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

    https://github.com/apache/spark/pull/21366#discussion_r195512808
  
    --- Diff: 
resource-managers/kubernetes/core/src/main/scala/org/apache/spark/scheduler/cluster/k8s/ExecutorPodsPollingSnapshotSource.scala
 ---
    @@ -0,0 +1,65 @@
    +/*
    + * 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.scheduler.cluster.k8s
    +
    +import java.util.concurrent.{Future, ScheduledExecutorService, TimeUnit}
    +
    +import io.fabric8.kubernetes.client.KubernetesClient
    +import scala.collection.JavaConverters._
    +
    +import org.apache.spark.SparkConf
    +import org.apache.spark.deploy.k8s.Config._
    +import org.apache.spark.deploy.k8s.Constants._
    +import org.apache.spark.util.ThreadUtils
    +
    +private[spark] class ExecutorPodsPollingSnapshotSource(
    +    conf: SparkConf,
    +    kubernetesClient: KubernetesClient,
    +    snapshotsStore: ExecutorPodsSnapshotsStore,
    +    pollingExecutor: ScheduledExecutorService) {
    +
    +  private val pollingInterval = 
conf.get(KUBERNETES_EXECUTOR_API_POLLING_INTERVAL)
    +
    +  private var pollingFuture: Future[_] = _
    +
    +  def start(applicationId: String): Unit = {
    +    require(pollingFuture == null, "Cannot start polling more than once.")
    +    pollingFuture = pollingExecutor.scheduleWithFixedDelay(
    +      new PollRunnable(applicationId), pollingInterval, pollingInterval, 
TimeUnit.MILLISECONDS)
    +  }
    +
    +  def stop(): Unit = {
    +    if (pollingFuture != null) {
    +      pollingFuture.cancel(true)
    +      pollingFuture = null
    +    }
    +    ThreadUtils.shutdown(pollingExecutor)
    +  }
    +
    +  private class PollRunnable(applicationId: String) extends Runnable {
    +    override def run(): Unit = {
    +      snapshotsStore.replaceSnapshot(kubernetesClient
    --- End diff --
    
    Not strictly why that's done here but a side-effect I suppose. Really the 
snapshots store should push an initial empty snapshot to all subscribers when 
it starts, and the unit tests do check for that - it's the responsibility of 
the snapshots store.


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