Github user JoshRosen commented on a diff in the pull request:
https://github.com/apache/spark/pull/2931#discussion_r19485339
--- Diff:
streaming/src/main/scala/org/apache/spark/streaming/rdd/HDFSBackedBlockRDD.scala
---
@@ -0,0 +1,92 @@
+/*
+ * 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.streaming.rdd
+
+import scala.reflect.ClassTag
+
+import org.apache.hadoop.conf.Configuration
+
+import org.apache.spark.rdd.BlockRDD
+import org.apache.spark.storage.{BlockId, StorageLevel}
+import org.apache.spark.streaming.util.{WriteAheadLogFileSegment,
HdfsUtils, WriteAheadLogRandomReader}
+import org.apache.spark._
+
+private[streaming]
+class HDFSBackedBlockRDDPartition(
+ val blockId: BlockId,
+ val index: Int,
+ val segment: WriteAheadLogFileSegment
+ ) extends Partition
+
+private[streaming]
+class HDFSBackedBlockRDD[T: ClassTag](
+ @transient sc: SparkContext,
+ @transient hadoopConfiguration: Configuration,
+ @transient override val blockIds: Array[BlockId],
+ @transient val segments: Array[WriteAheadLogFileSegment],
+ val storeInBlockManager: Boolean,
+ val storageLevel: StorageLevel
+ ) extends BlockRDD[T](sc, blockIds) {
+
+ require(blockIds.length == segments.length,
+ "Number of block ids must be the same as number of segments!")
+
+ // Hadoop Configuration is not serializable, so broadcast it as a
serializable.
+ val broadcastedHadoopConf = sc.broadcast(new
SerializableWritable(hadoopConfiguration))
+
+ override def getPartitions: Array[Partition] = {
+ assertValid()
+ (0 until blockIds.size).map { i =>
+ new HDFSBackedBlockRDDPartition(blockIds(i), i, segments(i))
+ }.toArray
+ }
+
+ override def compute(split: Partition, context: TaskContext):
Iterator[T] = {
+ assertValid()
+ val hadoopConf = broadcastedHadoopConf.value.value
+ val blockManager = SparkEnv.get.blockManager
+ val partition = split.asInstanceOf[HDFSBackedBlockRDDPartition]
+ val blockId = partition.blockId
+ blockManager.get(blockId) match {
+ // Data is in Block Manager, grab it from there.
+ case Some(block) =>
+ block.data.asInstanceOf[Iterator[T]]
+ // Data not found in Block Manager, grab it from HDFS
+ case None =>
+ logInfo("Reading partition data from write ahead log " +
partition.segment.path)
+ val reader = new WriteAheadLogRandomReader(partition.segment.path,
hadoopConf)
+ val dataRead = reader.read(partition.segment)
+ reader.close()
+ // Currently, we support storing the data to BM only in serialized
form and not in
+ // deserialized form
+ if (storeInBlockManager) {
+ blockManager.putBytes(blockId, dataRead, storageLevel)
+ }
+ dataRead.rewind()
+ blockManager.dataDeserialize(blockId,
dataRead).asInstanceOf[Iterator[T]]
+ }
+ }
+
+ override def getPreferredLocations(split: Partition): Seq[String] = {
+ val partition = split.asInstanceOf[HDFSBackedBlockRDDPartition]
+ val locations = getBlockIdLocations()
+ locations.getOrElse(partition.blockId,
--- End diff --
For very deeply-nested versions of this pattern, where you want to select
the first non-None value among many alternatives while lazily computing them, I
like doing something like
```scala
def sparkPreferredLocations = getBlockIdLocations().get(partition.blockId)
def hdfsPreferredLocations = HdfsUtils.getBlockLocations(...)
sparkPreferredLocations.orElse(hdfsPreferredLocations).getOrElse(Array[String].empty)
```
I kind of like this because it flattens the nested structure so the code
reads as "here are the alternatives to choose among, defined in order of their
precedence, and here's the line that tries them in order." I'm not sure
whether this is more or less confusing than the nesting for new Scala users,
though. There's probably an even nicer version of this without all of the
intermediate `orElse` calls, but I guess that's only really necessary if you're
picking among _many_ alternatives; it might be excessively confusing here.
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