Eureka源码学习（三）— Eureka Server源码解析

InstanceRegistry

InstanceRegistry是Eureka Server中注册表管理的核心接口。在根据类图可以发现它实现了LookUpService和LeaseManager接口。

LeaseManager主要用于维护实例的注册、续租、下线和清理，而LookupService提供对服务实例进行检索的功能。

实例的居住证Lease

初始化

初始化赋值注册时间和上次更新时间

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

服务下线

// 下线实例
public void cancel() {
  if (evictionTimestamp <= 0) {
    evictionTimestamp = System.currentTimeMillis();
  }
}

// 判断是否已经清理
public boolean isExpired() {
    return isExpired(0l);
}

// 判断是否已经清理
public boolean isExpired(long additionalLeaseMs) {
        // 在cancel中可以发现evictionTimestamp被设置为大于0的清理时时间戳
            // additionalLeaseMs
            // ① 清理定时任务中入参为0
            // 
        return (evictionTimestamp > 0 || System.currentTimeMillis() > (lastUpdateTimestamp + duration + additionalLeaseMs));
}

Eureka Server服务器用ConcurrentHashMap维护实例注册信息。

// ConcurrentHashMap<应用名, Map<实例id, Lease<实例信息>>>
private final ConcurrentHashMap>> registry
        = new ConcurrentHashMap>>();

租约更新

@PUT
public Response renewLease(
        @HeaderParam(PeerEurekaNode.HEADER_REPLICATION) String isReplication,
        @QueryParam("overriddenstatus") String overriddenStatus,
        @QueryParam("status") String status,
        @QueryParam("lastDirtyTimestamp") String lastDirtyTimestamp) {
    // 是否是同步节点来的，避免
    boolean isFromReplicaNode = "true".equals(isReplication);
    // 进行续租操作
    boolean isSuccess = registry.renew(app.getName(), id, isFromReplicaNode);

    // Not found in the registry, immediately ask for a register
    if (!isSuccess) {
        // 续租失败，该实例未注册，返回404
        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;
    // 防止并发冲突
    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 {
        // 成功续租
        response = Response.ok().build();
    }
    logger.debug("Found (Renew): {} - {}; reply status={}", app.getName(), id, response.getStatus());
    return response;
}

AbstractInstanceRegistry#renew()

对于续租的方法，不想注册一样需要整个实例信息，只需要实例名称和对应的实例id即可完成续租。

public boolean renew(String appName, String id, boolean isReplication) {
    RENEW.increment(isReplication);
    Map> gMap = registry.get(appName);
    Lease 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 {
        // 获取实例信息
        InstanceInfo instanceInfo = leaseToRenew.getHolder();
        if (instanceInfo != null) {
            // touchASGCache(instanceInfo.getASGName());
            InstanceStatus overriddenInstanceStatus = this.getOverriddenInstanceStatus(
                    instanceInfo, leaseToRenew, isReplication);
            //  如果得到的服务实例最后状态是UNKNOW，取消续约直接return
            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;
            }
            // 服务状态不一致。
            if (!instanceInfo.getStatus().equals(overriddenInstanceStatus)) {
                logger.info(
                        "The instance status {} is different from overridden instance status {} for instance {}. Hence setting the status to overridden status", instanceInfo.getStatus().name(),
                                instanceInfo.getOverriddenStatus().name(),
                                instanceInfo.getId());
                instanceInfo.setStatusWithoutDirty(overriddenInstanceStatus);

            }
        }
        // 统计每分钟续租的次数，用于自我保护机制
        renewsLastMin.increment();
        // 更新上次更新时间
        leaseToRenew.renew();
        return true;
    }
}

Lease#renew()

// 更新上次更新时间
public void renew() {
  lastUpdateTimestamp = System.currentTimeMillis() + duration;
}

过期租约清理

在AbstractInstanceRegistry中存在EvictionTask用于定时执行exict(0)来服务剔除，默认为60秒一次。但是如果Eureka Server处于自我保护状态，则无法进行清理操作。在进入自我保护状态后，在Eureka Client处，如果向Eureka Server注册失败，将快速超时并尝试与其他的Eureka Server进行通信。“自我保护机制”的设计大大提高了Eureka的可用性。

public void evict(long additionalLeaseMs) {
    logger.debug("Running the evict task");
        // 判断是否打开了自我保护机制
    // isLeaseExpirationEnabled == true 代表未开启自我保护机制
    if (!isLeaseExpirationEnabled()) {
        logger.debug("DS: lease expiration is currently disabled.");
        return;
    }

    // 遍历resgistry保存的注册信息，用expiredLeases收集所有过期的实例信息
    List> expiredLeases = new ArrayList<>();
    for (Entry>> groupEntry : registry.entrySet()) {
        // 对每个应用的实例列表判断是否需要清理
        Map> leaseMap = groupEntry.getValue();
        if (leaseMap != null) {
            for (Entry> leaseEntry : leaseMap.entrySet()) {
                Lease lease = leaseEntry.getValue();
                if (lease.isExpired(additionalLeaseMs) && lease.getHolder() != null) {
                    expiredLeases.add(lease);
                }
            }
        }
    }

    // To compensate for GC pauses or drifting local time, we need to use current registry size as a base for
    // triggering self-preservation. Without that we would wipe out full registry.
    int registrySize = (int) getLocalRegistrySize(); // 获取注册的实例数
    int registrySizeThreshold = (int) (registrySize * serverConfig.getRenewalPercentThreshold()); // 获得阈值 0.85（default）*size
    // 计算清理数量限制 evictionLimit = 
    // 注册实例数 - 注册实例数 * 0.85（default） = 0.15 * 注册实例数
    int evictionLimit = registrySize - registrySizeThreshold; 

    // 在limit和需要清理的数量中取较小值
    int toEvict = Math.min(expiredLeases.size(), evictionLimit);
    // 存在需要清理的实例，这里使用随机算法进行删除
    if (toEvict > 0) {
        logger.info("Evicting {} items (expired={}, evictionLimit={})", toEvict, expiredLeases.size(), evictionLimit);

        // 用随机算法cancel实例
        // 根据前面的保护机制，最大下线Math.min(expiredLeases.size(), evictionLimit)个实例
        Random random = new Random(System.currentTimeMillis());
        for (int i = 0; i < toEvict; i++) {
            // Pick a random item (Knuth shuffle algorithm)
            int next = i + random.nextInt(expiredLeases.size() - i);
            Collections.swap(expiredLeases, i, next);
            Lease lease = expiredLeases.get(i);

            String appName = lease.getHolder().getAppName();
            String id = lease.getHolder().getId();
            EXPIRED.increment();
            logger.warn("DS: Registry: expired lease for {}/{}", appName, id);
            // !! 注意，服务是逐个清理的
            internalCancel(appName, id, false);
        }
    }
}