Github user viirya commented on a diff in the pull request:
https://github.com/apache/spark/pull/7021#discussion_r33343168
--- Diff:
core/src/main/scala/org/apache/spark/util/CheckpointingIterator.scala ---
@@ -0,0 +1,143 @@
+/*
+ * 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.util
+
+import java.io.IOException
+
+import scala.reflect.ClassTag
+
+import org.apache.hadoop.fs.FileSystem
+import org.apache.hadoop.fs.Path
+
+import org.apache.spark._
+import org.apache.spark.broadcast.Broadcast
+import org.apache.spark.rdd.CheckpointRDD
+import org.apache.spark.serializer.SerializationStream
+
+/**
+ * Wrapper around an iterator which writes checkpoint data to HDFS while
running action on
+ * a RDD to support checkpointing RDD.
+ */
+private[spark] class CheckpointingIterator[A: ClassTag, +I <: Iterator[A]](
+ sub: I,
+ path: String,
+ broadcastedConf: Broadcast[SerializableConfiguration],
+ partitionId: Int,
+ context: TaskContext,
+ blockSize: Int = -1) extends Iterator[A] with Logging {
+
+ val env = SparkEnv.get
+ var fs: FileSystem = null
+ val bufferSize = env.conf.getInt("spark.buffer.size", 65536)
+ var serializeStream: SerializationStream = null
+
+ var finalOutputPath: Path = null
+ var tempOutputPath: Path = null
+
+ def init(): this.type = {
+ val outputDir = new Path(path)
+ fs = outputDir.getFileSystem(broadcastedConf.value.value)
+
+ val finalOutputName = CheckpointRDD.splitIdToFile(partitionId)
+ finalOutputPath = new Path(outputDir, finalOutputName)
+ tempOutputPath =
+ new Path(outputDir, "." + finalOutputName + "-attempt-" +
context.attemptNumber)
+
+ if (fs.exists(tempOutputPath)) {
+ // There are more than one iterator of the RDD is consumed.
+ // Don't checkpoint data in this iterator.
+ doCheckpoint = false
+ return this
+ }
+
+ val fileOutputStream = if (blockSize < 0) {
+ fs.create(tempOutputPath, false, bufferSize)
+ } else {
+ // This is mainly for testing purpose
+ fs.create(tempOutputPath, false, bufferSize,
fs.getDefaultReplication, blockSize)
+ }
+ val serializer = env.serializer.newInstance()
+ serializeStream = serializer.serializeStream(fileOutputStream)
+ this
+ }
+
+ def completion(): Unit = {
+ if (!doCheckpoint) {
+ return
+ }
+
+ serializeStream.close()
+
+ if (!fs.rename(tempOutputPath, finalOutputPath)) {
+ if (!fs.exists(finalOutputPath)) {
+ logInfo("Deleting tempOutputPath " + tempOutputPath)
+ fs.delete(tempOutputPath, false)
+ throw new IOException("Checkpoint failed: failed to save output of
task: "
+ + context.attemptNumber + " and final output path does not
exist")
+ } else {
+ // Some other copy of this task must've finished before us and
renamed it
+ logInfo("Final output path " + finalOutputPath + " already exists;
not overwriting it")
+ fs.delete(tempOutputPath, false)
+ }
+ }
+ }
+
+ def checkpointing(item: A): Unit = {
+ serializeStream.writeObject(item)
+ }
+
+ override def next(): A = {
+ val item = sub.next()
+ if (doCheckpoint) {
+ checkpointing(item)
+ }
+ // If this the latest item, call hasNext will write to final output
early.
+ hasNext
--- End diff --
Sometimes the rdd.iterator will not be consumed to call `hasNext` until it
returns false, e.g., we already know the number of elements and call next() at
exact times of that number. If so, we need to write the checkpoint data to
final output path early when we return the latest element.
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