Spring Cloud Nacos（二）--Nacos客户端的数据的加载流程
Nacos客户端的数据的加载流程
Nacos的外部化配置的实现? NacosPropertySourceLocator。
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Spring cloud Nacos配置的加载
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Spring Cloud Nacos配置变更
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@RefreshScope - Spring Cloud中提供的能力，在Spring中只提供了@Scope
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Nacos Confi core
- 客户端配置的加载
- 客户端配置的动态刷新
- 客户端配置的本地快照
- 服务端配置存储(存储到内存/数据库)
- 数据动态变更比较（客户端对请求的数据进行分片，通过本地比较+探测+最终服务端数据的获取)
- 数据同步，基于广摇的方式发送数据变更事件
配置数据的最终加载，是基于configService.getConfig，Nacos提供的SDK来实现的。
String getConfig(String dataId, String group, long timeoutMs) throws NacosException;
关于Nacos SDK的使用教程： https://nacos.io/zh-cn/docs/sdk.html
NacosConfigService.getConfig
@Override
public String getConfig(String dataId, String group, long timeoutMs) throws NacosException {
return getConfigInner(namespace, dataId, group, timeoutMs);
}
private String getConfigInner(String tenant, String dataId, String group, long timeoutMs) throws NacosException {
//获取group，如果为空，则为default-group
group = blank2defaultGroup(group);
//验证请求参数
ParamUtils.checkKeyParam(dataId, group);
//设置响应结果
ConfigResponse cr = new ConfigResponse();
cr.setDataId(dataId);
cr.setTenant(tenant);
cr.setGroup(group);
// 优先使用本地配置
String content = LocalConfigInfoProcessor.getFailover(worker.getAgentName(), dataId, group, tenant);
//如果本地缓存中的内容不为空
if (content != null) {
LOGGER.warn("[{}] [get-config] get failover ok, dataId={}, group={}, tenant={}, config={}",
worker.getAgentName(), dataId, group, tenant, ContentUtils.truncateContent(content));
//把内容设置到cr中,响应结果中。
cr.setContent(content);
//获取容灾配置的encryptedDataKey
String encryptedDataKey = LocalEncryptedDataKeyProcessor
.getEncryptDataKeyFailover(agent.getName(), dataId, group, tenant);
//保存到cr
cr.setEncryptedDataKey(encryptedDataKey);
//执行过滤(目前好像没有实现)
configFilterChainManager.doFilter(null, cr);
//返回文件content
content = cr.getContent();
return content;
}
//如果本地文件中不存在相关内容，则发起远程调用
try {
ConfigResponse response = worker.getServerConfig(dataId, group, tenant, timeoutMs, false);
cr.setContent(response.getContent());
cr.setEncryptedDataKey(response.getEncryptedDataKey());
configFilterChainManager.doFilter(null, cr);
content = cr.getContent();
return content;
} catch (NacosException ioe) {
if (NacosException.NO_RIGHT == ioe.getErrCode()) {
throw ioe;
}
LOGGER.warn("[{}] [get-config] get from server error, dataId={}, group={}, tenant={}, msg={}",
worker.getAgentName(), dataId, group, tenant, ioe.toString());
}
content = LocalConfigInfoProcessor.getSnapshot(worker.getAgentName(), dataId, group, tenant);
//把响应内容返回
if (content != null) {
//如果出现NacosException，且不是403异常，则尝试通过本地的快照文件去获取配置进行返回。
LOGGER.warn("[{}] [get-config] get snapshot ok, dataId={}, group={}, tenant={}, config={}",
worker.getAgentName(), dataId, group, tenant, ContentUtils.truncateContent(content));
}
cr.setContent(content);
String encryptedDataKey = LocalEncryptedDataKeyProcessor
.getEncryptDataKeySnapshot(agent.getName(), dataId, group, tenant);
cr.setEncryptedDataKey(encryptedDataKey);
configFilterChainManager.doFilter(null, cr);
content = cr.getContent();
return content;
}
从本地缓存读取配置
默认情况下，nacos先从本地缓存的配置中读取文件：
C:\Users\snail\nacos\config\fixed-121.5.102.117_8848_nacos
如果本地缓存内容存在，则返回内容数据，否则返回空值。
public static String getFailover(String serverName, String dataId, String group, String tenant) {
File localPath = getFailoverFile(serverName, dataId, group, tenant);
if (!localPath.exists() || !localPath.isFile()) {
return null;
}
try {
return readFile(localPath);
} catch (IOException ioe) {
LOGGER.error("[" + serverName + "] get failover error, " + localPath, ioe);
return null;
}
}
从指定文件目录下读取文件内容。
static File getFailoverFile(String serverName, String dataId, String group, String tenant) {
File tmp = new File(LOCAL_SNAPSHOT_PATH, serverName + SUFFIX);
tmp = new File(tmp, FAILOVER_FILE_CHILD_1);
if (StringUtils.isBlank(tenant)) {
tmp = new File(tmp, FAILOVER_FILE_CHILD_2);
} else {
tmp = new File(tmp, FAILOVER_FILE_CHILD_3);
tmp = new File(tmp, tenant);
}
return new File(new File(tmp, group), dataId);
}
ClientWorker.getServerConfig
ClientWorker，表示客户端的一个工作类，它负责和服务端交互。
public ConfigResponse getServerConfig(String dataId, String group, String tenant, long readTimeout, boolean notify)
throws NacosException {
if (StringUtils.isBlank(group)) {
group = Constants.DEFAULT_GROUP;
}
return this.agent.queryConfig(dataId, group, tenant, readTimeout, notify);
}
@Override
public ConfigResponse queryConfig(String dataId, String group, String tenant, long readTimeouts, boolean notify)
throws NacosException {
//构建请求参数
ConfigQueryRequest request = ConfigQueryRequest.build(dataId, group, tenant);
request.putHeader(NOTIFY_HEADER, String.valueOf(notify));
//构建一个grpc
RpcClient rpcClient = getOneRunningClient();
if (notify) {
CacheData cacheData = cacheMap.get().get(GroupKey.getKeyTenant(dataId, group, tenant));
if (cacheData != null) {
rpcClient = ensureRpcClient(String.valueOf(cacheData.getTaskId()));
}
}
//发起远程调用
ConfigQueryResponse response = (ConfigQueryResponse) requestProxy(rpcClient, request, readTimeouts);
ConfigResponse configResponse = new ConfigResponse();
//根据响应结果实现不同的处理
//如果响应成功，保存快照到本地，并返回响应内容
if (response.isSuccess()) {
LocalConfigInfoProcessor.saveSnapshot(this.getName(), dataId, group, tenant, response.getContent());
configResponse.setContent(response.getContent());
//配置文件的类型，如text、json、yaml等
String configType;
if (StringUtils.isNotBlank(response.getContentType())) {
configType = response.getContentType();
} else {
configType = ConfigType.TEXT.getType();
}
//设置到configResponse中，后续要根据文件类型实现不同解析策略
configResponse.setConfigType(configType);
//获取加密数据的key
String encryptedDataKey = response.getEncryptedDataKey();
//保存
LocalEncryptedDataKeyProcessor
.saveEncryptDataKeySnapshot(agent.getName(), dataId, group, tenant, encryptedDataKey);
configResponse.setEncryptedDataKey(encryptedDataKey);
return configResponse;
} else if (response.getErrorCode() == ConfigQueryResponse.CONFIG_NOT_FOUND) {
LocalConfigInfoProcessor.saveSnapshot(this.getName(), dataId, group, tenant, null);
LocalEncryptedDataKeyProcessor.saveEncryptDataKeySnapshot(agent.getName(), dataId, group, tenant, null);
return configResponse;
} else if (response.getErrorCode() == ConfigQueryResponse.CONFIG_QUERY_CONFLICT) {
LOGGER.error(
"[{}] [sub-server-error] get server config being modified concurrently, dataId={}, group={}, "
+ "tenant={}", this.getName(), dataId, group, tenant);
throw new NacosException(NacosException.CONFLICT,
"data being modified, dataId=" + dataId + ",group=" + group + ",tenant=" + tenant);
} else {
LOGGER.error("[{}] [sub-server-error] dataId={}, group={}, tenant={}, code={}", this.getName(), dataId,
group, tenant, response);
throw new NacosException(response.getErrorCode(),
"http error, code=" + response.getErrorCode() + ",msg=" + response.getMessage() + ",dataId=" + dataId + ",group=" + group
+ ",tenant=" + tenant);
}
}
ClientWorker的getServerConfig，这里的代码和nacos1.x已经完全不同了，nacos1.x是直接使用http请求来实现获取远程配置的，nacos2.0采用了grpc来实现客户端与服务端的交互
- nacos1.x
@Override
public HttpRestResult<String> httpGet(String path, Map<String, String> headers, Map<String, String> paramValues,
String encode, long readTimeoutMs) throws Exception {
final long endTime = System.currentTimeMillis() + readTimeoutMs;
injectSecurityInfo(paramValues); //注入安全信息
//获取当前服务器地址
String currentServerAddr = serverListMgr.getCurrentServerAddr();
//获取最大重试次数，默认3次
int maxRetry = this.maxRetry;
//配置HttpClient的属性，默认的readTimeOut超时时间是3s
HttpClientConfig httpConfig = HttpClientConfig.builder()
.setReadTimeOutMillis(Long.valueOf(readTimeoutMs).intValue())
.setConTimeOutMillis(ConfigHttpClientManager.getInstance().getConnectTimeoutOrDefault(100)).build();
do {
try {
//设置header
Header newHeaders = getSpasHeaders(paramValues, encode);
if (headers != null) {
newHeaders.addAll(headers);
}
//构建query查询条件
Query query = Query.newInstance().initParams(paramValues);
//发起http请求
HttpRestResult<String> result = NACOS_RESTTEMPLATE
.get(getUrl(currentServerAddr, path), httpConfig, newHeaders, query, String.class);
//如果请求失败
if (isFail(result)) {
LOGGER.error("[NACOS ConnectException] currentServerAddr: {}, httpCode: {}",
serverListMgr.getCurrentServerAddr(), result.getCode());
} else {
// Update the currently available server addr
serverListMgr.updateCurrentServerAddr(currentServerAddr);
return result;
}
} catch (ConnectException connectException) {
LOGGER.error("[NACOS ConnectException httpGet] currentServerAddr:{}, err : {}",
serverListMgr.getCurrentServerAddr(), connectException.getMessage());
} catch (SocketTimeoutException socketTimeoutException) {
LOGGER.error("[NACOS SocketTimeoutException httpGet] currentServerAddr:{}， err : {}",
serverListMgr.getCurrentServerAddr(), socketTimeoutException.getMessage());
} catch (Exception ex) {
LOGGER.error("[NACOS Exception httpGet] currentServerAddr: " + serverListMgr.getCurrentServerAddr(),
ex);
throw ex;
}
//如果服务端列表有多个，并且当前请求失败，则尝试用下一个地址进行重试
if (serverListMgr.getIterator().hasNext()) {
currentServerAddr = serverListMgr.getIterator().next();
} else {
maxRetry--; //重试次数递减
if (maxRetry < 0) {
throw new ConnectException(
"[NACOS HTTP-GET] The maximum number of tolerable server reconnection errors has been reached");
}
serverListMgr.refreshCurrentServerAddr();
}
} while (System.currentTimeMillis() <= endTime);
LOGGER.error("no available server");
throw new ConnectException("no available server");
}
- nacos2.x
private Response requestProxy(RpcClient rpcClientInner, Request request, long timeoutMills) throws NacosException {
try {
request.putAllHeader(super.getSecurityHeaders(resourceBuild(request)));
request.putAllHeader(super.getCommonHeader());
} catch (Exception e) {
throw new NacosException(NacosException.CLIENT_INVALID_PARAM, e);
}
JsonObject asJsonObjectTemp = new Gson().toJsonTree(request).getAsJsonObject();
asJsonObjectTemp.remove("headers");
asJsonObjectTemp.remove("requestId");
boolean limit = Limiter.isLimit(request.getClass() + asJsonObjectTemp.toString());
if (limit) {
throw new NacosException(NacosException.CLIENT_OVER_THRESHOLD,
"More than client-side current limit threshold");
}
return rpcClientInner.request(request, timeoutMills);
}
public Response request(Request request, long timeoutMills) throws NacosException {
int retryTimes = 0;
Response response;
Exception exceptionThrow = null;
long start = System.currentTimeMillis();
//获取最大重试次数，默认3次
while (retryTimes < RETRY_TIMES && System.currentTimeMillis() < timeoutMills + start) {
boolean waitReconnect = false;
try {
if (this.currentConnection == null || !isRunning()) {
waitReconnect = true;
throw new NacosException(NacosException.CLIENT_DISCONNECT,
"Client not connected, current status:" + rpcClientStatus.get());
}
//GrpcConnection
response = this.currentConnection.request(request, timeoutMills);
if (response == null) {
throw new NacosException(SERVER_ERROR, "Unknown Exception.");
}
if (response instanceof ErrorResponse) {
if (response.getErrorCode() == NacosException.UN_REGISTER) {
synchronized (this) {
waitReconnect = true;
if (rpcClientStatus.compareAndSet(RpcClientStatus.RUNNING, RpcClientStatus.UNHEALTHY)) {
LoggerUtils.printIfErrorEnabled(LOGGER,
"Connection is unregistered, switch server, connectionId = {}, request = {}",
currentConnection.getConnectionId(), request.getClass().getSimpleName());
switchServerAsync();
}
}
}
throw new NacosException(response.getErrorCode(), response.getMessage());
}
// return response.
lastActiveTimeStamp = System.currentTimeMillis();
return response;
} catch (Exception e) {
if (waitReconnect) {
try {
// wait client to reconnect.
Thread.sleep(Math.min(100, timeoutMills / 3));
} catch (Exception exception) {
// Do nothing.
}
}
LoggerUtils.printIfErrorEnabled(LOGGER, "Send request fail, request = {}, retryTimes = {}, errorMessage = {}", request, retryTimes, e.getMessage());
exceptionThrow = e;
}
retryTimes++;
}
if (rpcClientStatus.compareAndSet(RpcClientStatus.RUNNING, RpcClientStatus.UNHEALTHY)) {
switchServerAsyncOnRequestFail();
}
if (exceptionThrow != null) {
throw (exceptionThrow instanceof NacosException) ? (NacosException) exceptionThrow
: new NacosException(SERVER_ERROR, exceptionThrow);
} else {
throw new NacosException(SERVER_ERROR, "Request fail, unknown Error");
}
}
Nacos Server端的配置获取
客户端向服务端加载配置，调用的接口是：/nacos/v1/cs/configs ， 于是，在Nacos的源码中找到该接口
定位到Nacos源码中的ConfigController.getConfig中的方法，代码如下:
@GetMapping
@Secured(action = ActionTypes.READ, parser = ConfigResourceParser.class)
public void getConfig(HttpServletRequest request, HttpServletResponse response,
@RequestParam("dataId") String dataId, @RequestParam("group") String group,
@RequestParam(value = "tenant", required = false, defaultValue = StringUtils.EMPTY) String tenant,
@RequestParam(value = "tag", required = false) String tag)
throws IOException, ServletException, NacosException {
// check tenant
ParamUtils.checkTenant(tenant);
//租户，也就是namespaceid
tenant = NamespaceUtil.processNamespaceParameter(tenant);
// check params
//检查请求参数是否为空
ParamUtils.checkParam(dataId, group, "datumId", "content");
ParamUtils.checkParam(tag);
//获取请求的ip
final String clientIp = RequestUtil.getRemoteIp(request);
//加载配置
inner.doGetConfig(request, response, dataId, group, tenant, tag, clientIp);
}
inner.doGetConfig
public String doGetConfig(HttpServletRequest request, HttpServletResponse response, String dataId, String group, String tenant, String tag, String clientIp) throws IOException, ServletException {
final String groupKey = GroupKey2.getKey(dataId, group, tenant);
String autoTag = request.getHeader("Vipserver-Tag");
//请求端的应用名称
String requestIpApp = RequestUtil.getAppName(request);
//尝试获取当前请求配置的读锁（避免读写冲突）
int lockResult = tryConfigReadLock(groupKey);
//请求端的ip
final String requestIp = RequestUtil.getRemoteIp(request);
boolean isBeta = false;
//lockResult>0 ，表示CacheItem(也就是缓存的配置项)不为空，并且已经加了读锁，意味着这个缓存数据不能被删除。
//lockResult=0 ,表示cacheItem为空，不需要加读锁
//lockResult=-1 , 表示加锁失败，存在冲突。
if (lockResult > 0) {
// LockResult > 0 means cacheItem is not null and other thread can`t delete this cacheItem
FileInputStream fis = null;
try {
String md5 = Constants.NULL;
long lastModified = 0L;
//从本地缓存中，根据groupKey获取CacheItem
CacheItem cacheItem = ConfigCacheService.getContentCache(groupKey);
//判断是否是beta发布，也就是测试版本
if (cacheItem.isBeta() && cacheItem.getIps4Beta().contains(clientIp)) {
isBeta = true;
}
//获取配置文件的类型
final String configType =
(null != cacheItem.getType()) ? cacheItem.getType() : FileTypeEnum.TEXT.getFileType();
response.setHeader("Config-Type", configType);
//返回文件类型的枚举对象
FileTypeEnum fileTypeEnum = FileTypeEnum.getFileTypeEnumByFileExtensionOrFileType(configType);
String contentTypeHeader = fileTypeEnum.getContentType();
response.setHeader(HttpHeaderConsts.CONTENT_TYPE, contentTypeHeader);
File file = null;
ConfigInfoBase configInfoBase = null;
PrintWriter out = null;
//如果是测试配置
if (isBeta) {
md5 = cacheItem.getMd54Beta();
lastModified = cacheItem.getLastModifiedTs4Beta();
if (PropertyUtil.isDirectRead()) {
configInfoBase = persistService.findConfigInfo4Beta(dataId, group, tenant);
} else {
//从磁盘中获取文件，得到的是一个完整的File
file = DiskUtil.targetBetaFile(dataId, group, tenant);
}
response.setHeader("isBeta", "true");
} else {
//判断tag标签是否为空，tag对应的是nacos配置中心的标签选项
if (StringUtils.isBlank(tag)) {
if (isUseTag(cacheItem, autoTag)) {
if (cacheItem.tagMd5 != null) {
md5 = cacheItem.tagMd5.get(autoTag);
}
if (cacheItem.tagLastModifiedTs != null) {
lastModified = cacheItem.tagLastModifiedTs.get(autoTag);
}
if (PropertyUtil.isDirectRead()) {
configInfoBase = persistService.findConfigInfo4Tag(dataId, group, tenant, autoTag);
} else {
file = DiskUtil.targetTagFile(dataId, group, tenant, autoTag);
}
response.setHeader("Vipserver-Tag",
URLEncoder.encode(autoTag, StandardCharsets.UTF_8.displayName()));
//直接走这个逻辑（默认不会配置tag属性）
} else {
//获取缓存的md5
md5 = cacheItem.getMd5();
//获取最后更新时间
lastModified = cacheItem.getLastModifiedTs();
//判断是否是stamdalone模式且使用的是derby数据库，如果是，则从derby数据库加载数据
if (PropertyUtil.isDirectRead()) {
configInfoBase = persistService.findConfigInfo(dataId, group, tenant);
} else {
//否则，如果是数据库或者集群模式，先从本地磁盘得到文件
file = DiskUtil.targetFile(dataId, group, tenant);
}
//如果本地磁盘文件为空，并且configInfoBase为空，则表示配置数据不存在，直接返回null
if (configInfoBase == null && fileNotExist(file)) {
// FIXME CacheItem
// No longer exists. It is impossible to simply calculate the push delayed. Here, simply record it as - 1.
ConfigTraceService.logPullEvent(dataId, group, tenant, requestIpApp, -1,
ConfigTraceService.PULL_EVENT_NOTFOUND, -1, requestIp);
// pullLog.info("[client-get] clientIp={}, {},
// no data",
// new Object[]{clientIp, groupKey});
response.setStatus(HttpServletResponse.SC_NOT_FOUND);
response.getWriter().println("config data not exist");
return HttpServletResponse.SC_NOT_FOUND + "";
}
}
} else {
//如果tag不为空，说明配置文件设置了tag标签
if (cacheItem.tagMd5 != null) {
md5 = cacheItem.tagMd5.get(tag);
}
if (cacheItem.tagLastModifiedTs != null) {
Long lm = cacheItem.tagLastModifiedTs.get(tag);
if (lm != null) {
lastModified = lm;
}
}
if (PropertyUtil.isDirectRead()) {
configInfoBase = persistService.findConfigInfo4Tag(dataId, group, tenant, tag);
} else {
file = DiskUtil.targetTagFile(dataId, group, tenant, tag);
}
if (configInfoBase == null && fileNotExist(file)) {
// FIXME CacheItem
// No longer exists. It is impossible to simply calculate the push delayed. Here, simply record it as - 1.
ConfigTraceService.logPullEvent(dataId, group, tenant, requestIpApp, -1,
ConfigTraceService.PULL_EVENT_NOTFOUND, -1, requestIp);
// pullLog.info("[client-get] clientIp={}, {},
// no data",
// new Object[]{clientIp, groupKey});
response.setStatus(HttpServletResponse.SC_NOT_FOUND);
response.getWriter().println("config data not exist");
return HttpServletResponse.SC_NOT_FOUND + "";
}
}
}
//把获取的数据结果设置到response中返回
response.setHeader(Constants.CONTENT_MD5, md5);
// Disable cache.
response.setHeader("Pragma", "no-cache");
response.setDateHeader("Expires", 0);
response.setHeader("Cache-Control", "no-cache,no-store");
if (PropertyUtil.isDirectRead()) {
response.setDateHeader("Last-Modified", lastModified);
} else {
fis = new FileInputStream(file);
response.setDateHeader("Last-Modified", file.lastModified());
}
//如果是单机模式，直接把数据写回到客户端
if (PropertyUtil.isDirectRead()) {
out = response.getWriter();
out.print(configInfoBase.getContent());
out.flush();
out.close();
//否则，通过trasferTo
} else {
fis.getChannel()
.transferTo(0L, fis.getChannel().size(), Channels.newChannel(response.getOutputStream()));
}
LogUtil.PULL_CHECK_LOG.warn("{}|{}|{}|{}", groupKey, requestIp, md5, TimeUtils.getCurrentTimeStr());
final long delayed = System.currentTimeMillis() - lastModified;
// TODO distinguish pull-get && push-get
/*
Otherwise, delayed cannot be used as the basis of push delay directly,
because the delayed value of active get requests is very large.
*/
ConfigTraceService.logPullEvent(dataId, group, tenant, requestIpApp, lastModified,
ConfigTraceService.PULL_EVENT_OK, delayed, requestIp);
} finally {
//释放锁
releaseConfigReadLock(groupKey);
IoUtils.closeQuietly(fis);
}
//说明缓存为空
} else if (lockResult == 0) {
// FIXME CacheItem No longer exists. It is impossible to simply calculate the push delayed. Here, simply record it as - 1.
ConfigTraceService
.logPullEvent(dataId, group, tenant, requestIpApp, -1, ConfigTraceService.PULL_EVENT_NOTFOUND, -1,
requestIp);
response.setStatus(HttpServletResponse.SC_NOT_FOUND);
response.getWriter().println("config data not exist");
return HttpServletResponse.SC_NOT_FOUND + "";
} else {
PULL_LOG.info("[client-get] clientIp={}, {}, get data during dump", clientIp, groupKey);
response.setStatus(HttpServletResponse.SC_CONFLICT);
response.getWriter().println("requested file is being modified, please try later.");
return HttpServletResponse.SC_CONFLICT + "";
}
return HttpServletResponse.SC_OK + "";
}
persistService.findConfigInfo
从derby数据库中获取数据内容,这个就是一个基本的数据查询操作。
@Override
public ConfigInfo findConfigInfo(final String dataId, final String group, final String tenant) {
final String tenantTmp = StringUtils.isBlank(tenant) ? StringUtils.EMPTY : tenant;
final String sql = "SELECT ID,data_id,group_id,tenant_id,app_name,content,md5,type FROM config_info "
+ " WHERE data_id=? AND group_id=? AND tenant_id=?";
final Object[] args = new Object[] {dataId, group, tenantTmp};
return databaseOperate.queryOne(sql, args, CONFIG_INFO_ROW_MAPPER);
}
DiskUtil.targetFile
从磁盘目录中获取目标文件，直接根据dataId/group/tenant ，查找指定目录下的文件即可
public static File targetFile(String dataId, String group, String tenant) {
File file = null;
if (StringUtils.isBlank(tenant)) {
file = new File(EnvUtil.getNacosHome(), BASE_DIR);
} else {
file = new File(EnvUtil.getNacosHome(), TENANT_BASE_DIR);
file = new File(file, tenant);
}
file = new File(file, group);
file = new File(file, dataId);
return file;
}
客户端配置缓存更新
当客户端拿到配置后，需要动态刷新，从而保证数据和服务器端是一致的，这个过程是如何实现的呢？
Nacos采用长轮训机制来实现数据变更的同步，原理如下：
整体工作流程如下：
-
客户端发起长轮训请求
-
服务端收到请求以后，先比较服务端缓存中的数据是否相同，如果不同，则直接返回
-
如果相同，则通过schedule延迟29.5s之后再执行比较
-
为了保证当服务端在29.5s之内发生数据变化能够及时通知给客户端，服务端采用事件订阅的方式来监听服务端本地数据变化的事件，一旦收到事件，则触发DataChangeTask的通知，并且遍历allStubs队列中的ClientLongPolling,把结果写回到客户端，就完成了一次数据的推送
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如果 DataChangeTask 任务完成了数据的 “推送” 之后，ClientLongPolling 中的调度任务又开始执行了怎么办呢？ 很简单，只要在进行 “推送” 操作之前，先将原来等待执行的调度任务取消掉就可以了，这样就防止了推送操作写完响应数据之后，调度任务又去写响应数据，这时肯定会报错的。所以，在ClientLongPolling方法中，最开始的一个步骤就是删除订阅事件。
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