Eureka源码深度刨析-(4)EurekaClient服务发现

“不积跬步，无以至千里。”

Eureka client服务注册之后，需要从Server服务器那里拉取最新的注册表信息到本地存储起来，供后续的业务使用，这个服务注册表，我们之前也看过了，是一个ConcurrentHashMap数据结构，Key是服务名称，Value是一个Map，为什么是Map，这个好理解，一个服务，可以部署在多台机器上面，那么就会有多个实例信息，即InstanceInfo，所以Map的Key就是服务实例Id，Value就是服务实例信息，即InstanceInfo，只不过用Lease对象包了一下而已，放在Lease的一个holder对象，这个holder的类型，就是Lease的泛型，即InstanceInfo。

这一篇文章先来看看client端抓取注册表的逻辑，其实看完了eureka源码发现，服务发现这个功能真正复杂的地方是在eureka server端，那里有一套很优秀的设计思路，就是多级缓存机制。

首先要明白一点，eureka client第一次启动的时候，一定是从server端一次性抓取全量的注册表信息，在本地进行缓存，后续会每隔30s从server端抓取增量的注册表信息，跟本地缓存进行合并，当然这个间隔时间都是可以配置的，在application.yml文件中。

作为client端，在启动的时候就会自动抓取全量注册表，

if (clientConfig.shouldFetchRegistry()) {
            try {
                boolean primaryFetchRegistryResult = fetchRegistry(false);
                ...
            }
}

这个shouldFetchRegistry()默认为ture，说白了，除了单机的eureka server以后，配置的都是ture，然后看看fetchRegistry(false)这个方法，这是去执行拉取注册表，

private boolean fetchRegistry(boolean forceFullRegistryFetch) {
    Stopwatch tracer = FETCH_REGISTRY_TIMER.start();

    try {
        // If the delta is disabled or if it is the first time, get all
        // applications
        Applications applications = getApplications();

        if (clientConfig.shouldDisableDelta()
            || (!Strings.isNullOrEmpty(clientConfig.getRegistryRefreshSingleVipAddress()))
            || forceFullRegistryFetch
            || (applications == null)
            || (applications.getRegisteredApplications().size() == 0)
            || (applications.getVersion() == -1)) //Client application does not have latest library supporting delta
        {
            logger.info("Disable delta property : {}", clientConfig.shouldDisableDelta());
            logger.info("Single vip registry refresh property : {}", clientConfig.getRegistryRefreshSingleVipAddress());
            logger.info("Force full registry fetch : {}", forceFullRegistryFetch);
            logger.info("Application is null : {}", (applications == null));
            logger.info("Registered Applications size is zero : {}",
                        (applications.getRegisteredApplications().size() == 0));
            logger.info("Application version is -1: {}", (applications.getVersion() == -1));
            getAndStoreFullRegistry();
        } else {
            getAndUpdateDelta(applications);
        }
        applications.setAppsHashCode(applications.getReconcileHashCode());
        logTotalInstances();
    } catch (Throwable e) {
        logger.info(PREFIX + "{} - was unable to refresh its cache! This periodic background refresh will be retried in {} seconds. status = {} stacktrace = {}",
                    appPathIdentifier, clientConfig.getRegistryFetchIntervalSeconds(), e.getMessage(), ExceptionUtils.getStackTrace(e));
        return false;
    } finally {
        if (tracer != null) {
            tracer.stop();
        }
    }

看到这个方法先获取本地的Application缓存，Applications就是代指所有的服务，而一个Application中包含了自己这个服务所有的InstanceInfo信息。

Applications applications = getApplications();

private final AtomicReference<Applications> localRegionApps = new AtomicReference<Applications>();

@Override
public Applications getApplications() {
    return localRegionApps.get();
}

然后进行一大串的if判断，是否要进行全量拉取注册表，这里这么搞不是很好，把大量的判断放在if里面，会导致代码不够简洁，可读性、可维护性都很差，一般我们设计业务模块的时候也不会这么写的，如果是我来写这块，会把if里的整个逻辑封装成一个方法，来判断是否全量拉取，shouldGetAllRegistry()，类似的一个方法，会比较好一些。

if (clientConfig.shouldDisableDelta()
    || (!Strings.isNullOrEmpty(clientConfig.getRegistryRefreshSingleVipAddress()))
    || forceFullRegistryFetch
    || (applications == null)
    || (applications.getRegisteredApplications().size() == 0)
    || (applications.getVersion() == -1)) //Client application does not have latest library supporting delta
{
    logger.info("Disable delta property : {}", clientConfig.shouldDisableDelta());
    logger.info("Single vip registry refresh property : {}", clientConfig.getRegistryRefreshSingleVipAddress());
    logger.info("Force full registry fetch : {}", forceFullRegistryFetch);
    logger.info("Application is null : {}", (applications == null));
    logger.info("Registered Applications size is zero : {}",
                (applications.getRegisteredApplications().size() == 0));
    logger.info("Application version is -1: {}", (applications.getVersion() == -1));
    getAndStoreFullRegistry();
} else {
    getAndUpdateDelta(applications);
}

第一次肯定会全量抓取了，clientConfig.getRegistryRefreshSingleVipAddress()这行代码默认是null，因为我们没有配置registryRefreshSingleVipAddress这个值，所以取默认值。

那么就会调用queryClient这个组件的getApplications方法，发送rest请求到server端，然后拿到响应结果。

private EurekaHttpClient queryClient;

private void getAndStoreFullRegistry() throws Throwable {
    long currentUpdateGeneration = fetchRegistryGeneration.get();

    logger.info("Getting all instance registry info from the eureka server");

    Applications apps = null;
    EurekaHttpResponse<Applications> httpResponse = clientConfig.getRegistryRefreshSingleVipAddress() == null
        ? eurekaTransport.queryClient.getApplications(remoteRegionsRef.get())
        : eurekaTransport.queryClient.getVip(clientConfig.getRegistryRefreshSingleVipAddress(), remoteRegionsRef.get());
    if (httpResponse.getStatusCode() == Status.OK.getStatusCode()) {
        apps = httpResponse.getEntity();
    }
    logger.info("The response status is {}", httpResponse.getStatusCode());
    ... ...
}

同样的，使用的是jersey2的客户端工具类，

@Override
public EurekaHttpResponse<Applications> getApplications(String... regions) {
    return getApplicationsInternal("apps/", regions);
}

调用jersey client，发送http请求（http://localhost:8080/v2/apps），GET请求，调用eureka server的getContainers restful接口，获取全量注册表，缓存在自己的本地。

private EurekaHttpResponse<Applications> getApplicationsInternal(String urlPath, String[] regions) {
    Response response = null;
    try {
        WebTarget webTarget = jerseyClient.target(serviceUrl).path(urlPath);
        if (regions != null && regions.length > 0) {
            webTarget = webTarget.queryParam("regions", StringUtil.join(regions));
        }
        Builder requestBuilder = webTarget.request();
        addExtraProperties(requestBuilder);
        addExtraHeaders(requestBuilder);
        response = requestBuilder.accept(MediaType.APPLICATION_JSON_TYPE).get();

        Applications applications = null;
        if (response.getStatus() == Status.OK.getStatusCode() && response.hasEntity()) {
            applications = response.readEntity(Applications.class);
        }
        return anEurekaHttpResponse(response.getStatus(), applications).headers(headersOf(response)).build();
    } finally {
        if (logger.isDebugEnabled()) {
            logger.debug("Jersey2 HTTP GET {}/{}; statusCode={}", serviceUrl, urlPath, response == null ? "N/A" : response.getStatus());
        }
        if (response != null) {
            response.close();
        }
    }
}