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:
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 getDelegationToken(Text renewer)
throws IOException {
assert dtService != null;
Token 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:
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 nnAddrs) {
// this cloning logic is only used by hdfs
Text haService = HAUtil.buildTokenServiceForLogicalUri(haUri,
HdfsConstants.HDFS_URI_SCHEME);
Token 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 specificToken =
new Token.PrivateToken(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。