Github user steveloughran commented on a diff in the pull request:

    https://github.com/apache/spark/pull/8744#discussion_r39653540
  
    --- Diff: 
yarn/history/src/main/scala/org/apache/spark/deploy/history/yarn/YarnHistoryService.scala
 ---
    @@ -0,0 +1,981 @@
    +/*
    + * 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.history.yarn
    +
    +import java.net.{ConnectException, URI}
    +import java.util.concurrent.LinkedBlockingQueue
    +import java.util.concurrent.atomic.{AtomicBoolean, AtomicInteger}
    +
    +import scala.collection.mutable.LinkedList
    +import scala.util.control.NonFatal
    +
    +import org.apache.hadoop.conf.Configuration
    +import org.apache.hadoop.security.UserGroupInformation
    +import org.apache.hadoop.service.{AbstractService, Service}
    +import org.apache.hadoop.yarn.api.records.{ApplicationAttemptId, 
ApplicationId}
    +import org.apache.hadoop.yarn.api.records.timeline.{TimelineDomain, 
TimelineEntity, TimelinePutResponse}
    +import org.apache.hadoop.yarn.client.api.TimelineClient
    +import org.apache.hadoop.yarn.conf.YarnConfiguration
    +
    +import org.apache.spark.deploy.history.yarn.YarnTimelineUtils._
    +import org.apache.spark.scheduler._
    +import org.apache.spark.scheduler.cluster.YarnExtensionService
    +import org.apache.spark.util.{SystemClock, Clock, Utils}
    +import org.apache.spark.{Logging, SparkContext}
    +
    +/**
    + * Implements a Hadoop service with the init/start logic replaced by that
    + * of the YarnService.
    + * <p>
    + * As [[AbstractService]] implements `close()`, routing
    + * to its `stop` method, calling `close()` is sufficient
    + * to stop the service instance.
    + * <p>
    + * However, when registered to receive spark events, the service will 
continue to
    + * receive them until the spark context is stopped. Events received when 
this service
    + * is in a `STOPPED` state will be discarded.
    + */
    +private[spark] class YarnHistoryService  extends AbstractService("History 
Service")
    +  with YarnExtensionService with Logging {
    +
    +  import org.apache.spark.deploy.history.yarn.YarnHistoryService._
    +  /**
    +   * Spark context; valid once started
    +   */
    +  private var sparkContext: SparkContext = _
    +
    +  /** application ID. */
    +  private var _applicationId: ApplicationId = _
    +
    +  /** attempt ID this will be null if the service is started in 
yarn-client mode */
    +  private var _attemptId: Option[ApplicationAttemptId] = None
    +
    +  /** YARN timeline client */
    +  private var timelineClient: Option[TimelineClient] = None
    +
    +  /** registered event listener */
    +  private var listener: Option[YarnEventListener] = None
    +
    +  /** Applicaton name */
    +  private var appName: String = _
    +
    +  /** Application ID from the spark start event */
    +  private var sparkApplicationId: Option[String] = None
    +
    +  /** Optional Attempt ID from the spark start event */
    +  private var sparkApplicationAttemptId: Option[String] = None
    +
    +  /** user name as derived from `SPARK_USER` env var or `UGI` */
    +  private var userName: String = Utils.getCurrentUserName
    +
    +  /**
    +   * Clock for recording time
    +   */
    +  private val clock: Clock = new SystemClock()
    +
    +  /**
    +   * Start time of the application, as received in the start event.
    +   */
    +  private var startTime: Long = _
    +
    +  /** number of events to batch up before posting*/
    +  private var batchSize: Int = DEFAULT_BATCH_SIZE
    +
    +  /** queue of actions*/
    +  private val actionQueue = new LinkedBlockingQueue[QueuedAction]
    +
    +  /** cache layer to handle timeline client failure.*/
    +  private var entityList = new LinkedList[TimelineEntity]
    +
    +  /** current entity; wil be created on demand. */
    +  private var curEntity: Option[TimelineEntity] = None
    +
    +  /** Has a start event been processed? */
    +  private val appStartEventProcessed = new AtomicBoolean(false)
    +
    +  /* has the application event event been processed */
    +  private val appEndEventProcessed = new AtomicBoolean(false)
    +
    +  /** How many events have been received in the current entity */
    +  private var currentEventCount = 0
    +
    +  /** counter of events processed -that is have been through 
handleEvent()*/
    +  private val eventsProcessed: AtomicInteger = new AtomicInteger(0)
    +
    +  /** counter of events queued. */
    +  private val eventsQueued: AtomicInteger = new AtomicInteger(0)
    +
    +  /** how many event postings failed? */
    +  private val eventPostFailures: AtomicInteger = new AtomicInteger(0)
    +
    +  /** how many flushes have taken place? */
    +  private val flushCount = new AtomicInteger(0)
    +
    +  /** Event handler */
    +  private var eventHandlingThread: Option[Thread] = None
    +
    +  /**
    +   * Flag to indicate the thread is stopped; events aren't being
    +   * processed.
    +   */
    +  private val stopped: AtomicBoolean = new AtomicBoolean(true)
    +
    +  /**
    +   * boolean to track whether a thread is active or not, for tests to
    +   * monitor and see if the thread has completed.
    +   */
    +  private val postThreadActive: AtomicBoolean = new AtomicBoolean(false)
    +
    +  /**
    +   * object used for a lock on entity operations
    +   */
    +  private val entityLock: AnyRef = new AnyRef
    +
    +  /**
    +   * How long to wait for shutdown before giving up
    +   */
    +  private var maxTimeToWaitOnShutdown: Long = SHUTDOWN_WAIT_TIME
    +
    +  /** Domain ID for entities: may be null */
    +  private var domainId: String = null
    +
    +  /** URI to timeline web application -valid after `serviceStart()` */
    +  private var timelineWebappAddress: URI = _
    +
    +  /**
    +   * Create a timeline client and start it. This does not update the
    +   * `timelineClient` field, though it does verify that the field
    +   * is unset.
    +   *
    +   * The method is private to the package so that tests can access it, 
which
    +   * some of the mock tests do to override the timeline client creation.
    +   * @return the timeline client
    +   */
    +  private [yarn] def createTimelineClient(): TimelineClient = {
    +    require(timelineClient.isEmpty, "timeline client already set")
    +    YarnTimelineUtils.createTimelineClient(sparkContext)
    +  }
    +
    +  /**
    +   * Get the timeline client.
    +   * @return the client
    +   * @throws Exception if the timeline client is not currently running
    +   */
    +  def getTimelineClient: TimelineClient = {
    +    timelineClient.getOrElse(throw new Exception("Timeline client not 
running"))
    +  }
    +
    +  /**
    +   * Get the total number of processed events
    +   * @return counter of events processed
    +   */
    +  def getEventsProcessed: Int = {
    +    eventsProcessed.get
    +  }
    +
    +  /**
    +   * Get the total number of events queued
    +   * @return the total event count
    +   */
    +  def getEventsQueued: Int = {
    +    eventsQueued.get
    +  }
    +
    +  /**
    +   * Get the current size of the queue
    +   * @return the current queue length
    +   */
    +  def getQueueSize: Int = {
    +    actionQueue.size
    +  }
    +
    +  /**
    +   * Get the current batch size
    +   * @return the batch size
    +   */
    +  def getBatchSize: Int = {
    +    batchSize
    +  }
    +
    +  /**
    +   * Get the total number of failed posts events
    +   * @return counter of timeline post operations which failed
    +   */
    +  def getEventPostFailures: Int = {
    +    eventPostFailures.get
    +  }
    +
    +  /**
    +   * is the asynchronous posting thread active?
    +   * @return true if the post thread has started; false if it has not 
yet/ever started, or
    +   *         if it has finished.
    +   */
    +  def isPostThreadActive: Boolean = {
    +    postThreadActive.get
    +  }
    +
    +  /**
    +   * The YARN application ID of this history service
    +   * @return the application ID provided when the service started
    +   */
    +  def applicationId: ApplicationId = { _applicationId }
    +
    +  /**
    +   * The YARN attempt ID of this history service
    +   * @return the attempt ID provided when the service started
    +   */
    +  def attemptId: Option[ApplicationAttemptId] = { _attemptId }
    +
    +  /**
    +   * Reset the timeline client
    +   * <p>
    +   * 1. Stop the timeline client service if running.
    +   * 2. set the `timelineClient` field to `None`
    +   */
    +  def stopTimelineClient(): Unit = {
    +    stopOptionalService(timelineClient)
    +    timelineClient = None
    +  }
    +
    +  /**
    +   * Create the timeline domain
    +   * @return a domain string or null
    +   */
    +  private def createTimelineDomain(): String = {
    +    val sparkConf = sparkContext.getConf
    +    val aclsFlag = sparkConf.getOption("spark.acls.enable")
    +    val aclsOn = aclsFlag.getOrElse(
    +      sparkConf.get("spark.ui.acls.enable", "false")).toBoolean
    +    if (!aclsOn) {
    +      logDebug("ACLs are disabled; not creating the timeline domain")
    +      return null
    +    }
    +    val predefDomain = sparkConf.getOption(TIMELINE_DOMAIN)
    +    if (predefDomain.isDefined) {
    +      return predefDomain.get
    +    }
    +    val current = UserGroupInformation.getCurrentUser.getShortUserName
    +    val adminAcls  = stringToSet(sparkConf.get("spark.admin.acls", ""))
    +    val viewAcls = stringToSet(sparkConf.get("spark.ui.view.acls", ""))
    +    val modifyAcls = stringToSet(sparkConf.get("spark.modify.acls", ""))
    +
    +    val readers = (adminAcls ++ modifyAcls ++ 
viewAcls).foldLeft(current)(_ + " " + _)
    +    val writers = (adminAcls ++ modifyAcls).foldLeft(current)(_ + " " + _)
    +    var domainId = DOMAIN_ID_PREFIX + _applicationId
    +    logInfo(s"Creating domain $domainId with" +
    +      s" readers: readers  and writers: $writers")
    +
    +    // create the timeline domain with the reader and writer permissions
    +    val timelineDomain = new TimelineDomain()
    +    timelineDomain.setId(domainId)
    +    timelineDomain.setReaders(readers)
    +    timelineDomain.setWriters(writers)
    +    try {
    +      getTimelineClient.putDomain(timelineDomain)
    +    } catch {
    +      case e: Exception => {
    +        logError("cannot create the domain")
    +        // fallback to default
    +        domainId = null
    +      }
    +    }
    +    domainId
    +  }
    +
    +  /**
    +   * Start the service, calling the service's `init()` and `start()` 
actions in the
    +   * correct order
    +   * @param context spark context
    +   * @param appId YARN application ID
    +   * @param attemptId YARN attempt ID if known. Otherwise, `None`.
    +   * @return true if the service is hooked up to the timeline service; 
that is: it is live
    +   */
    +  def start(context: SparkContext,
    +      appId: ApplicationId,
    +      attemptId: Option[ApplicationAttemptId]): Boolean = {
    +    require(context != null, "Null context parameter")
    +    bindToYarnApplication(appId, attemptId)
    +    
    +    val yarnConf = new YarnConfiguration(context.hadoopConfiguration)
    +    // the init() operation checks the state machine & prevents invocation 
out of sequence
    +    init(yarnConf)
    +    this.sparkContext = context
    +
    +    // work out the attempt ID from the YARN attempt ID. No attempt, 
assume "1".
    +    // this is assumed by the AM, which uses it when creating a path to an 
attempt
    +    val attempt1 = attemptId match {
    +      case Some(attempt) => attempt.getAttemptId.toString
    +      case None => CLIENT_BACKEND_ATTEMPT_ID
    +    }
    +    setContextAppAndAttemptInfo(Some(appId.toString), Some(attempt1))
    +    batchSize = sparkContext.conf.getInt(BATCH_SIZE, batchSize)
    +
    +    start()
    +    if (timelineServiceEnabled) {
    +      true
    +    } else {
    +      logInfo("Yarn timeline service not available, disabling client.")
    +      false
    +    }
    +  }
    +
    +  /**
    +   * Service start.
    +   *
    +   * If the timeline client is enabled,
    +   * Create the timeline client, the timeline domain, and the event 
handling thread.
    +   *
    +   * Irrespective of the timeline enabled flag, the service will attempt 
to register
    +   * as a listener for events. They will merely be discarded.
    +   */
    +  override protected def serviceStart {
    +    require(sparkContext != null, "No spark context set")
    +    val conf: Configuration = getConfig
    +    if (timelineServiceEnabled) {
    +      timelineWebappAddress = getTimelineEndpoint(conf)
    +      logInfo(s"Starting $this")
    +      logInfo(s"Spark events will be published to the Timeline service at 
$timelineWebappAddress")
    +      timelineClient = Some(createTimelineClient)
    +      domainId = createTimelineDomain
    +      // declare that the processing is started
    +      stopped.set(false)
    +      eventHandlingThread = Some(new Thread(new Dequeue(), 
"HistoryEventHandlingThread"))
    +      eventHandlingThread.get.start
    +    } else {
    +      logInfo("Timeline service is disabled")
    +    }
    +    // irrespective of state, hook up to the listener
    +    val registered = registerListener
    +    if (registered) {
    +      logInfo(s"History Service listening for events: $this")
    +    } else {
    +      logInfo(s"History Service is not listening for events: $this")
    +    }
    +    super.serviceStart()
    +  }
    +
    +  /**
    +   * Check the service configuration to see if the timeline service is 
enabled
    +   * @return true if `YarnConfiguration.TIMELINE_SERVICE_ENABLED`
    +   *         is set.
    +   */
    +  def timelineServiceEnabled: Boolean = {
    +    YarnTimelineUtils.timelineServiceEnabled(getConfig)
    +  }
    +
    +  /**
    +   * Return a summary of the service state to help diagnose problems
    +   * during test runs, possibly even production
    +   * @return a summary of the current service state
    +   */
    +  override def toString: String = {
    +        s"YarnHistoryService for application $applicationId attempt 
$attemptId;" +
    +        s" endpoint=$timelineWebappAddress;" +
    +        s" bonded to ATS=$bondedToATS;" +
    +        s" listening=$listening;" +
    +        s" batchSize=$batchSize;" +
    +        s" flush count=$getFlushCount;" +
    +        s" current queue size=$getQueueSize;" +
    +        s" total number queued=$getEventsQueued, 
processed=$getEventsProcessed;" +
    +        s" post failures=$getEventPostFailures;"
    +  }
    +
    +  /**
    +   * Is the service listening to events from the spark context?
    +   * @return true if it has registered as a listener
    +   */
    +  def listening: Boolean = {
    +    listener.isDefined
    +  }
    +
    +  /**
    +   * Is the service hooked up to an ATS server. This does not
    +   * check the validity of the link, only whether or not the service
    +   * has been set up to talk to ATS.
    +   * @return true if the service has a timeline client
    +   */
    +  def bondedToATS: Boolean = {
    +    timelineClient.isDefined
    +  }
    +
    +  /**
    +   * Set the YARN binding information. This is called during startup. It 
is private
    +   * to the package so that tests may update this data
    +   * @param appId YARN application ID
    +   * @param attemptId optional attempt ID
    +   */
    +  private[yarn] def bindToYarnApplication(appId: ApplicationId,
    +      attemptId: Option[ApplicationAttemptId]): Unit = {
    +    require(appId != null, "Null appId parameter")
    +    this._applicationId = appId
    +    this._attemptId = attemptId
    +  }
    +  
    +  /**
    +   * Set the "spark" application and attempt information -the information
    +   * provided in the start event. The attempt ID here may be `None`; even
    +   * if set it may only be unique amongst the attempts of this application.
    +   * That is: not unique enough to be used as the entity ID
    +   * @param appId application ID
    +   * @param attemptId attempt ID
    +   */
    +  private def setContextAppAndAttemptInfo(appId: Option[String],
    +      attemptId: Option[String]): Unit = {
    +    logDebug(s"Setting application ID to $appId; attempt ID to $attemptId")
    +    sparkApplicationId = appId
    +    sparkApplicationAttemptId = attemptId
    +  }
    +
    +  /**
    +   * Add the listener if it is not disabled.
    +   * This is accessible in the same package purely for testing
    +   * @return true if the register was enabled
    +   */
    +  private [yarn] def registerListener: Boolean = {
    +    assert(sparkContext != null, "Null context")
    +    if (sparkContext.conf.getBoolean(REGISTER_LISTENER, true)) {
    +      logDebug("Registering listener to spark context")
    +      val l = new YarnEventListener(sparkContext, this)
    +      listener = Some(l)
    +      sparkContext.listenerBus.addListener(l)
    +      true
    +    } else {
    +      false
    +    }
    +  }
    +
    +  /**
    +   * Queue an action, or if the service's `stopped` flag
    +   * is set, discard it
    +   * @param action action to process
    +   * @return true if the event was queued
    +   */
    +  def enqueue(action: QueuedAction): Boolean = {
    +    if (!stopped.get) {
    +      innerEnqueue(action)
    +      true
    +    } else {
    +      if (timelineServiceEnabled) {
    +        // if the timeline service was ever enabled, log the fact the event
    +        // is being discarded. Don't do this if it was not, as it will
    +        // only make the logs noisy.
    +        logInfo(s"History service stopped; ignoring queued event : 
$action")
    +      }
    +      false
    +    }
    +  }
    +
    +  /**
    +   * Inner operation to queue the event. This does not check for service 
state
    +   * @param event event to queue
    +   */
    +  private def innerEnqueue(event: QueuedAction) = {
    +    eventsQueued.incrementAndGet
    +    logDebug(s"Enqueue $event")
    +    actionQueue.add(event)
    +  }
    +
    +  /**
    +   * Stop the service; this triggers flushing the queue and, if not 
already processed,
    +   * a pushing out of an application end event.
    +   *
    +   * This operation will block for up to `maxTimeToWaitOnShutdown` 
milliseconds
    +   * to await the asynchronous action queue completing.
    +   */
    +  override protected def serviceStop {
    +    // if the queue is live
    +    if (!stopped.get) {
    +
    +      if (!appEndEventProcessed.get()) {
    +        // push out an application stop event if none has been received
    +        logDebug("Generating a SparkListenerApplicationEnd during service 
stop()")
    +        val current = now()
    +        innerEnqueue(new 
HandleSparkEvent(SparkListenerApplicationEnd(current), current))
    +      }
    +
    +      // flush
    +      logInfo(s"Shutting down: pushing out ${actionQueue.size} events")
    +      innerEnqueue(FlushTimelineEvents())
    +
    +      // stop operation
    +      postThreadActive synchronized {
    +        innerEnqueue(StopQueue())
    +        // now await that marker flag
    +        postThreadActive.wait(maxTimeToWaitOnShutdown)
    +      }
    +
    +      if (!actionQueue.isEmpty) {
    +        // likely cause is ATS not responding.
    +        // interrupt the thread, albeit at the risk of app stop events
    +        // being lost. There's not much else that can be done if
    +        // ATS is being unresponsive
    +        logWarning(s"Did not finish flushing actionQueue before " +
    +          s"stopping ATSService, eventQueueBacklog= ${actionQueue.size}")
    +        eventHandlingThread.foreach(_.interrupt())
    +      }
    +      stopTimelineClient
    +      logInfo(s"Stopped: $this")
    +    }
    +  }
    +
    +  /**
    +   * Get the current entity, creating one on demand if needed, setting up 
its entity Id
    +   * from the current appID and attempt ID values, filters from the 
current name and user
    +   * @return the current entity
    +   */
    +  private def getOrCreateCurrentEntity() = {
    +    curEntity.getOrElse {
    +      currentEventCount = 0
    +      val entity = createTimelineEntity(
    +        _applicationId,
    +        _attemptId,
    +        sparkApplicationId,
    +        sparkApplicationAttemptId,
    +        appStartEventProcessed.get(),
    +        appName,
    +        userName)
    +      curEntity = Some(entity)
    +      logDebug(s"Demand creation of new entity ${describeEntity(entity)}")
    +      entity
    +    }
    +  }
    +
    +  /**
    +   * Perform any preflight checks.
    +   * @param entity timeline entity to review.
    +   */
    +  private[yarn] def preflightCheck(entity: TimelineEntity): Unit = {
    +    require(entity.getStartTime != null,
    +      s"No start time in ${describeEntity(entity)}")
    +  }
    +
    +  /**
    +   * If there is any available entity to be sent, push to timeline server
    +   */
    +  private def flushEntity(): Unit = {
    +    if (entityList.nonEmpty) {
    +      val count = flushCount.incrementAndGet()
    +      logDebug(s"flushEntity #$count: list size ${entityList.size}")
    +      var client = getTimelineClient
    +      val lastUpdated = now()
    +      // attempt to post each entity, removing from the list all of those
    +      // which were successfully posted. The remainder are retained for
    +      // followon attempts
    +      entityList = entityList.filter { entity => {
    +          if (entity == null) {
    +            false
    +          } else {
    +            entity.addOtherInfo(FIELD_LAST_UPDATED, lastUpdated)
    +            if (domainId != null) {
    +              entity.setDomainId(domainId)
    +            }
    +            val entityDescription = describeEntity(entity)
    +            logDebug(s"About to POST $entityDescription")
    +            try { 
    +              val response: TimelinePutResponse = 
client.putEntities(entity)
    +              val errors = response.getErrors
    +              if (!response.getErrors.isEmpty) {
    +                val err = errors.get(0)
    +                eventPostFailures.addAndGet(errors.size())
    +                if (err.getErrorCode != 0) {
    +                  logError(s"Failed to post 
${entityDescription}\n:${describeError(err)}")
    +                }
    +              } else {
    +                // successful submission
    +                logDebug(s"entity successfully posted: $entityDescription")
    +              }
    +              // whatever the outcome, this request is not re-issued
    +              false
    +            } catch {
    +              case e: ConnectException =>
    +                eventPostFailures.incrementAndGet
    +                logWarning(s"Connection exception submitting 
$entityDescription\n$e", e)
    +                true
    +
    +              case e: RuntimeException =>
    +                // this is probably a retry timeout event; some Hadoop 
versions don't
    +                // rethrow the exception causing the problem, instead 
raising an RTE
    +                eventPostFailures.incrementAndGet
    +                logWarning(s"Runtime exception submitting 
$entityDescription\n$e", e)
    +                // same policy as before: retry on these
    +                true
    +
    +              case e: Exception =>
    +                // something else has gone wrong.
    +                eventPostFailures.incrementAndGet
    +                logWarning(s"Could not handle history entity: 
$entityDescription\n$e", e)
    +                false
    +            }
    +          }
    +        }
    +      }
    +      logDebug(s"after pushEntities: ${entityList.size}")
    +    }
    +  }
    +
    +  /**
    +   * Process the action placed in the queue
    +   * @param action action
    +   * @return true if the queue processor must now exit
    +   */
    +  private def processAction(action: QueuedAction): Boolean = {
    +    action match {
    +
    +      case StopQueue() =>
    +        logDebug("Stop queue action received")
    +        true
    +
    +      case event: HandleSparkEvent =>
    +        try {
    +          handleEvent(event)
    +          false
    +        } catch {
    +          case NonFatal(e) => false
    +        }
    +
    +      case FlushTimelineEvents() =>
    +        logDebug("Flush queue action received")
    +        flushCurrentEventsToATS
    +        false
    +    }
    +  }
    +
    +  /**
    +   * If the event reaches the batch size or flush is true, push events to 
ATS.
    +   *
    +   * @param event event. If null no event is queued, but the post-queue 
flush logic
    +   *              still applies
    +   * @return true if the event should stop the service.
    +   */
    +  private def handleEvent(event: HandleSparkEvent): Unit = {
    +    var push = false
    +    // if we receive a new appStart event, we always push
    +    // not much contention here, only happens when service is stopped
    +    entityLock synchronized {
    +      logDebug(s"Processing event $event")
    +      if (eventsProcessed.get() % 1000 == 0) {
    +        logDebug(s"${eventsProcessed} events are processed")
    +      }
    +      val sparkEvent = event.sparkEvent
    +      sparkEvent match {
    +        case start: SparkListenerApplicationStart =>
    +          // we already have all information,
    +          // flush it for old one to switch to new one
    +          logDebug(s"Handling application start event: $event")
    +          appName = start.appName
    +          userName = start.sparkUser
    +          startTime = start.time
    +          if (startTime == 0) {
    +            startTime = now()
    +          }
    +          setContextAppAndAttemptInfo(
    +            start.appId,
    +            start.appAttemptId)
    +          // flush old entity
    +          resetCurrentEntity
    +          appStartEventProcessed.set(true)
    --- End diff --
    
    You can get events coming in before the app start, in particular 
{{onEnvironmentUpdate}} comes in. It's currently going out as an event using 
the entity built from the Yarn app id (which is guaranteed to be unique 
cluster-wide, and known from the moment the service starts). 
    
    Some tests did re-use the queue, but they should all have been moved off it 
with the move to multi-attempts (and fixed entity events).
    
    That's an interesting q. about event history without the full list of 
events ... I don't think there's any test for it.
    



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