#总结
eureka内部实际上是分为三个实例的，分别是Client，Server和Instance，
Eureka-Client启动，创建Instance实例，封装成对象，推送给server，server接收Instance对象，返回实例集合，并签订租约，client定时发动续租请求到server，server维护各个instance实例，服务之间调用不通过eureka。
#Eureka-Client
client启动类实例顺序

EurekaInstanceConfigBean读取配置-> InstanceInfoFactory创建实例-> EurekaClientAutoConfiguration封装实例转换成-> ApplicationInfoManager 结合 EurekaClientConfigBean -> DiscoveryClient300行，调构造方法，并且发起注册

eureka原生Instance实例配置类

spring cloud 实现了这个接口也就是EurekaInstanceConfigBean读取配置文件，

与此同时 EurekaClientConfig spring cloud 提供的实现是EurekaClientConfigBean获取注册Instance的信息

配置初始化之后，EurekaClientAutoConfiguration(也是spring cloud 自己实现的一个类)会调用eurekaApplicationInfoManager来创建一个ApplicationInfoManager实例

创建完实例ApplicationInfoManager之后就到了client中最重要的类DiscoveryClient，这个类就是client向sever注册的直通车，他的构造方法内部

完成了一系列的操作，例如注册，启动心跳线程池，获取server端Instance实例集合。
接下来我们重点说说这个类：
DiscoveryClient类的父类EurekaClient继承了LookupService接口，该接口定义了client从server获取实例的一系列方法

public interface LookupService<T> {

    /**
     *通过appname获取实例
     */
    Application getApplication(String appName);

    /**
     *获取实例列表，这个方法就是client端缓存服务数据的来源
     */
    Applications getApplications();

    /**
     * 通过serverId获取实例
     */
    List<InstanceInfo> getInstancesById(String id);

    /**
     * 从下一个eureka获取实例
     */
    InstanceInfo getNextServerFromEureka(String virtualHostname, boolean secure);
}

启动client debug之，发现DiscoveryClient实现类里它里面存放了从server同步过来的Instance实例

他还实现了

DiscoveryClient的

方法，定时去刷server的Instance实例列表，
DiscoveryClient的构造方法

会去把当前服务的实例注册上去，并启动定时任务



注册服务到server

这里的register实际上调用的是AbstractJerseyEurekaHttpClient的register

可以看到他实际上是给server发送了一个注册的请求，这样client端的启动就结束了。
#Eureka-Server
sever启动首先就是用EurekaServerConfigBean，EurekaInstanceConfigBean，EurekaClientConfigBean 从配置文件读取配置，由于server端自己也是一个client，所以他也会走一遍client端的注册流程
配置完这些后，spring cloud 的 EurekaServerInitializerConfiguration会启动调用start方法，调用EurekaServerBootstrap的contextInitialized方法

执行完这个，server端就算是启动完成了。由于server端这两个方法比较复杂，并且对于我们理解原理没什么帮助，主需要理解他的字面意思就行。
#租约相关
LeaseManager这个接口定义了租约相关的操作，Lease类定义租约对象

public interface LeaseManager<T> {
    //注册
    void register(T r, int leaseDuration, boolean isReplication);
    //注销
    boolean cancel(String appName, String id, boolean isReplication);
    //续租
    boolean renew(String appName, String id, boolean isReplication);
    //删除过期服务
    void evict();
   
}

spring cloud 的实现 InstanceRegistry这个类实现了租约相关的操作如注册，注销等等，接下来我们来谈谈注册
#注册

#Eureka-Client端
Eureka-Client 向 Eureka-Server 发起注册应用实例需要符合如下条件：
1.配置 eureka.registration.enabled = true，Eureka-Client 向 Eureka-Server 发起注册应用实例的开关。
2.InstanceInfo 在 Eureka-Client 和 Eureka-Server 数据不一致。
每次 InstanceInfo 发生属性变化时，标记 isInstanceInfoDirty 属性为 true，表示 InstanceInfo 在 Eureka-Client 和 Eureka-Server 数据不一致，需要注册。另外，InstanceInfo 刚被创建时，在 Eureka-Server 不存在，也会被注册。
当符合条件时，InstanceInfo 不会立即向 Eureka-Server 注册，而是后台线程定时注册。
当 InstanceInfo 的状态( status ) 属性发生变化时，并且配置 eureka.shouldOnDemandUpdateStatusChange = true 时，立即向 Eureka-Server 注册。因为状态属性非常重要，一般情况下建议开启，当然默认情况也是开启的。

注册的方法在DiscoveryClient

会创建一个InstanceInfoReplicator应用实例复制器
定时检查 InstanceInfo 的状态( status ) 属性是否发生变化。若是，发起注册。实现代码如下：

// InstanceInfoReplicator.java
@Override
public void run() {
   try {
       // 刷新 应用实例信息
       discoveryClient.refreshInstanceInfo();
       // 判断 应用实例信息 是否数据不一致
       Long dirtyTimestamp = instanceInfo.isDirtyWithTime();
       if (dirtyTimestamp != null) {
           // 发起注册
           discoveryClient.register();
           // 设置 应用实例信息 数据一致
           instanceInfo.unsetIsDirty(dirtyTimestamp);
       }
   } catch (Throwable t) {
       logger.warn("There was a problem with the instance info replicator", t);
   } finally {
       // 提交任务，并设置该任务的 Future
       Future next = scheduler.schedule(this, replicationIntervalSeconds, TimeUnit.SECONDS);
       scheduledPeriodicRef.set(next);
   }
}

// InstanceInfo.java
public synchronized long setIsDirtyWithTime() {
   setIsDirty();
   return lastDirtyTimestamp;
}

public synchronized void unsetIsDirty(long unsetDirtyTimestamp) {
   if (lastDirtyTimestamp <= unsetDirtyTimestamp) {
       isInstanceInfoDirty = false;
   } else {
   }
}

如果发现状态变化
会调用DiscoveryClient的

默认调用AbstractJerseyEurekaHttpClient的注册方法，发送请求给server发起注册

@Override
    public EurekaHttpResponse<Void> register(InstanceInfo info) {
        String urlPath = "apps/" + info.getAppName();
        ClientResponse response = null;
        try {
            Builder resourceBuilder = jerseyClient.resource(serviceUrl).path(urlPath).getRequestBuilder();
            addExtraHeaders(resourceBuilder);
            response = resourceBuilder
                    .header("Accept-Encoding", "gzip")
                    .type(MediaType.APPLICATION_JSON_TYPE)
                    .accept(MediaType.APPLICATION_JSON)
                    .post(ClientResponse.class, info);
            return anEurekaHttpResponse(response.getStatus()).headers(headersOf(response)).build();
        } finally {
            if (logger.isDebugEnabled()) {
                logger.debug("Jersey HTTP POST {}/{} with instance {}; statusCode={}", serviceUrl, urlPath, info.getId(),
                        response == null ? "N/A" : response.getStatus());
            }
            if (response != null) {
                response.close();
            }
        }
    }

并且注册完成后，server会返回当前注册到eureka的实例的集合，client会把他缓存在自己的内存中，定时刷新
#eureka-server端
ApplicationResource 类用来接收client端发来的请求

@Produces({"application/xml", "application/json"})
public class ApplicationResource {

    @POST
    @Consumes({"application/json", "application/xml"})
    public Response addInstance(InstanceInfo info,
                                @HeaderParam(PeerEurekaNode.HEADER_REPLICATION) String isReplication) {
        // 校验参数是否合法
        logger.debug("Registering instance {} (replication={})", info.getId(), isReplication);
        // validate that the instanceinfo contains all the necessary required fields
        if (isBlank(info.getId())) {
            return Response.status(400).entity("Missing instanceId").build();
        } else if (isBlank(info.getHostName())) {
            return Response.status(400).entity("Missing hostname").build();
        } else if (isBlank(info.getIPAddr())) {
            return Response.status(400).entity("Missing ip address").build();
        } else if (isBlank(info.getAppName())) {
            return Response.status(400).entity("Missing appName").build();
        } else if (!appName.equals(info.getAppName())) {
            return Response.status(400).entity("Mismatched appName, expecting " + appName + " but was " + info.getAppName()).build();
        } else if (info.getDataCenterInfo() == null) {
            return Response.status(400).entity("Missing dataCenterInfo").build();
        } else if (info.getDataCenterInfo().getName() == null) {
            return Response.status(400).entity("Missing dataCenterInfo Name").build();
        }

        // AWS 相关，跳过
        // handle cases where clients may be registering with bad DataCenterInfo with missing data
        DataCenterInfo dataCenterInfo = info.getDataCenterInfo();
        if (dataCenterInfo instanceof UniqueIdentifier) {
            String dataCenterInfoId = ((UniqueIdentifier) dataCenterInfo).getId();
            if (isBlank(dataCenterInfoId)) {
                boolean experimental = "true".equalsIgnoreCase(serverConfig.getExperimental("registration.validation.dataCenterInfoId"));
                if (experimental) {
                    String entity = "DataCenterInfo of type " + dataCenterInfo.getClass() + " must contain a valid id";
                    return Response.status(400).entity(entity).build();
                } else if (dataCenterInfo instanceof AmazonInfo) {
                    AmazonInfo amazonInfo = (AmazonInfo) dataCenterInfo;
                    String effectiveId = amazonInfo.get(AmazonInfo.MetaDataKey.instanceId);
                    if (effectiveId == null) {
                        amazonInfo.getMetadata().put(AmazonInfo.MetaDataKey.instanceId.getName(), info.getId());
                    }
                } else {
                    logger.warn("Registering DataCenterInfo of type {} without an appropriate id", dataCenterInfo.getClass());
                }
            }
        }

        // 注册应用实例信息
        registry.register(info, "true".equals(isReplication));

        // 返回 204 成功
        return Response.status(204).build();  // 204 to be backwards compatible
    }

}

最终调用到PeerAwareInstanceRegistryImpl的register方法

@Override
public void register(final InstanceInfo info, final boolean isReplication) {
   // 租约过期时间
   int leaseDuration = Lease.DEFAULT_DURATION_IN_SECS;
   if (info.getLeaseInfo() != null && info.getLeaseInfo().getDurationInSecs() > 0) {
       leaseDuration = info.getLeaseInfo().getDurationInSecs();
   }
   // 注册应用实例信息
   super.register(info, leaseDuration, isReplication);
   // Eureka-Server 复制
   replicateToPeers(Action.Register, info.getAppName(), info.getId(), info, null, isReplication);
}

实现注册需要用到租约的Lease是定义租约的实例

public class Lease<T> {

    /**
     * 实体
     */
    private T holder;
    /**
     * 注册时间戳
     */
    private long registrationTimestamp;
    /**
     * 开始服务时间戳
     */
    private long serviceUpTimestamp;
    /**
     * 取消注册时间戳
     */
    private long evictionTimestamp;
    /**
     * 最后更新时间戳
     */
    // Make it volatile so that the expiration task would see this quicker
    private volatile long lastUpdateTimestamp;
    /**
     * 租约持续时长，单位：毫秒
     */
    private long duration;

    public Lease(T r, int durationInSecs) {
        holder = r;
        registrationTimestamp = System.currentTimeMillis();
        lastUpdateTimestamp = registrationTimestamp;
        duration = (durationInSecs * 1000);
    }
    
}

最终AbstractInstanceRegistry的register长这样

 1:  public void register(InstanceInfo registrant, int leaseDuration, boolean isReplication) {
 2:     try {
 3:         // 获取读锁
 4:         read.lock();
 5:         Map<String, Lease<InstanceInfo>> gMap = registry.get(registrant.getAppName());
 6:         // 增加 注册次数 到 监控
 7:         REGISTER.increment(isReplication);
 8:         // 获得 应用实例信息 对应的 租约
 9:         if (gMap == null) {
10:             final ConcurrentHashMap<String, Lease<InstanceInfo>> gNewMap = new ConcurrentHashMap<String, Lease<InstanceInfo>>();
11:             gMap = registry.putIfAbsent(registrant.getAppName(), gNewMap); // 添加 应用
12:             if (gMap == null) { // 添加 应用 成功
13:                 gMap = gNewMap;
14:             }
15:         }
16:         Lease<InstanceInfo> existingLease = gMap.get(registrant.getId());
17:         // Retain the last dirty timestamp without overwriting it, if there is already a lease
18:         if (existingLease != null && (existingLease.getHolder() != null)) { // 已存在时，使用数据不一致的时间大的应用注册信息为有效的
19:             Long existingLastDirtyTimestamp = existingLease.getHolder().getLastDirtyTimestamp(); // Server 注册的 InstanceInfo
20:             Long registrationLastDirtyTimestamp = registrant.getLastDirtyTimestamp(); // Client 请求的 InstanceInfo
21:             logger.debug("Existing lease found (existing={}, provided={}", existingLastDirtyTimestamp, registrationLastDirtyTimestamp);
22: 
23:             // this is a > instead of a >= because if the timestamps are equal, we still take the remote transmitted
24:             // InstanceInfo instead of the server local copy.
25:             if (existingLastDirtyTimestamp > registrationLastDirtyTimestamp) {
26:                 logger.warn("There is an existing lease and the existing lease's dirty timestamp {} is greater" +
27:                         " than the one that is being registered {}", existingLastDirtyTimestamp, registrationLastDirtyTimestamp);
28:                 logger.warn("Using the existing instanceInfo instead of the new instanceInfo as the registrant");
29:                 registrant = existingLease.getHolder();
30:             }
31:         } else {
32:             // The lease does not exist and hence it is a new registration
33:             // 【自我保护机制】增加 `numberOfRenewsPerMinThreshold` 、`expectedNumberOfRenewsPerMin`
34:             synchronized (lock) {
35:                 if (this.expectedNumberOfRenewsPerMin > 0) {
36:                     // Since the client wants to cancel it, reduce the threshold
37:                     // (1
38:                     // for 30 seconds, 2 for a minute)
39:                     this.expectedNumberOfRenewsPerMin = this.expectedNumberOfRenewsPerMin + 2;
40:                     this.numberOfRenewsPerMinThreshold =
41:                             (int) (this.expectedNumberOfRenewsPerMin * serverConfig.getRenewalPercentThreshold());
42:                 }
43:             }
44:             logger.debug("No previous lease information found; it is new registration");
45:         }
46:         // 创建 租约
47:         Lease<InstanceInfo> lease = new Lease<InstanceInfo>(registrant, leaseDuration);
48:         if (existingLease != null) { // 若租约已存在，设置 租约的开始服务的时间戳
49:             lease.setServiceUpTimestamp(existingLease.getServiceUpTimestamp());
50:         }
51:         // 添加到 租约映射
52:         gMap.put(registrant.getId(), lease);
53:         // 添加到 最近注册的调试队列
54:         synchronized (recentRegisteredQueue) {
55:             recentRegisteredQueue.add(new Pair<Long, String>(
56:                     System.currentTimeMillis(),
57:                     registrant.getAppName() + "(" + registrant.getId() + ")"));
58:         }
59:         // 添加到 应用实例覆盖状态映射（Eureka-Server 初始化使用）
60:         // This is where the initial state transfer of overridden status happens
61:         if (!InstanceStatus.UNKNOWN.equals(registrant.getOverriddenStatus())) {
62:             logger.debug("Found overridden status {} for instance {}. Checking to see if needs to be add to the "
63:                             + "overrides", registrant.getOverriddenStatus(), registrant.getId());
64:             if (!overriddenInstanceStatusMap.containsKey(registrant.getId())) {
65:                 logger.info("Not found overridden id {} and hence adding it", registrant.getId());
66:                 overriddenInstanceStatusMap.put(registrant.getId(), registrant.getOverriddenStatus());
67:             }
68:         }
69:         InstanceStatus overriddenStatusFromMap = overriddenInstanceStatusMap.get(registrant.getId());
70:         if (overriddenStatusFromMap != null) {
71:             logger.info("Storing overridden status {} from map", overriddenStatusFromMap);
72:             registrant.setOverriddenStatus(overriddenStatusFromMap);
73:         }
74: 
75:         // 获得应用实例最终状态，并设置应用实例的状态
76:         // Set the status based on the overridden status rules
77:         InstanceStatus overriddenInstanceStatus = getOverriddenInstanceStatus(registrant, existingLease, isReplication);
78:         registrant.setStatusWithoutDirty(overriddenInstanceStatus);
79: 
80:         // 设置 租约的开始服务的时间戳（只有第一次有效）
81:         // If the lease is registered with UP status, set lease service up timestamp
82:         if (InstanceStatus.UP.equals(registrant.getStatus())) {
83:             lease.serviceUp();
84:         }
85:         // 设置 应用实例信息的操作类型 为 添加
86:         registrant.setActionType(ActionType.ADDED);
87:         // 添加到 最近租约变更记录队列
88:         recentlyChangedQueue.add(new RecentlyChangedItem(lease));
89:         // 设置 租约的最后更新时间戳
90:         registrant.setLastUpdatedTimestamp();
91:         // 设置 响应缓存 过期
92:         invalidateCache(registrant.getAppName(), registrant.getVIPAddress(), registrant.getSecureVipAddress());
93:         logger.info("Registered instance {}/{} with status {} (replication={})",
94:                 registrant.getAppName(), registrant.getId(), registrant.getStatus(), isReplication);
95:     } finally {
96:         // 释放锁
97:         read.unlock();
98:     }
99: }

#续租
注册了租约以后 Eureka-Client 固定间隔向 Eureka-Server 发起续租( renew )，避免租约过期。默认情况下，租约有效期为 90 秒，续租频率为 30 秒。两者比例为 1 : 3 ，保证在网络异常等情况下，有三次重试的机会。
client端的续租任务

private void initScheduledTasks() {

    // 向 Eureka-Server 心跳（续租）执行器
    if (clientConfig.shouldRegisterWithEureka()) {
        int renewalIntervalInSecs = instanceInfo.getLeaseInfo().getRenewalIntervalInSecs(); // 续租频率
        int expBackOffBound = clientConfig.getHeartbeatExecutorExponentialBackOffBound(); //
        logger.info("Starting heartbeat executor: " + "renew interval is: " + renewalIntervalInSecs);

        // Heartbeat timer  这个heart线程就是发送续租请求的
        scheduler.schedule(
               new TimedSupervisorTask(
                       "heartbeat",
                       scheduler,
                       heartbeatExecutor,
                       renewalIntervalInSecs,
                       TimeUnit.SECONDS,
                       expBackOffBound,
                       new HeartbeatThread()
               ),
               renewalIntervalInSecs, TimeUnit.SECONDS);
               
          // ... 省略无关代码
     }
     // ... 省略无关代码
}

这个定时任务在DiscoveryClient的构造方法中启动按照配置定时向server发送续租请求。
调用这个线程run方法

// DiscoveryClient.java
/**
* 最后成功向 Eureka-Server 心跳时间戳
*/
private volatile long lastSuccessfulHeartbeatTimestamp = -1;

private class HeartbeatThread implements Runnable {

   public void run() {
       if (renew()) {
           lastSuccessfulHeartbeatTimestamp = System.currentTimeMillis();
       }
   }
}

renew方法

// DiscoveryClient.java
boolean renew() {
   EurekaHttpResponse<InstanceInfo> httpResponse;
   try {
       httpResponse = eurekaTransport.registrationClient.sendHeartBeat(instanceInfo.getAppName(), instanceInfo.getId(), instanceInfo, null);
       logger.debug("{} - Heartbeat status: {}", PREFIX + appPathIdentifier, httpResponse.getStatusCode());
       if (httpResponse.getStatusCode() == 404) {
           REREGISTER_COUNTER.increment();
           logger.info("{} - Re-registering apps/{}", PREFIX + appPathIdentifier, instanceInfo.getAppName());
           long timestamp = instanceInfo.setIsDirtyWithTime();
           // 发起注册
           boolean success = register();
           if (success) {
               instanceInfo.unsetIsDirty(timestamp);
           }
           return success;
       }
       return httpResponse.getStatusCode() == 200;
   } catch (Throwable e) {
       logger.error("{} - was unable to send heartbeat!", PREFIX + appPathIdentifier, e);
       return false;
   }
}

sendHeartBeat

// AbstractJerseyEurekaHttpClient.java
@Override
public EurekaHttpResponse<InstanceInfo> sendHeartBeat(String appName, String id, InstanceInfo info, InstanceStatus overriddenStatus) {
   String urlPath = "apps/" + appName + '/' + id;
   ClientResponse response = null;
   try {
       WebResource webResource = jerseyClient.resource(serviceUrl)
               .path(urlPath)
               .queryParam("status", info.getStatus().toString())
               .queryParam("lastDirtyTimestamp", info.getLastDirtyTimestamp().toString());
       if (overriddenStatus != null) {
           webResource = webResource.queryParam("overriddenstatus", overriddenStatus.name());
       }
       Builder requestBuilder = webResource.getRequestBuilder();
       addExtraHeaders(requestBuilder);
       response = requestBuilder.put(ClientResponse.class);
       EurekaHttpResponseBuilder<InstanceInfo> eurekaResponseBuilder = anEurekaHttpResponse(response.getStatus(), InstanceInfo.class).headers(headersOf(response));
       if (response.hasEntity()) {
           eurekaResponseBuilder.entity(response.getEntity(InstanceInfo.class));
       }
       return eurekaResponseBuilder.build();
   } finally {
       if (logger.isDebugEnabled()) {
           logger.debug("Jersey HTTP PUT {}/{}; statusCode={}", serviceUrl, urlPath, response == null ? "N/A" : response.getStatus());
       }
       if (response != null) {
           response.close();
       }
   }
}

#Eureka-Server 接收续租
接收续租调用的是InstanceResource的renewLease方法

@PUT
    public Response renewLease(
            @HeaderParam(PeerEurekaNode.HEADER_REPLICATION) String isReplication,
            @QueryParam("overriddenstatus") String overriddenStatus,
            @QueryParam("status") String status,
            @QueryParam("lastDirtyTimestamp") String lastDirtyTimestamp) {
        boolean isFromReplicaNode = "true".equals(isReplication);
        boolean isSuccess = registry.renew(app.getName(), id, isFromReplicaNode);

        // Not found in the registry, immediately ask for a register
        if (!isSuccess) {
            logger.warn("Not Found (Renew): {} - {}", app.getName(), id);
            return Response.status(Status.NOT_FOUND).build();
        }
        // Check if we need to sync based on dirty time stamp, the client
        // instance might have changed some value
        Response response = null;
        if (lastDirtyTimestamp != null && serverConfig.shouldSyncWhenTimestampDiffers()) {
            response = this.validateDirtyTimestamp(Long.valueOf(lastDirtyTimestamp), isFromReplicaNode);
            // Store the overridden status since the validation found out the node that replicates wins
            if (response.getStatus() == Response.Status.NOT_FOUND.getStatusCode()
                    && (overriddenStatus != null)
                    && !(InstanceStatus.UNKNOWN.name().equals(overriddenStatus))
                    && isFromReplicaNode) {
                registry.storeOverriddenStatusIfRequired(app.getAppName(), id, InstanceStatus.valueOf(overriddenStatus));
            }
        } else {
            response = Response.ok().build();
        }
        logger.debug("Found (Renew): {} - {}; reply status={}" + app.getName(), id, response.getStatus());
        return response;
    }

这里调用了LeaseManager#renew方法这个接口在spring cloud里的实现是InstanceRegistry
最终调用PeerAwareInstanceRegistryImpl#renew方法

// PeerAwareInstanceRegistryImpl.java
public boolean renew(final String appName, final String id, final boolean isReplication) {
   if (super.renew(appName, id, isReplication)) { // 续租
       // Eureka-Server 复制
       replicateToPeers(Action.Heartbeat, appName, id, null, null, isReplication);
       return true;
   }
   return false;
}

这里继续往上调AbstractInstanceRegistry的renew方法

public boolean renew(String appName, String id, boolean isReplication) {
        RENEW.increment(isReplication);
        Map<String, Lease<InstanceInfo>> gMap = registry.get(appName);
        Lease<InstanceInfo> leaseToRenew = null;
        if (gMap != null) {
            leaseToRenew = gMap.get(id);
        }
        if (leaseToRenew == null) {
            RENEW_NOT_FOUND.increment(isReplication);
            logger.warn("DS: Registry: lease doesn't exist, registering resource: {} - {}", appName, id);
            return false;
        } else {
            InstanceInfo instanceInfo = leaseToRenew.getHolder();
            if (instanceInfo != null) {
                // touchASGCache(instanceInfo.getASGName());
                InstanceStatus overriddenInstanceStatus = this.getOverriddenInstanceStatus(
                        instanceInfo, leaseToRenew, isReplication);
                if (overriddenInstanceStatus == InstanceStatus.UNKNOWN) {
                    logger.info("Instance status UNKNOWN possibly due to deleted override for instance {}"
                            + "; re-register required", instanceInfo.getId());
                    RENEW_NOT_FOUND.increment(isReplication);
                    return false;
                }
                if (!instanceInfo.getStatus().equals(overriddenInstanceStatus)) {
                    Object[] args = {
                            instanceInfo.getStatus().name(),
                            instanceInfo.getOverriddenStatus().name(),
                            instanceInfo.getId()
                    };
                    logger.info(
                            "The instance status {} is different from overridden instance status {} for instance {}. "
                                    + "Hence setting the status to overridden status", args);
                    instanceInfo.setStatus(overriddenInstanceStatus);
                }
            }
            renewsLastMin.increment();
            leaseToRenew.renew();
            return true;
        }
    }

#下线
Eureka-Client 发起下线，需要满足如下条件才可发起：
配置 eureka.registration.enabled = true ，应用实例开启注册开关。默认为 false 。
配置 eureka.shouldUnregisterOnShutdown = true ，应用实例开启关闭时下线开关。默认为 true 。
调用的是DiscoveryClient

// DiscoveryClient.java
public synchronized void shutdown() {

    // ... 省略无关代码

    // If APPINFO was registered
    if (applicationInfoManager != null
         && clientConfig.shouldRegisterWithEureka() // eureka.registration.enabled = true
         && clientConfig.shouldUnregisterOnShutdown()) { // eureka.shouldUnregisterOnShutdown = true
        applicationInfoManager.setInstanceStatus(InstanceStatus.DOWN);
        unregister();//这里设置实例的status为DOWN
    }
}

Eureka-Server 接收下线，InstanceResource#cancelLease

@DELETE
public Response cancelLease(
       @HeaderParam(PeerEurekaNode.HEADER_REPLICATION) String isReplication) {
   // 下线
   boolean isSuccess = registry.cancel(app.getName(), id, "true".equals(isReplication));

   if (isSuccess) { // 下线成功
       logger.debug("Found (Cancel): " + app.getName() + " - " + id);
       return Response.ok().build();
   } else { // 下线成功
       logger.info("Not Found (Cancel): " + app.getName() + " - " + id);
       return Response.status(Status.NOT_FOUND).build();
   }
}

最终调用到AbstractInstanceRegistry的internalCancel方法

 protected boolean internalCancel(String appName, String id, boolean isReplication) {
        try {
            read.lock();
            CANCEL.increment(isReplication);
            Map<String, Lease<InstanceInfo>> gMap = registry.get(appName);
            Lease<InstanceInfo> leaseToCancel = null;
            if (gMap != null) {
                leaseToCancel = gMap.remove(id);
            }
            synchronized (recentCanceledQueue) {
                recentCanceledQueue.add(new Pair<Long, String>(System.currentTimeMillis(), appName + "(" + id + ")"));
            }
            InstanceStatus instanceStatus = overriddenInstanceStatusMap.remove(id);
            if (instanceStatus != null) {
                logger.debug("Removed instance id {} from the overridden map which has value {}", id, instanceStatus.name());
            }
            if (leaseToCancel == null) {
                CANCEL_NOT_FOUND.increment(isReplication);
                logger.warn("DS: Registry: cancel failed because Lease is not registered for: {}/{}", appName, id);
                return false;
            } else {
                leaseToCancel.cancel();
                InstanceInfo instanceInfo = leaseToCancel.getHolder();
                String vip = null;
                String svip = null;
                if (instanceInfo != null) {
                    instanceInfo.setActionType(ActionType.DELETED);
                    recentlyChangedQueue.add(new RecentlyChangedItem(leaseToCancel));
                    instanceInfo.setLastUpdatedTimestamp();
                    vip = instanceInfo.getVIPAddress();
                    svip = instanceInfo.getSecureVipAddress();
                }
                invalidateCache(appName, vip, svip);
                logger.info("Cancelled instance {}/{} (replication={})", appName, id, isReplication);
                return true;
            }
        } finally {
            read.unlock();
        }
    }

#过期
正常情况下，应用实例下线时候会主动向 Eureka-Server 发起下线请求。但实际情况下，应用实例可能异常崩溃，又或者是网络异常等原因，导致下线请求无法被成功提交。
介于这种情况，通过 Eureka-Client 心跳延长租约，配合 Eureka-Server 清理超时的租约解决上述异常。
AbstractInstanceRegistry.EvictionTask在eureka-server启动时初始化定时任务

// AbstractInstanceRegistry.java
/**
* 清理租约过期任务
*/
private final AtomicReference<EvictionTask> evictionTaskRef = new AtomicReference<EvictionTask>();

protected void postInit() {
   // .... 省略无关代码

   // 初始化 清理租约过期任务
   if (evictionTaskRef.get() != null) {
       evictionTaskRef.get().cancel();
   }
   evictionTaskRef.set(new EvictionTask());
   evictionTimer.schedule(evictionTaskRef.get(),
           serverConfig.getEvictionIntervalTimerInMs(),
           serverConfig.getEvictionIntervalTimerInMs());
}

配置 eureka.evictionIntervalTimerInMs ，清理租约过期任务执行频率，单位：毫秒。默认，60000 毫秒。
eureka启动流程大体上就是这样，如有补充欢迎指正。