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

    https://github.com/apache/spark/pull/19681#discussion_r150091458
  
    --- Diff: 
sql/core/src/main/scala/org/apache/spark/sql/execution/ui/SQLAppStatusStore.scala
 ---
    @@ -0,0 +1,172 @@
    +/*
    + * 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.sql.execution.ui
    +
    +import java.lang.{Long => JLong}
    +import java.util.Date
    +
    +import scala.collection.JavaConverters._
    +import scala.collection.mutable.ArrayBuffer
    +
    +import com.fasterxml.jackson.databind.annotation.JsonDeserialize
    +
    +import org.apache.spark.{JobExecutionStatus, SparkConf}
    +import org.apache.spark.scheduler.SparkListener
    +import org.apache.spark.status.AppStatusPlugin
    +import org.apache.spark.status.KVUtils.KVIndexParam
    +import org.apache.spark.ui.SparkUI
    +import org.apache.spark.util.Utils
    +import org.apache.spark.util.kvstore.KVStore
    +
    +/**
    + * Provides a view of a KVStore with methods that make it easy to query 
SQL-specific state. There's
    + * no state kept in this class, so it's ok to have multiple instances of 
it in an application.
    + */
    +private[sql] class SQLAppStatusStore(
    +    store: KVStore,
    +    listener: Option[SQLAppStatusListener] = None) {
    +
    +  def executionsList(): Seq[SQLExecutionUIData] = {
    +    store.view(classOf[SQLExecutionUIData]).asScala.toSeq
    +  }
    +
    +  def execution(executionId: Long): Option[SQLExecutionUIData] = {
    +    try {
    +      Some(store.read(classOf[SQLExecutionUIData], executionId))
    +    } catch {
    +      case _: NoSuchElementException => None
    +    }
    +  }
    +
    +  def executionsCount(): Long = {
    +    store.count(classOf[SQLExecutionUIData])
    +  }
    +
    +  def executionMetrics(executionId: Long): Map[Long, String] = {
    +    val exec = store.read(classOf[SQLExecutionUIData], executionId)
    +    Option(exec.metricValues)
    +      .orElse(listener.map(_.executionMetrics(executionId)))
    +      .getOrElse(Map())
    --- End diff --
    
    is there a race here when the execution ends?
    
    - T1 (UI thread, calling this method): execution hasn't ended, so 
`exec.metricValues` is null
    - T2 (listener): execution ends, drops execution from `liveExecutions`
    - T1: `_.executionMetrics(executionId)` throws an exception because its 
dropped from the `liveExecutions`


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