从远端服务器获取变更数据的主要模式有两种：推（push）和拉（pull）。Push 模式简单来说就是服务端主动将数据变更信息推送给客户端，这种模式优点是时效性好，服务端数据发生变更可以立马通知到客户端，但这种模式需要服务端维持与客户端的心跳连接，会增加服务端实现的复杂度，服务端也需要占用更多的资源来维持与客户端的连接。
而 Pull 模式则是客户端主动去服务器请求数据，例如，每间隔10ms就向服务端发起请求获取数据。显而易见pull模式存在时效性问题。请求的间隔也不太好设置，间隔太短，对服务器请求压力过大。间隔时间过长，那么必然会造成时效性很差。而且如果配置长时间不更新，并且存在大量的客户端就会产生大量无效的pull请求。
Nacos 没有采用上述的两种模式，而是采用了长轮询方式结合了推和拉的优点：
image.png
- 长轮询也是轮询，因此 Nacos 客户端会默认每10ms向服务端发起请求，当客户端请求服务端时会在请求头上携带长轮询的超时时间，默认是30s。而服务端接收到该请求时会hang住请求，为了防止客户端超时会在请求头携带的超时时间上减去500ms，因此默认会hang住请求29.5s。在这期间如果服务端发生了配置变更会产生相应的事件，监听到该事件后，会响应对应的客户端。这样一来客户端不会频繁发起轮询请求，而服务端也不需要维持与客户端的心跳，兼备了时效性和复杂度。
如果你觉得源码枯燥的话，可以选择不看后半部分的源码，先通过这张流程图去了解Nacos动态刷新机制的流程：
image.png
Nacos Config 长轮询源码剖析
首先，打开 com.alibaba.cloud.nacos.NacosConfigBootstrapConfiguration 这个类，从类名也可以看出该类是Nacos Config的启动配置类，是Nacos Config自动装配的入口。在该类中的 nacosConfigManager 方法实例化了一个 NacosConfigManager 对象，并注册到容器中：
@Bean
@ConditionalOnMissingBean
public NacosConfigManager nacosConfigManager(
NacosConfigProperties nacosConfigProperties) {
return new NacosConfigManager(nacosConfigProperties);
}
在 NacosConfigManager 的构造器中调用了 createConfigService 方法，这是一个静态方法用来创建 ConfigService 对象的单例。
/**
* Compatible with old design,It will be perfected in the future.
*/
static ConfigService createConfigService(
NacosConfigProperties nacosConfigProperties) {
// 双重检查锁模式的单例
if (Objects.isNull(service)) {
synchronized (NacosConfigManager.class) {
try {
if (Objects.isNull(service)) {
service = NacosFactory.createConfigService(
nacosConfigProperties.assembleConfigServiceProperties());
}
}
catch (NacosException e) {
log.error(e.getMessage());
throw new NacosConnectionFailureException(
nacosConfigProperties.getServerAddr(), e.getMessage(), e);
}
}
}
return service;
}
ConfigService 的具体实现是 NacosConfigService，在该类的构造器中主要初始化了 HttpAgent 和 ClientWorker 对象。ClientWorker 的构造器中则初始化了几个线程池：
public ClientWorker(final HttpAgent agent, final ConfigFilterChainManager configFilterChainManager,
final Properties properties) {
this.agent = agent;
this.configFilterChainManager = configFilterChainManager;
// Initialize the timeout parameter
init(properties);
// 创建具有定时执行功能的单线程池，用于定时执行 checkConfigInfo 方法
this.executor = Executors.newScheduledThreadPool(1, new ThreadFactory() {
@Override
public Thread newThread(Runnable r) {
Thread t = new Thread(r);
t.setName("com.alibaba.nacos.client.Worker." + agent.getName());
t.setDaemon(true);
return t;
}
});
// 创建具有定时执行功能的且线程数与cpu核数相对应的线程池，用于根据需要动态刷新的配置文件执行 LongPollingRunnable，因此长轮询任务是可以有多个并行的
this.executorService = Executors
.newScheduledThreadPool(Runtime.getRuntime().availableProcessors(), new ThreadFactory() {
@Override
public Thread newThread(Runnable r) {
Thread t = new Thread(r);
t.setName("com.alibaba.nacos.client.Worker.longPolling." + agent.getName());
t.setDaemon(true);
return t;
}
});
// 每10ms执行一次 checkConfigInfo 方法
this.executor.scheduleWithFixedDelay(new Runnable() {
@Override
public void run() {
try {
checkConfigInfo();
} catch (Throwable e) {
LOGGER.error("[" + agent.getName() + "] [sub-check] rotate check error", e);
}
}
}, 1L, 10L, TimeUnit.MILLISECONDS);
}
private void init(Properties properties) {
// 长轮询的超时时间，默认为30秒，此参数会被放到请求头中带到服务端，服务端会根据该参数去做长轮询的hold
timeout = Math.max(ConvertUtils.toInt(properties.getProperty(PropertyKeyConst.CONFIG_LONG_POLL_TIMEOUT),
Constants.CONFIG_LONG_POLL_TIMEOUT), Constants.MIN_CONFIG_LONG_POLL_TIMEOUT);
taskPenaltyTime = ConvertUtils
.toInt(properties.getProperty(PropertyKeyConst.CONFIG_RETRY_TIME), Constants.CONFIG_RETRY_TIME);
this.enableRemoteSyncConfig = Boolean
.parseBoolean(properties.getProperty(PropertyKeyConst.ENABLE_REMOTE_SYNC_CONFIG));
}
/**
* Check config info.
*/
public void checkConfigInfo() {
// Dispatch taskes.
// 获取需要监听的文件数量
int listenerSize = cacheMap.size();
// Round up the longingTaskCount.
// 默认一个 LongPollingRunnable 可以处理监听3k个配置文件的变化，超过3k个才会创建新的 LongPollingRunnable
int longingTaskCount = (int) Math.ceil(listenerSize / ParamUtil.getPerTaskConfigSize());
if (longingTaskCount > currentLongingTaskCount) {
for (int i = (int) currentLongingTaskCount; i < longingTaskCount; i++) {
// The task list is no order.So it maybe has issues when changing.
executorService.execute(new LongPollingRunnable(i));
}
currentLongingTaskCount = longingTaskCount;
}
}
LongPollingRunnable 类主要用于检查本地配置，以及长轮询地去服务端获取变更配置的 dataid 和 group，其代码位于 com.alibaba.nacos.client.config.impl.ClientWorker 类，代码如下：
class LongPollingRunnable implements Runnable {
private final int taskId;
public LongPollingRunnable(int taskId) {
this.taskId = taskId;
}
@Override
public void run() {
List cacheDatas = new ArrayList();
List inInitializingCacheList = new ArrayList();
try {
// check failover config
// 遍历本地缓存的配置
for (CacheData cacheData : cacheMap.values()) {
if (cacheData.getTaskId() == taskId) {
cacheDatas.add(cacheData);
try {
// 检查本地配置
checkLocalConfig(cacheData);
if (cacheData.isUseLocalConfigInfo()) {
cacheData.checkListenerMd5();
}
} catch (Exception e) {
LOGGER.error("get local config info error", e);
}
}
}
// check server config
// 通过长轮询检查服务端配置
List changedGroupKeys = checkUpdateDataIds(cacheDatas, inInitializingCacheList);
if (!CollectionUtils.isEmpty(changedGroupKeys)) {
LOGGER.info("get changedGroupKeys:" + changedGroupKeys);
}
for (String groupKey : changedGroupKeys) {
String[] key = GroupKey.parseKey(groupKey);
String dataId = key[0];
String group = key[1];
String tenant = null;
if (key.length == 3) {
tenant = key[2];
}
try {
String[] ct = getServerConfig(dataId, group, tenant, 3000L);
CacheData cache = cacheMap.get(GroupKey.getKeyTenant(dataId, group, tenant));
cache.setContent(ct[0]);
if (null != ct[1]) {
cache.setType(ct[1]);
}
LOGGER.info("[{}] [data-received] dataId={}, group={}, tenant={}, md5={}, content={}, type={}",
agent.getName(), dataId, group, tenant, cache.getMd5(),
ContentUtils.truncateContent(ct[0]), ct[1]);
} catch (NacosException ioe) {
String message = String
.format("[%s] [get-update] get changed config exception. dataId=%s, group=%s, tenant=%s",
agent.getName(), dataId, group, tenant);
LOGGER.error(message, ioe);
}
}
for (CacheData cacheData : cacheDatas) {
if (!cacheData.isInitializing() || inInitializingCacheList
.contains(GroupKey.getKeyTenant(cacheData.dataId, cacheData.group, cacheData.tenant))) {
cacheData.checkListenerMd5();
cacheData.setInitializing(false);
}
}
inInitializingCacheList.clear();
executorService.execute(this);
} catch (Throwable e) {
// If the rotation training task is abnormal, the next execution time of the task will be punished
LOGGER.error("longPolling error : ", e);
executorService.schedule(this, taskPenaltyTime, TimeUnit.MILLISECONDS);
}
}
}
上面有个 checkUpdateDataIds 方法，用于获取发生变更了的配置文件的dataId列表，它同样位于 ClientWorker 内。如下：
/**
* Fetch the dataId list from server.
*
* @param cacheDatas CacheDatas for config infomations.
* @param inInitializingCacheList initial cache lists.
* @return String include dataId and group (ps: it maybe null).
* @throws Exception Exception.
*/
List checkUpdateDataIds(List cacheDatas, List inInitializingCacheList) throws Exception {
// 拼接出配置文件的唯一标识
StringBuilder sb = new StringBuilder();
for (CacheData cacheData : cacheDatas) {
if (!cacheData.isUseLocalConfigInfo()) {
sb.append(cacheData.dataId).append(WORD_SEPARATOR);
sb.append(cacheData.group).append(WORD_SEPARATOR);
if (StringUtils.isBlank(cacheData.tenant)) {
sb.append(cacheData.getMd5()).append(LINE_SEPARATOR);
} else {
sb.append(cacheData.getMd5()).append(WORD_SEPARATOR);
sb.append(cacheData.getTenant()).append(LINE_SEPARATOR);
}
if (cacheData.isInitializing()) {
// It updates when cacheData occours in cacheMap by first time.
inInitializingCacheList
.add(GroupKey.getKeyTenant(cacheData.dataId, cacheData.group, cacheData.tenant));
}
}
}
boolean isInitializingCacheList = !inInitializingCacheList.isEmpty();
return checkUpdateConfigStr(sb.toString(), isInitializingCacheList);
}
/**
* Fetch the updated dataId list from server.
*
* @param probeUpdateString updated attribute string value.
* @param isInitializingCacheList initial cache lists.
* @return The updated dataId list(ps: it maybe null).
* @throws IOException Exception.
*/
List checkUpdateConfigStr(String probeUpdateString, boolean isInitializingCacheList) throws Exception {
Map params = new HashMap(2);
params.put(Constants.PROBE_MODIFY_REQUEST, probeUpdateString);
Map headers = new HashMap(2);
// 长轮询的超时时间
headers.put("Long-Pulling-Timeout", "" + timeout);
// told server do not hang me up if new initializing cacheData added in
if (isInitializingCacheList) {
headers.put("Long-Pulling-Timeout-No-Hangup", "true");
}
if (StringUtils.isBlank(probeUpdateString)) {
return Collections.emptyList();
}
try {
// In order to prevent the server from handling the delay of the client's long task,
// increase the client's read timeout to avoid this problem.
long readTimeoutMs = timeout + (long) Math.round(timeout >> 1);
// 向服务端发起一个http请求，该请求在服务端配置没有变更的情况下默认会hang住30s
HttpRestResult result = agent
.httpPost(Constants.CONFIG_CONTROLLER_PATH + "/listener", headers, params, agent.getEncode(),
readTimeoutMs);
if (result.ok()) {
setHealthServer(true);
// 响应状态是成功则解析响应体得到 dataId、group、tenant 等信息并返回
return parseUpdateDataIdResponse(result.getData());
} else {
setHealthServer(false);
LOGGER.error("[{}] [check-update] get changed dataId error, code: {}", agent.getName(),
result.getCode());
}
} catch (Exception e) {
setHealthServer(false);
LOGGER.error("[" + agent.getName() + "] [check-update] get changed dataId exception", e);
throw e;
}
return Collections.emptyList();
}
客户端对 listener 接口的请求会进入到服务端的 com.alibaba.nacos.config.server.controller.ConfigController#listener 方法进行处理，该方法主要是调用了 com.alibaba.nacos.config.server.controller.ConfigServletInner#doPollingConfig 方法。代码如下：
/**
* 轮询接口
*/
public String doPollingConfig(HttpServletRequest request, HttpServletResponse response,
Map clientMd5Map, int probeRequestSize)
throws IOException, ServletException {
// 如果支持长轮询则进入长轮询的流程
if (LongPollingService.isSupportLongPolling(request)) {
longPollingService.addLongPollingClient(request, response, clientMd5Map, probeRequestSize);
return HttpServletResponse.SC_OK + "";
}
// else 兼容短轮询逻辑
List changedGroups = MD5Util.compareMd5(request, response, clientMd5Map);
// 兼容短轮询result
String oldResult = MD5Util.compareMd5OldResult(changedGroups);
String newResult = MD5Util.compareMd5ResultString(changedGroups);
String version = request.getHeader(Constants.CLIENT_VERSION_HEADER);
if (version == null) {
version = "2.0.0";
}
int versionNum = Protocol.getVersionNumber(version);
/**
* 2.0.4版本以前, 返回值放入header中
*/
if (versionNum < START_LONGPOLLING_VERSION_NUM) {
response.addHeader(Constants.PROBE_MODIFY_RESPONSE, oldResult);
response.addHeader(Constants.PROBE_MODIFY_RESPONSE_NEW, newResult);
} else {
request.setAttribute("content", newResult);
}
// 禁用缓存
response.setHeader("Pragma", "no-cache");
response.setDateHeader("Expires", 0);
response.setHeader("Cache-Control", "no-cache,no-store");
response.setStatus(HttpServletResponse.SC_OK);
return HttpServletResponse.SC_OK + "";
}
我们主要关注上面的 com.alibaba.nacos.config.server.service.LongPollingService#addLongPollingClient 长轮询流程的方法。代码如下：
public void addLongPollingClient(HttpServletRequest req, HttpServletResponse rsp, Map clientMd5Map,
int probeRequestSize) {
String str = req.getHeader(LongPollingService.LONG_POLLING_HEADER);
String noHangUpFlag = req.getHeader(LongPollingService.LONG_POLLING_NO_HANG_UP_HEADER);
String appName = req.getHeader(RequestUtil.CLIENT_APPNAME_HEADER);
String tag = req.getHeader("Vipserver-Tag");
int delayTime = SwitchService.getSwitchInteger(SwitchService.FIXED_DELAY_TIME, 500);
/**
* 提前500ms返回响应，为避免客户端超时 @qiaoyi.dingqy 2013.10.22改动 add delay time for LoadBalance
*/
long timeout = Math.max(10000, Long.parseLong(str) - delayTime);
if (isFixedPolling()) {
timeout = Math.max(10000, getFixedPollingInterval());
// do nothing but set fix polling timeout
} else {
long start = System.currentTimeMillis();
List changedGroups = MD5Util.compareMd5(req, rsp, clientMd5Map);
if (changedGroups.size() > 0) {
generateResponse(req, rsp, changedGroups);
LogUtil.clientLog.info("{}|{}|{}|{}|{}|{}|{}",
System.currentTimeMillis() - start, "instant", RequestUtil.getRemoteIp(req), "polling",
clientMd5Map.size(), probeRequestSize, changedGroups.size());
return;
} else if (noHangUpFlag != null && noHangUpFlag.equalsIgnoreCase(TRUE_STR)) {
LogUtil.clientLog.info("{}|{}|{}|{}|{}|{}|{}", System.currentTimeMillis() - start, "nohangup",
RequestUtil.getRemoteIp(req), "polling", clientMd5Map.size(), probeRequestSize,
changedGroups.size());
return;
}
}
String ip = RequestUtil.getRemoteIp(req);
// 一定要由HTTP线程调用，否则离开后容器会立即发送响应
final AsyncContext asyncContext = req.startAsync();
// AsyncContext.setTimeout()的超时时间不准，所以只能自己控制
asyncContext.setTimeout(0L);
// 在 ClientLongPolling 的 run 方法会将 ClientLongPolling 实例（携带了本次请求的相关信息）放入 allSubs 中，然后会在29.5s后再执行另一个 Runnable，该 Runnable 用于等待29.5s后依旧没有相应的配置变更时对客户端进行响应，并将相应的 ClientLongPolling 实例从 allSubs 中移出
scheduler.execute(
new ClientLongPolling(asyncContext, clientMd5Map, ip, probeRequestSize, timeout, appName, tag));
}
而 LongPollingService 实现了 AbstractEventListener，也就是说能接收事件通知，在其 com.alibaba.nacos.config.server.service.LongPollingService#onEvent 方法中可以看到，它关注的是 LocalDataChangeEvent 事件：
@Override
public void onEvent(Event event) {
if (isFixedPolling()) {
// ignore
} else {
if (event instanceof LocalDataChangeEvent) {
LocalDataChangeEvent evt = (LocalDataChangeEvent)event;
scheduler.execute(new DataChangeTask(evt.groupKey, evt.isBeta, evt.betaIps));
}
}
}
在nacos上修改配置后就会产生 LocalDataChangeEvent 事件，此时 LongPollingService 也就能监听到，当收到该事件时就会遍历 allSubs，找到匹配的请求并将 groupKey 返回给客户端。具体代码在 DataChangeTask 中：
class DataChangeTask implements Runnable {
@Override
public void run() {
try {
ConfigService.getContentBetaMd5(groupKey);
for (Iterator iter = allSubs.iterator(); iter.hasNext(); ) {
ClientLongPolling clientSub = iter.next();
if (clientSub.clientMd5Map.containsKey(groupKey)) {
// 如果beta发布且不在beta列表直接跳过
if (isBeta && !betaIps.contains(clientSub.ip)) {
continue;
}
// 如果tag发布且不在tag列表直接跳过
if (StringUtils.isNotBlank(tag) && !tag.equals(clientSub.tag)) {
continue;
}
getRetainIps().put(clientSub.ip, System.currentTimeMillis());
iter.remove(); // 删除订阅关系
LogUtil.clientLog.info("{}|{}|{}|{}|{}|{}|{}",
(System.currentTimeMillis() - changeTime),
"in-advance",
RequestUtil.getRemoteIp((HttpServletRequest)clientSub.asyncContext.getRequest()),
"polling",
clientSub.clientMd5Map.size(), clientSub.probeRequestSize, groupKey);
clientSub.sendResponse(Arrays.asList(groupKey));
}
}
} catch (Throwable t) {
LogUtil.defaultLog.error("data change error:" + t.getMessage(), t.getCause());
}
}
DataChangeTask(String groupKey) {
this(groupKey, false, null);
}
DataChangeTask(String groupKey, boolean isBeta, List betaIps) {
this(groupKey, isBeta, betaIps, null);
}
DataChangeTask(String groupKey, boolean isBeta, List betaIps, String tag) {
this.groupKey = groupKey;
this.isBeta = isBeta;
this.betaIps = betaIps;
this.tag = tag;
}
final String groupKey;
final long changeTime = System.currentTimeMillis();
final boolean isBeta;
final List betaIps;
final String tag;
}
当客户端收到变更的dataid+group后，就会去服务端获取最新的配置数据，并更新本地数据 cacheData，然后发送数据变更事件，整个流程结束。
- 获取服务端最新配置数据的方法：
com.alibaba.nacos.client.config.impl.ClientWorker#getServerConfig - 发送数据变更事件的方法：
com.alibaba.nacos.client.config.impl.CacheData#checkListenerMd5
最后附上一张流程与源码的对应图：
nacos config刷新类图.png