Eureka server啟動流程圖和原始碼分析

流程圖

原始碼決議

由流程圖看出，Eureka server啟動時會做兩件事

1.服務同步

因為eureka服務可以搭建集群，所以每個服務節點啟動時，會從相鄰的eureka節點中同步實體資料，原始碼如下：

 @Override
    public int syncUp() {
        // Copy entire entry from neighboring DS node
        int count = 0;
        //當我們配置register-with-eureka: true時候，getRegistrySyncRetries的回傳值為5
        for (int i = 0; ((i < serverConfig.getRegistrySyncRetries()) && (count == 0)); i++) {
            if (i > 0) {
                try {
                    //從第二次開始，休眠30秒
                    Thread.sleep(serverConfig.getRegistrySyncRetryWaitMs());
                } catch (InterruptedException e) {
                    logger.warn("Interrupted during registry transfer..");
                    break;
                }
            }
            //eurekaClient是當前節點的最近一個節點，在啟動時會初始化
            Applications apps = eurekaClient.getApplications();
            //回圈節點上的實體
            for (Application app : apps.getRegisteredApplications()) {
                for (InstanceInfo instance : app.getInstances()) {
                    try {
                        if (isRegisterable(instance)) {
                            //將實體同步到本地記憶體
                            register(instance, instance.getLeaseInfo().getDurationInSecs(), true);
                            count++;
                        }
                    } catch (Throwable t) {
                        logger.error("During DS init copy", t);
                    }
                }
            }
        }
        return count;
    }

當然，若我們的eureka服務為單機的話，可以設定以下配置

eureka：
  client:
    fetch-registry: false   #是否把eureka服務當成client注冊到配置中心
    register-with-eureka: false  #eureka服務是否需要讀取配置中心的實體

2.服務剔除

當完成服務同步后，eureka會開啟服務剔除的定時任務，原始碼如下：

protected void postInit() {
        renewsLastMin.start();
        if (evictionTaskRef.get() != null) {
            evictionTaskRef.get().cancel();
        }
        evictionTaskRef.set(new EvictionTask());
        //執行java Timer 定時器任務
        evictionTimer.schedule(evictionTaskRef.get(),
                serverConfig.getEvictionIntervalTimerInMs(), //60秒后執行任務，可配置
                serverConfig.getEvictionIntervalTimerInMs()); //每次執行完任務后，間隔60秒再執行一次
    }

EvictionTask實作了Runnable方法，其run方法會呼叫evict方法，evict方法原始碼如下

//additionalLeaseMs為當前時間
public void evict(long additionalLeaseMs) {
        logger.debug("Running the evict task");

        if (!isLeaseExpirationEnabled()) {
            logger.debug("DS: lease expiration is currently disabled.");
            return;
        }

        // We collect first all expired items, to evict them in random order. For large eviction sets,
        // if we do not that, we might wipe out whole apps before self preservation kicks in. By randomizing it,
        // the impact should be evenly distributed across all applications.
        List<Lease<InstanceInfo>> expiredLeases = new ArrayList<>();
        //回圈實體
        for (Entry<String, Map<String, Lease<InstanceInfo>>> groupEntry : registry.entrySet()) {
            Map<String, Lease<InstanceInfo>> leaseMap = groupEntry.getValue();
            if (leaseMap != null) {
                for (Entry<String, Lease<InstanceInfo>> leaseEntry : leaseMap.entrySet()) {
                    Lease<InstanceInfo> lease = leaseEntry.getValue();
                    //如果該實體最后一次心跳續約記錄日期與當前時間間隔大于90秒，則add到List，后續執行剔除操作
                    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.
        //設定最大剔除數量
        int registrySize = (int) getLocalRegistrySize();
        int registrySizeThreshold = (int) (registrySize * serverConfig.getRenewalPercentThreshold());
        int evictionLimit = registrySize - registrySizeThreshold;

        int toEvict = Math.min(expiredLeases.size(), evictionLimit);
        if (toEvict > 0) {
            logger.info("Evicting {} items (expired={}, evictionLimit={})", toEvict, expiredLeases.size(), evictionLimit);
            //隨機剔除實體
            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<InstanceInfo> 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);
                internalCancel(appName, id, false);
            }
        }
    }