Github user vanzin commented on a diff in the pull request:
https://github.com/apache/spark/pull/22624#discussion_r223767268
--- Diff:
core/src/main/scala/org/apache/spark/deploy/security/AbstractCredentialRenewer.scala
---
@@ -0,0 +1,224 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.spark.deploy.security
+
+import java.io.File
+import java.security.PrivilegedExceptionAction
+import java.util.concurrent.{ScheduledExecutorService, TimeUnit}
+import java.util.concurrent.atomic.AtomicReference
+
+import org.apache.hadoop.conf.Configuration
+import org.apache.hadoop.security.{Credentials, UserGroupInformation}
+
+import org.apache.spark.SparkConf
+import org.apache.spark.deploy.SparkHadoopUtil
+import org.apache.spark.internal.Logging
+import org.apache.spark.internal.config._
+import org.apache.spark.rpc.RpcEndpointRef
+import
org.apache.spark.scheduler.cluster.CoarseGrainedClusterMessages.UpdateDelegationTokens
+import org.apache.spark.ui.UIUtils
+import org.apache.spark.util.ThreadUtils
+
+/**
+ * Base class for periodically updating delegation tokens needed by the
application.
+ *
+ * When configured with a principal and a keytab, this manager will make
sure long-running apps
+ * (such as Spark Streaming apps) can run without interruption while
accessing secured services. It
+ * periodically logs in to the KDC with user-provided credentials, and
contacts all the configured
+ * secure services to obtain delegation tokens to be distributed to the
rest of the application.
+ *
+ * This class will manage the kerberos login, by renewing the TGT when
needed. Because the UGI API
+ * does not expose the TTL of the TGT, a configuration controls how often
to check that a relogin is
+ * necessary. This is done reasonably often since the check is a no-op
when the relogin is not yet
+ * needed. The check period can be overridden in the configuration.
+ *
+ * New delegation tokens are created once 75% of the renewal interval of
the original tokens has
+ * elapsed. The new tokens are sent to the Spark driver endpoint once it's
registered with the AM.
+ * The driver is tasked with distributing the tokens to other processes
that might need them.
+ *
+ * This class can also be used when without a principal and keytab, in
which case token renewal will
--- End diff --
The goal is to make it simple for scheduler backends to use token
functionality. Adding more classes that the backends need to deal with
complicates things instead of making them simpler.
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