication) {        int leaseDuration = Lease.DEFAULT_DURATION_IN_SECS;
        if (info.getLeaseInfo() != null && info.getLeaseInfo().getDurationInSecs() > 0) {
            leaseDuration = info.getLeaseInfo().getDurationInSecs();
        }
        super.register(info, leaseDuration, isReplication);
        replicateToPeers(Action.Register, info.getAppName(), info.getId(), info, null, isReplication);
    }

2、具体的复制步骤

复制方法，复制eureka的所有action操作除了流量。
 private void replicateToPeers(Action action, String appName, String id,
                                  InstanceInfo info /* optional */,
                                  InstanceStatus newStatus /* optional */, boolean isReplication) {
        Stopwatch tracer = action.getTimer().start();
        try {
            if (isReplication) {
                numberOfReplicationsLastMin.increment();
            }
            // 如果已经复制过，就不再复制
            if (peerEurekaNodes == Collections.EMPTY_LIST || isReplication) {
                return;
            }
		//遍历eureka集群中的所有节点，进行复制操作
            for (final PeerEurekaNode node : peerEurekaNodes.getPeerEurekaNodes()) {
                // If the url represents this host, do not replicate to yourself.
                if (peerEurekaNodes.isThisMyUrl(node.getServiceUrl())) {
                    continue;
                }
                replicateInstanceActionsToPeers(action, appName, id, info, newStatus, node);
            }
        } finally {
            tracer.stop();
        }
    }

Tips：

        Eureka Server的复制不会进行第二次，可以参考上面的代码。

        判断isReplication的值，如果复制过，则不再复制。如果没有复制过，遍历集群中的node节点个数（即Eureka Server集群），依次进行复制操作。

具体的复制action，包括取消、注册、心跳、状态更新等。

 private void replicateInstanceActionsToPeers(Action action, String appName,                                                 String id, InstanceInfo info, InstanceStatus newStatus,
                                                 PeerEurekaNode node) {
        try {
            InstanceInfo infoFromRegistry = null;
            CurrentRequestVersion.set(Version.V2);
            switch (action) {
                case Cancel:
                    node.cancel(appName, id);
                    break;
                case Heartbeat:
                    InstanceStatus overriddenStatus = overriddenInstanceStatusMap.get(id);
                    infoFromRegistry = getInstanceByAppAndId(appName, id, false);
                    node.heartbeat(appName, id, infoFromRegistry, overriddenStatus, false);
                    break;
                case Register:
                    node.register(info);
                    break;
                case StatusUpdate:
                    infoFromRegistry = getInstanceByAppAndId(appName, id, false);
                    node.statusUpdate(appName, id, newStatus, infoFromRegistry);
                    break;
                case DeleteStatusOverride:
                    infoFromRegistry = getInstanceByAppAndId(appName, id, false);
                    node.deleteStatusOverride(appName, id, infoFromRegistry);
                    break;
            }
        } catch (Throwable t) {
            logger.error("Cannot replicate information to {} for action {}", node.getServiceUrl(), action.name(), t);
        }
    }

思考：
         peerEurekaNodes如何获取集群中的所有node节点？

首先，在PeerAwareInstanceRegistryImpl类中的init方法中找到了peerEurekaNodes的赋值。

public void init(PeerEurekaNodes peerEurekaNodes) throws Exception {
        this.numberOfReplicationsLastMin.start();
        this.peerEurekaNodes = peerEurekaNodes;
        initializedResponseCache();
        scheduleRenewalThresholdUpdateTask();
        initRemoteRegionRegistry();

        try {
            Monitors.registerObject(this);
        } catch (Throwable e) {
            logger.warn("Cannot register the JMX monitor for the InstanceRegistry :", e);
        }
    }

在其父类中AwsInstanceRegistry也有其实现。

public void init(PeerEurekaNodes peerEurekaNodes) throws Exception {
        super.init(peerEurekaNodes);
        this.awsAsgUtil = new AwsAsgUtil(serverConfig, clientConfig, this);
        // We first check if the instance is STARTING or DOWN, then we check explicit overrides,
        // then we see if our ASG is UP, then we check the status of a potentially existing lease.
        this.instanceStatusOverrideRule = new FirstMatchWinsCompositeRule(new DownOrStartingRule(),
                new OverrideExistsRule(overriddenInstanceStatusMap), new AsgEnabledRule(this.awsAsgUtil),
                new LeaseExistsRule());
    }

继续往上找，DefaultEurekaServerContext类中，

 @PostConstruct
    @Override
    public void initialize() throws Exception {
        logger.info("Initializing ...");
        peerEurekaNodes.start();
        registry.init(peerEurekaNodes);
        logger.info("Initialized");
    }

最后，EurekaBootStrap中进行了调用。

 protected void initEurekaServerContext() throws Exception {
  
	//省略配置信息
        logger.info("Initializing the eureka client...");
  	
	//创建node组
        PeerEurekaNodes peerEurekaNodes = new PeerEurekaNodes(
                registry,
                eurekaServerConfig,
                eurekaClientConfig,
                serverCodecs,
                applicationInfoManager
        );

        serverContext = new DefaultEurekaServerContext(
                eurekaServerConfig,
                serverCodecs,
                registry,
                peerEurekaNodes,
                applicationInfoManager
        );

        EurekaServerContextHolder.initialize(serverContext);

        serverContext.initialize();
        logger.info("Initialized server context");

        // Copy registry from neighboring eureka node
        int registryCount = registry.syncUp();
        registry.openForTraffic(applicationInfoManager, registryCount);

        // Register all monitoring statistics.
        EurekaMonitors.registerAllStats();
    }