Github user rxin commented on a diff in the pull request:
https://github.com/apache/spark/pull/2931#discussion_r19573331
--- Diff:
streaming/src/main/scala/org/apache/spark/streaming/rdd/WriteAheadLogBackedBlockRDD.scala
---
@@ -0,0 +1,124 @@
+/*
+ * 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._
+import org.apache.spark.rdd.BlockRDD
+import org.apache.spark.storage.{BlockId, StorageLevel}
+import org.apache.spark.streaming.util.{HdfsUtils,
WriteAheadLogFileSegment, WriteAheadLogRandomReader}
+
+/**
+ * Partition class for
[[org.apache.spark.streaming.rdd.WriteAheadLogBackedBlockRDD]].
+ * It contains information about the id of the blocks having this
partition's data and
+ * the segment of the write ahead log that backs the partition.
+ * @param index index of the partition
+ * @param blockId id of the block having the partition data
+ * @param segment segment of the write ahead log having the partition data
+ */
+private[streaming]
+class WriteAheadLogBackedBlockRDDPartition(
+ val index: Int,
+ val blockId: BlockId,
+ val segment: WriteAheadLogFileSegment
+ ) extends Partition
+
+
+/**
+ * This class represents a special case of the BlockRDD where the data
blocks in
+ * the block manager are also backed by segments in write ahead logs. For
reading
+ * the data, this RDD first looks up the blocks by their ids in the block
manager.
+ * If it does not find them, it looks up the corresponding file segment.
+ *
+ * @param sc SparkContext
+ * @param hadoopConfig Hadoop configuration
+ * @param blockIds Ids of the blocks that contains this RDD's data
+ * @param segments Segments in write ahead logs that contain this RDD's
data
+ * @param storeInBlockManager Whether to store in the block manager after
reading from the segment
+ * @param storageLevel storage level to store when storing in block manager
+ * (applicable when storeInBlockManager = true)
+ */
+private[streaming]
+class WriteAheadLogBackedBlockRDD[T: ClassTag](
+ @transient sc: SparkContext,
+ @transient hadoopConfig: Configuration,
+ @transient override val blockIds: Array[BlockId],
+ @transient val segments: Array[WriteAheadLogFileSegment],
+ val storeInBlockManager: Boolean,
+ val storageLevel: StorageLevel
--- End diff --
nitpick: the common style in spark is
```scala
val storageLevel: StorageLevel)
extends BlockRDD[T](sc, blockIds) {
```
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