什么是delegation token

delegation token其实就是hadoop里一种轻量级认证方法，作为kerberos认证的一种补充。理论上只使用kerberos来认证是足够了，为什么hadoop还要自己开发一套使用delegation token的认证方式呢？这是因为如果在一个很大的分布式系统当中，如果每个节点访问某个服务的时候都使用kerberos来作为认证方式，那么势必对KDC造成很大的压力，KDC就会成为一个系统的瓶颈。

与kerberos的区别

kerberos认证需三方参与，client， kdc， server三方协作完成认证。通常包括三个子过程：

• client向kdc申请TGT（Ticket Granting Ticket），TGT包含client信息及client和KDC之间的 session key两部分信息，并且使用KDC的master key加密
• client使用TGT向KDC申请访问某个服务的Ticket，Ticket包含client信息及client和server之间的session key两部分信息，并且使用server的master key加密。
• client使用Ticket访问某个服务。

Delegation token的认证只需要两方参与，client和server。在server端生成token并发送给client端。client使用该token访问server，server对该token进行认证。
Delegation token可以传递给其它服务使用，这也是它叫delegation token的原因。比如在client端获取到hdfs delegation token后，可以分发到Mapper端和Reducer端。这样Map，Reduce就不用在通过Kerberos认证而直接使用该token访问hdfs。同时，delegation token可以指定更新者（renewer），比如yarn，或者自己。token快要过期的时候需要更新，更新的时候只涉及更新者和server端。其它任何使用该token的人都不会受到影响。

delegation token 期限

delegation token有过期时间，需要定期刷新才能保证token有效。但是刷新次数不是无限的，也就是说每个token都有个最大生存时间，超过该时间，该token就失效。比如token每个24小时需要刷新一次，否则就失效。同时每个token最大生命值为7天，那么七天后该token就不能在被使用。

delegation 包含的内容

Token.java

  private byte[] identifier;
  private byte[] password;
  private Text kind;
  private Text service;
  private TokenRenewer renewer;

其中identifiertoken的标识，password用于server端认证该token。kind参数表示该token的类型，比如HDFS_DELEGATION_TOKEN，service表示该token访问的服务，比如ha-hdfs:<nameservice>，renewer表示刷新者。
以上几个部分是client拿到的token包含的内容。token的失效时间、owner、realuser等信息存放在server端。

delegation 生命周期

上图展示了在yarn应用中，delegation token的生命流程。
1）client端首先通过Kerberos 认证方式访问namenode，获取DT（delegation token）
2）client向yarn提交应用，并且把DT传给RM。同时会指定yarn作为该token的renewer。
3）rm选一个节点启动Am，随后AM向RM申请资源，将worker contaiern都启动。这一步中DT都会分发到相应的container中。
4）所有的工作节点都使用DT去访问hdfs
5）当工作结束后，RM释放该DT。

delegation token过期应该怎么办

delegation token会失效，集群默认配置是renew的间隔为一天，token最大生存时间为7天。对于像mapreduce这种批处理任务可能不会面临token失效的问题，但对于spark streaming, storm等这种长时运行应用来说，不得不面临一个问题：token存在最大生命周期。当token达到其最大生命周期的时候，比如七天，所有的工作节点（比如spark streaming的executor）中使用的token都会失效，此时在使用该token去访问hdfs就会被namenode拒绝，导致应用异常退出。

一种解决思路是将keytab文件分发给Am及每个container，让am和container去访问kdc来认证，但这种方式会造成文章开头所说的问题：对KDC造成很大的访问压力，导致KDC会误认为自己遭受了DDos攻击，从而影响程序性能。
另一种解决思路是先由client把keytab文件放到hdfs上。然后在Am中使用keytab登录，并申请delegation token。Am在启动worker的时候把该token分发给相应的容器。当token快要过期的时候，Am重新登录一次，并重新获取delegation token，并告知所有的worker使用更新后的token访问服务。

spark中怎么解决delegation token过期问题

spark使用的就是第二种解决思路，接下来详细分析下spark1.6是怎么解决token过期问题的。
spark 为了解决DT失效问题，加了两个参数"--keytab"和"--principal"，分别指定用于kerberos登录的keytab文件和principal。spark中用于提交yarn任务的类为Client

org.apache.spark.deploy.yarn.Client

def submitApplication(): ApplicationId = {
   var appId: ApplicationId = null
   try {
     launcherBackend.connect()
     // Setup the credentials before doing anything else,
     // so we have don't have issues at any point.
     setupCredentials()
     yarnClient.init(yarnConf)
     yarnClient.start()

     logInfo("Requesting a new application from cluster with %d NodeManagers"
       .format(yarnClient.getYarnClusterMetrics.getNumNodeManagers))

     // Get a new application from our RM
     val newApp = yarnClient.createApplication()
     val newAppResponse = newApp.getNewApplicationResponse()
     appId = newAppResponse.getApplicationId()
     reportLauncherState(SparkAppHandle.State.SUBMITTED)
     launcherBackend.setAppId(appId.toString())

     // Verify whether the cluster has enough resources for our AM
     verifyClusterResources(newAppResponse)

     // Set up the appropriate contexts to launch our AM
     val containerContext = createContainerLaunchContext(newAppResponse)
     val appContext = createApplicationSubmissionContext(newApp, containerContext)

     // Finally, submit and monitor the application
     logInfo(s"Submitting application ${appId.getId} to ResourceManager")
     yarnClient.submitApplication(appContext)
     appId
   } catch {
     case e: Throwable =>
       if (appId != null) {
         cleanupStagingDir(appId)
       }
       throw e
   }

submitApplication是提交yarn任务的入口，该函数的最开始调用了setupCredentials函数用于设置Credential。

def setupCredentials(): Unit = {
    loginFromKeytab = args.principal != null || sparkConf.contains("spark.yarn.principal")
    ...
    // Defensive copy of the credentials
    credentials = new Credentials(UserGroupInformation.getCurrentUser.getCredentials)
  }

先判断参数中是否包含“--principal”，如果包含，则成员变量logingFromKeytab置为true。这个成员变量用于后面的一些判断。另外就是获取当前ugi中包含的Credentials对象。
设置完credentials后，调用yarn的接口创建一个Application，并取得appid，然后需要创建containerLaunchContext，这是走的yarn程序的标准流程，进一步分析该函数

private def createContainerLaunchContext(newAppResponse: GetNewApplicationResponse)
    : ContainerLaunchContext = {
    logInfo("Setting up container launch context for our AM")
    val appId = newAppResponse.getApplicationId
    val appStagingDir = getAppStagingDir(appId)
    val pySparkArchives =
      if (sparkConf.getBoolean("spark.yarn.isPython", false)) {
        findPySparkArchives()
      } else {
        Nil
      }
    val launchEnv = setupLaunchEnv(appStagingDir, pySparkArchives)
    val localResources = prepareLocalResources(appStagingDir, pySparkArchives)

    // Set the environment variables to be passed on to the executors.
    distCacheMgr.setDistFilesEnv(launchEnv)
    distCacheMgr.setDistArchivesEnv(launchEnv)

    val amContainer = Records.newRecord(classOf[ContainerLaunchContext])
    amContainer.setLocalResources(localResources.asJava)
    amContainer.setEnvironment(launchEnv.asJava)
    ...
    setupSecurityToken(amContainer)

    amContainer
  }

这个函数很长，省略了一些代码，主要是设置container启动的命令行，环境变量，classpath之类的东西。在setupLaunchEnv函数里设置了后期在Am中用于通知executor的credentials文件

private def setupLaunchEnv(
      stagingDir: String,
      pySparkArchives: Seq[String]): HashMap[String, String] = {
    ...
    if (loginFromKeytab) {
      val remoteFs = FileSystem.get(hadoopConf)
      val stagingDirPath = new Path(remoteFs.getHomeDirectory, stagingDir)
      val credentialsFile = "credentials-" + UUID.randomUUID().toString
      sparkConf.set(
        "spark.yarn.credentials.file", new Path(stagingDirPath, credentialsFile).toString)
      logInfo(s"Credentials file set to: $credentialsFile")
      val renewalInterval = getTokenRenewalInterval(stagingDirPath)
      sparkConf.set("spark.yarn.token.renewal.interval", renewalInterval.toString)
    }

    ...
}

先判断loginFromKeytab是否为true。在前面我们看到只要在命令行中使用了"--principal"参数，loginFromKeytab就为true，这里声明了用于存放token的文件位置，默认为hdfs上位于/user/{user}/.sparkSgating/{appid}目录下的以credentials开头的文件。并把该文件位置放在sparkConf中，key为“spark.yarn.credentials.file”，这在后边会用到。同时这里获取了DT renew的间隔。也同样放在sparkConf中，key为“spark.yarn.token.renewal.interval”。
设置完登录环境后，进入到另一个函数prepareLocalResources，这个函数里边有一个关键的步骤：获取hdfs
delegation token

YarnSparkHadoopUtil.get.obtainTokensForNamenodes(nns, hadoopConf, credentials)

def obtainTokensForNamenodes(
    paths: Set[Path],
    conf: Configuration,
    creds: Credentials,
    renewer: Option[String] = None
  ): Unit = {
    if (UserGroupInformation.isSecurityEnabled()) {
      val delegTokenRenewer = renewer.getOrElse(getTokenRenewer(conf))
      paths.foreach { dst =>
        val dstFs = dst.getFileSystem(conf)
        logInfo("getting token for namenode: " + dst)
        dstFs.addDelegationTokens(delegTokenRenewer, creds)
      }
    }
  }

这个函数用于向namenode索取hdfs delegation token，并把该token添加到Credentials对象中。前面我们讲过，credentiasl对象的初值为UserGroupInformation.getCurrentUser.getCredentials, 而ugi中默认是不包含hdfs delegation token的。因此通过该函数会吧hdfs delegation token添加到credentials中。
然后回到createContainerLaunchContext， 准备工作都做好后，创建了amContainer，并调用setupSecurityToken函数给amContainer设置刚刚获取到的token。所以当Am起来后不需要通过kerberos认证，可以直接使用hdfs delegation token与namenode交互。

private def setupSecurityToken(amContainer: ContainerLaunchContext): Unit = {
    val dob = new DataOutputBuffer
    credentials.writeTokenStorageToStream(dob)
    amContainer.setTokens(ByteBuffer.wrap(dob.getData))
  }

至此，有关token的东西都准备好了，调用yarnClient.submitApplication(appContext)向yarn提交任务。yarn在收到请求后会先找一个机器启动AmContainer。yarn启动container的命令其实就是client传给yarn的。大概就是“bin/java xxx org.apache.spark.deploy.yarn.ApplicationMaster xxx”。Am的入口为ApplicationMaster的run函数。

org.apache.spark.deploy.yarn.ApplicationMaster

final def run(): Int = {
    try {
      ...
      // If the credentials file config is present, we must periodically renew tokens. So create
      // a new AMDelegationTokenRenewer
      if (sparkConf.contains("spark.yarn.credentials.file")) {
        delegationTokenRenewerOption = Some(new AMDelegationTokenRenewer(sparkConf, yarnConf))
        // If a principal and keytab have been set, use that to create new credentials for executors
        // periodically
        delegationTokenRenewerOption.foreach(_.scheduleLoginFromKeytab())
      }

      if (isClusterMode) {
        runDriver(securityMgr)
      } else {
        runExecutorLauncher(securityMgr)
      }
    } catch {
      case e: Exception =>
        // catch everything else if not specifically handled
        logError("Uncaught exception: ", e)
        finish(FinalApplicationStatus.FAILED,
          ApplicationMaster.EXIT_UNCAUGHT_EXCEPTION,
          "Uncaught exception: " + e)
    }
    exitCode
  }

这里看到了一个熟悉的配置“spark.yarn.credentials.file”， 还记得我们之前讲过该参数被设置为什么了吗？就是Am用于保存token文件的位置。所以，如果spark submit启动的时候传递了"--principal"参数，就会在sparkConf中生成一个“spark.yarn.credentials.file”配置，如果sparkConf中有“spark.yarn.credentials.file”配置，在AM中，也就是run函数中，会生成一个AMDelegationTokenRenewer对象。从名字也可以看出，这个对象就负责定期的更新token，将token写入到一个hdfs文件，然后executor从该文件中获取新的token从而防止token过期的作用了。

AMDelegationTokenRenewer

private[spark] def scheduleLoginFromKeytab(): Unit = {
    val principal = sparkConf.get("spark.yarn.principal")
    val keytab = sparkConf.get("spark.yarn.keytab")

    /**
     * Schedule re-login and creation of new tokens. If tokens have already expired, this method
     * will synchronously create new ones.
     */
    def scheduleRenewal(runnable: Runnable): Unit = {
      val credentials = UserGroupInformation.getCurrentUser.getCredentials
      val renewalInterval = hadoopUtil.getTimeFromNowToRenewal(sparkConf, 0.75, credentials)
      // Run now!
      if (renewalInterval <= 0) {
        logInfo("HDFS tokens have expired, creating new tokens now.")
        runnable.run()
      } else {
        logInfo(s"Scheduling login from keytab in $renewalInterval millis.")
        delegationTokenRenewer.schedule(runnable, renewalInterval, TimeUnit.MILLISECONDS)
      }
    }

    // This thread periodically runs on the driver to update the delegation tokens on HDFS.
    val driverTokenRenewerRunnable =
      new Runnable {
        override def run(): Unit = {
          try {
            writeNewTokensToHDFS(principal, keytab)
            cleanupOldFiles()
          } catch {
            case e: Exception =>
              // Log the error and try to write new tokens back in an hour
              logWarning("Failed to write out new credentials to HDFS, will try again in an " +
                "hour! If this happens too often tasks will fail.", e)
              delegationTokenRenewer.schedule(this, 1, TimeUnit.HOURS)
              return
          }
          scheduleRenewal(this)
        }
      }
    // Schedule update of credentials. This handles the case of updating the tokens right now
    // as well, since the renenwal interval will be 0, and the thread will get scheduled
    // immediately.
    scheduleRenewal(driverTokenRenewerRunnable)
  }

首先判断token是否快要过期了，如果是，则调用writeNewTokensToHDFS函数获取新的token，并写到hdfs上。否则，生成一个调度任务再一段时间后重新判断。

private def writeNewTokensToHDFS(principal: String, keytab: String): Unit = {
    // Keytab is copied by YARN to the working directory of the AM, so full path is
    // not needed.

    // HACK:
    // HDFS will not issue new delegation tokens, if the Credentials object
    // passed in already has tokens for that FS even if the tokens are expired (it really only
    // checks if there are tokens for the service, and not if they are valid). So the only real
    // way to get new tokens is to make sure a different Credentials object is used each time to
    // get new tokens and then the new tokens are copied over the the current user's Credentials.
    // So:
    // - we login as a different user and get the UGI
    // - use that UGI to get the tokens (see doAs block below)
    // - copy the tokens over to the current user's credentials (this will overwrite the tokens
    // in the current user's Credentials object for this FS).
    // The login to KDC happens each time new tokens are required, but this is rare enough to not
    // have to worry about (like once every day or so). This makes this code clearer than having
    // to login and then relogin every time (the HDFS API may not relogin since we don't use this
    // UGI directly for HDFS communication.
    logInfo(s"Attempting to login to KDC using principal: $principal")
    //1）重新登录kdc
    val keytabLoggedInUGI = UserGroupInformation.loginUserFromKeytabAndReturnUGI(principal, keytab)
    logInfo("Successfully logged into KDC.")
    val tempCreds = keytabLoggedInUGI.getCredentials
    val credentialsPath = new Path(credentialsFile)
    val dst = credentialsPath.getParent
    //2）使用新的登录身份信息向namenode拿hdfs delegation token，并添加到tempCreds中
    keytabLoggedInUGI.doAs(new PrivilegedExceptionAction[Void] {
      // Get a copy of the credentials
      override def run(): Void = {
        val nns = YarnSparkHadoopUtil.get.getNameNodesToAccess(sparkConf) + dst
        hadoopUtil.obtainTokensForNamenodes(nns, freshHadoopConf, tempCreds)
        null
      }
    })
    // Add the temp credentials back to the original ones.
   //3)将新获取的token添加到当前登录用户中
    UserGroupInformation.getCurrentUser.addCredentials(tempCreds)
    val remoteFs = FileSystem.get(freshHadoopConf)
    // If lastCredentialsFileSuffix is 0, then the AM is either started or restarted. If the AM
    // was restarted, then the lastCredentialsFileSuffix might be > 0, so find the newest file
    // and update the lastCredentialsFileSuffix.
    if (lastCredentialsFileSuffix == 0) {
      hadoopUtil.listFilesSorted(
        remoteFs, credentialsPath.getParent,
        credentialsPath.getName, SparkHadoopUtil.SPARK_YARN_CREDS_TEMP_EXTENSION)
        .lastOption.foreach { status =>
        lastCredentialsFileSuffix = hadoopUtil.getSuffixForCredentialsPath(status.getPath)
      }
    }
    val nextSuffix = lastCredentialsFileSuffix + 1
    val tokenPathStr =
      credentialsFile + SparkHadoopUtil.SPARK_YARN_CREDS_COUNTER_DELIM + nextSuffix
    val tokenPath = new Path(tokenPathStr)
    val tempTokenPath = new Path(tokenPathStr + SparkHadoopUtil.SPARK_YARN_CREDS_TEMP_EXTENSION)
    logInfo("Writing out delegation tokens to " + tempTokenPath.toString)
    val credentials = UserGroupInformation.getCurrentUser.getCredentials
    //4）将credentials信息写到目标文件中
    credentials.writeTokenStorageFile(tempTokenPath, freshHadoopConf)
    logInfo(s"Delegation Tokens written out successfully. Renaming file to $tokenPathStr")
    remoteFs.rename(tempTokenPath, tokenPath)
    logInfo("Delegation token file rename complete.")
    lastCredentialsFileSuffix = nextSuffix
  }

更新token的代码都在这个函数里了。其实就包括以下几个步骤
1）使用keytab和principal重新登录kerberos，并获取登录的ugi信息：keytabLoggedInUGI，注意，这里仅仅是kerberos的一个认证过程，还并不涉及到hdfs delegation token的东西，即keytabLoggedInUGI 中并不包含token信息。loginUserFromKeytabAndReturnUGI函数回返回一个新的用户对象，而不会影响当前登录的用户
2）获取keytabLoggedInUGI 中的credentials对象，然后使用keytabLoggedInUGI 身份去向namenode获取新的hdfs delegation token。并将token添加到一个临时的credentials对象中
3）将临时的credentials对象中的token添加到当前登录的ugi中。此时Am中使用token已经被更新，所以Am不会出现token expired问题，但是还需要把token更新到executor中。
4）生成token的存放目录，token存放目录为/user/{user}/.sparkStaging/${appid}, token文件名格式为“credentials-UUID-suffix”， suffix为后缀，按文件个数递增。token file默认保留五天。
至此，Am端token已经写到hdfs文件了。接下来就是executor端怎么读到最新的token文件，并把token更新到自己的ugi当中。
Executor启动过程这里就不在分析，主要是在AM当中会生成executor的启动信息及上下文，并发给NodeManager，由NodeManager来启动executor的container。在spark中，最终代表executor的类为：CoarseGrainedExecutorBackend

CoarseGrainedExecutorBackend

private def run(
      driverUrl: String,
      executorId: String,
      hostname: String,
      cores: Int,
      appId: String,
      workerUrl: Option[String],
      userClassPath: Seq[URL]) {

     ...
      if (driverConf.contains("spark.yarn.credentials.file")) {
        logInfo("Will periodically update credentials from: " +
          driverConf.get("spark.yarn.credentials.file"))
        SparkHadoopUtil.get.startExecutorDelegationTokenRenewer(driverConf)
      }

      ...
    }
  }

这里也是先判断在sparkConf中是否有"spark.yarn.credentials.file"配置，如果有，会生成一个ExecutorDelegationTokenUpdater对象，并调用其updateCredentialsIfRequired更新token

ExecutorDelegationTokenUpdater

try {
      val credentialsFilePath = new Path(credentialsFile)
      val remoteFs = FileSystem.get(freshHadoopConf)
      SparkHadoopUtil.get.listFilesSorted(
        remoteFs, credentialsFilePath.getParent,
        credentialsFilePath.getName, SparkHadoopUtil.SPARK_YARN_CREDS_TEMP_EXTENSION)
        .lastOption.foreach { credentialsStatus =>
        val suffix = SparkHadoopUtil.get.getSuffixForCredentialsPath(credentialsStatus.getPath)
        if (suffix > lastCredentialsFileSuffix) {
          logInfo("Reading new delegation tokens from " + credentialsStatus.getPath)
          val newCredentials = getCredentialsFromHDFSFile(remoteFs, credentialsStatus.getPath)
          lastCredentialsFileSuffix = suffix
          UserGroupInformation.getCurrentUser.addCredentials(newCredentials)
          logInfo("Tokens updated from credentials file.")
        } else {
          // Check every hour to see if new credentials arrived.
          logInfo("Updated delegation tokens were expected, but the driver has not updated the " +
            "tokens yet, will check again in an hour.")
          delegationTokenRenewer.schedule(executorUpdaterRunnable, 1, TimeUnit.HOURS)
          return
        }
      }
      val timeFromNowToRenewal =
        SparkHadoopUtil.get.getTimeFromNowToRenewal(
          sparkConf, 0.8, UserGroupInformation.getCurrentUser.getCredentials)
      if (timeFromNowToRenewal <= 0) {
        // We just checked for new credentials but none were there, wait a minute and retry.
        // This handles the shutdown case where the staging directory may have been removed(see
        // SPARK-12316 for more details).
        delegationTokenRenewer.schedule(executorUpdaterRunnable, 1, TimeUnit.MINUTES)
      } else {
        logInfo(s"Scheduling token refresh from HDFS in $timeFromNowToRenewal millis.")
        delegationTokenRenewer.schedule(
          executorUpdaterRunnable, timeFromNowToRenewal, TimeUnit.MILLISECONDS)
      }
    } catch {
      // Since the file may get deleted while we are reading it, catch the Exception and come
      // back in an hour to try again
      case NonFatal(e) =>
        logWarning("Error while trying to update credentials, will try again in 1 hour", e)
        delegationTokenRenewer.schedule(executorUpdaterRunnable, 1, TimeUnit.HOURS)
    }

这个函数就是executor端更新token的整个过程，包括几个步骤
1）获取hdfs上保存credentials目录下的最近更新的文件，并取出其suffix，与当前程序中保存的lastCredentialsFileSuffix 比较，如果比lastCredentialsFileSuffix 大，则表示AM端更新了token，需要重新读取token并更新。
2）如果Am端还没更新，则过一小时重试
3）token更新完后，会再次判断下次待更新的时间，并生成一个调度任务，到期执行更新操作。
在executor端更新其实就是把hdfs上的credentials文件读取出来，使用 UserGroupInformation.getCurrentUser.addCredentials(newCredentials)函数对当前的ugi添加新的token信息就可以了。
至此，spark上解决hdfs delegation token过期问题就分析完了。整个过程类似与下面这张图：

总结下来就是在Am端更新token信息，并把更新后的token写到hdfs，在executor端读取hdfs上更新的token，并更新到自己的ugi当中。按理说这样能解决token过期的问题了，但是用过spark streaming的同学可能会遇到一个奇怪的问题，即使在提交任务的时候带上了"--principal"参数，还是会遇到hdfs delegation token 过期的问题，那又是怎么一会事呢？下面继续分析hdfs的一个bug。

hdfs delegation token bug

如上文分析，Spark在Am端会去更新token，因此理论上来讲应该不会出现token过期问题了，但在我们使用过程中还是会出现token过期的情况，网上查了后说是hfds上的一个bug导致:hdfs-9276

https://issues.apache.org/jira/browse/HDFS-9276

理解这个bug要先知道一个概念，即Token的service字段是client从server端获取token后添加的，client用于区分不同服务的token，在server端根本没有service字段的概念。客户端通过FileSystem.addDelegationTokens函数向namenode申请hdfs delegation token。当从server端申请到token后，会给token设置service字段：

DFSClient.java

public Token<DelegationTokenIdentifier> getDelegationToken(Text renewer)
      throws IOException {
    assert dtService != null;
    Token<DelegationTokenIdentifier> token =
      namenode.getDelegationToken(renewer);

    if (token != null) {
      token.setService(this.dtService);
      LOG.info("Created " + DelegationTokenIdentifier.stringifyToken(token));
    } else {
      LOG.info("Cannot get delegation token from " + renewer);
    }
    return token;

  }

这里将service字段设置为dtService。在HA这种情况下，客户端使用nameservice访问hdfs，所以dtService的值为：ha-hdfs:<nameservice>。
这个service我们暂且称之为logicService。但是，client必须使用IP:PORT访问server。当client确定active的namenode后，怎么确定使用哪个token来和server端认证呢？之前讲过token的service字段用于区分不同的server，但是该字段里并不包含具体的ip和端口。为了解决这个问题，其实每次在new 一个DFSClient实例时，会把token拷贝两份，并把里面的service字段替换成具体的ip和端口：

HAUtil.java

public static void cloneDelegationTokenForLogicalUri(
      UserGroupInformation ugi, URI haUri,
      Collection<InetSocketAddress> nnAddrs) {
    // this cloning logic is only used by hdfs
    Text haService = HAUtil.buildTokenServiceForLogicalUri(haUri,
        HdfsConstants.HDFS_URI_SCHEME);
    Token<DelegationTokenIdentifier> haToken =
        tokenSelector.selectToken(haService, ugi.getTokens());
    if (haToken != null) {
      for (InetSocketAddress singleNNAddr : nnAddrs) {
        // this is a minor hack to prevent physical HA tokens from being
        // exposed to the user via UGI.getCredentials(), otherwise these
        // cloned tokens may be inadvertently propagated to jobs
        Token<DelegationTokenIdentifier> specificToken =
            new Token.PrivateToken<DelegationTokenIdentifier>(haToken);
        SecurityUtil.setTokenService(specificToken, singleNNAddr);
        Text alias = new Text(
            buildTokenServicePrefixForLogicalUri(HdfsConstants.HDFS_URI_SCHEME)
                + "//" + specificToken.getService());
        ugi.addToken(alias, specificToken);
        LOG.debug("Mapped HA service delegation token for logical URI " +
            haUri + " to namenode " + singleNNAddr);
      }
    } else {
      LOG.debug("No HA service delegation token found for logical URI " +
          haUri);
    }
  }

这样一来，在client端，每一个token其实就有三个拷贝，分别为一个HA token，和两个对应到具体namenode的namenode token。于是，client想和哪个namenode通信就能选择到相应的token了。
那hdfs-9276这个bug就很明显了，意思是当用户使用UserGroupInformation.getCurrentUser().addCredentials(credentials)方法更新token时，只能更新HA token，并不能更新两个namenode token。所以当client使用namenode 的ip和port选择到某个namenode token时，该token其实还是老的token，并没有被更新，因此使用该token去访问server端，就会被server拒绝，并提示token过期异常。
所以9276的patch把这个问题解决了（代码就不分析，感兴趣的可以自己去看下），当用户addCredentials的时候，会把HA token对应的两个namenode token也更新。细心的读者应该发现，当每次new一个DFSClient实例的时候，内部就会把HA token拷贝两份，生成新的两个namenode token，因此如果每次都new 一个DFSClient是可以绕过9276描述的问题的。
其实spark也是尝试这么做的，回到spark 在excutor端更新token的过程：

ExecutorDelegationTokenUpdater

try {
      val credentialsFilePath = new Path(credentialsFile)
      //此处获取了一个新的FileSystem对象，但此时ugi中的HA token还没有被更新
      val remoteFs = FileSystem.get(freshHadoopConf)
      SparkHadoopUtil.get.listFilesSorted(
        remoteFs, credentialsFilePath.getParent,
        credentialsFilePath.getName, SparkHadoopUtil.SPARK_YARN_CREDS_TEMP_EXTENSION)
        .lastOption.foreach { credentialsStatus =>
        val suffix = SparkHadoopUtil.get.getSuffixForCredentialsPath(credentialsStatus.getPath)
        if (suffix > lastCredentialsFileSuffix) {
          logInfo("Reading new delegation tokens from " + credentialsStatus.getPath)
          //从hdfs中读取HA token
          val newCredentials = getCredentialsFromHDFSFile(remoteFs, credentialsStatus.getPath)
          lastCredentialsFileSuffix = suffix
          //把新的HA token更新到ugi当中
          UserGroupInformation.getCurrentUser.addCredentials(newCredentials)
          logInfo("Tokens updated from credentials file.")
        } else {
          // Check every hour to see if new credentials arrived.
          logInfo("Updated delegation tokens were expected, but the driver has not updated the " +
            "tokens yet, will check again in an hour.")
          delegationTokenRenewer.schedule(executorUpdaterRunnable, 1, TimeUnit.HOURS)
          return
        }
      }
      val timeFromNowToRenewal =
        SparkHadoopUtil.get.getTimeFromNowToRenewal(
          sparkConf, 0.8, UserGroupInformation.getCurrentUser.getCredentials)
      if (timeFromNowToRenewal <= 0) {
        // We just checked for new credentials but none were there, wait a minute and retry.
        // This handles the shutdown case where the staging directory may have been removed(see
        // SPARK-12316 for more details).
        delegationTokenRenewer.schedule(executorUpdaterRunnable, 1, TimeUnit.MINUTES)
      } else {
      //更新完token后，马上调度了下一个更新任务，而这个任务要在更新的token快要过期时才会执行
        logInfo(s"Scheduling token refresh from HDFS in $timeFromNowToRenewal millis.")
        delegationTokenRenewer.schedule(
          executorUpdaterRunnable, timeFromNowToRenewal, TimeUnit.MILLISECONDS)
      }
    } catch {
      // Since the file may get deleted while we are reading it, catch the Exception and come
      // back in an hour to try again
      case NonFatal(e) =>
        logWarning("Error while trying to update credentials, will try again in 1 hour", e)
        delegationTokenRenewer.schedule(executorUpdaterRunnable, 1, TimeUnit.HOURS)
    }

在函数的开始获使用FileSystem.get(freshHadoopConf)获取remoteFs对象，其中freshHadoopConf的"fs.hdfs.impl.disable.cache"设置为true，表示新生产一个FileSystem对象。这里其实很明显就是想让绕过9276 bug。但是很可惜，用的地方不对。在新生产这个对象的时候，ugi中保存的token其实还并没有被更新。随后读取hdfs中新的token，并更新到ugi当中。然后便调度下一个任务了。可以看到在更新token后，没有在new 一个FileSystem，所以ugi中的namenode token就得不到更新，因此还是会出现token过期问题。

hdfs delegation token过期问题分析到此结束。