Github user cloud-fan commented on a diff in the pull request:
https://github.com/apache/spark/pull/19751#discussion_r157639459
--- Diff:
core/src/main/scala/org/apache/spark/status/ElementTrackingStore.scala ---
@@ -0,0 +1,160 @@
+/*
+ * 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.status
+
+import java.util.concurrent.TimeUnit
+
+import scala.collection.mutable.{HashMap, ListBuffer}
+
+import com.google.common.util.concurrent.MoreExecutors
+
+import org.apache.spark.SparkConf
+import org.apache.spark.util.{ThreadUtils, Utils}
+import org.apache.spark.util.kvstore._
+
+/**
+ * A KVStore wrapper that allows tracking the number of elements of
specific types, and triggering
+ * actions once they reach a threshold. This allows writers, for example,
to control how much data
+ * is stored by potentially deleting old data as new data is added.
+ *
+ * This store is used when populating data either from a live UI or an
event log. On top of firing
+ * triggers when elements reach a certain threshold, it provides two extra
bits of functionality:
+ *
+ * - a generic worker thread that can be used to run expensive tasks
asynchronously; the tasks can
+ * be configured to run on the calling thread when more determinism is
desired (e.g. unit tests).
+ * - a generic flush mechanism so that listeners can be notified about
when they should flush
+ * internal state to the store (e.g. after the SHS finishes parsing an
event log).
+ *
+ * The configured triggers are run on a separate thread by default; they
can be forced to run on
+ * the calling thread by setting the `ASYNC_TRACKING_ENABLED`
configuration to `false`.
+ */
+private[spark] class ElementTrackingStore(store: KVStore, conf: SparkConf)
extends KVStore {
+
+ import config._
+
+ private val triggers = new HashMap[Class[_], Seq[Trigger[_]]]()
--- End diff --
use a mutable map?
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