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== HadoopRDD

https://spark.apache.org/docs/latest/api/scala/index.html#org.apache.spark.rdd.HadoopRDD[HadoopRDD] is an RDD that provides core functionality for reading data stored in HDFS, a local file system (available on all nodes), or any Hadoop-supported file system URI using the older MapReduce API (https://hadoop.apache.org/docs/current/api/org/apache/hadoop/mapred/package-summary.html[org.apache.hadoop.mapred]).

HadoopRDD is created as a result of calling the following methods in ROOT:SparkContext.md[]:

  • hadoopFile
  • textFile (the most often used in examples!)
  • sequenceFile

Partitions are of type HadoopPartition.

When an HadoopRDD is computed, i.e. an action is called, you should see the INFO message Input split: in the logs.

scala> sc.textFile("README.md").count
...
15/10/10 18:03:21 INFO HadoopRDD: Input split: file:/Users/jacek/dev/oss/spark/README.md:0+1784
15/10/10 18:03:21 INFO HadoopRDD: Input split: file:/Users/jacek/dev/oss/spark/README.md:1784+1784
...

The following properties are set upon partition execution:

  • mapred.tip.id - task id of this task's attempt
  • mapred.task.id - task attempt's id
  • mapred.task.is.map as true
  • mapred.task.partition - split id
  • mapred.job.id

Spark settings for HadoopRDD:

  • spark.hadoop.cloneConf (default: false) - shouldCloneJobConf - should a Hadoop job configuration JobConf object be cloned before spawning a Hadoop job. Refer to https://issues.apache.org/jira/browse/SPARK-2546[[SPARK-2546] Configuration object thread safety issue]. When true, you should see a DEBUG message Cloning Hadoop Configuration.

You can register callbacks on spark-TaskContext.md[TaskContext].

HadoopRDDs are not checkpointed. They do nothing when checkpoint() is called.

[CAUTION]

FIXME

  • What are InputMetrics?
  • What is JobConf?
  • What are the InputSplits: FileSplit and CombineFileSplit? * What are InputFormat and Configurable subtypes?
  • What's InputFormat's RecordReader? It creates a key and a value. What are they?
  • What's Hadoop Split? input splits for Hadoop reads? See InputFormat.getSplits

=== [[getPreferredLocations]] getPreferredLocations Method

CAUTION: FIXME

=== [[getPartitions]] getPartitions Method

The number of partition for HadoopRDD, i.e. the return value of getPartitions, is calculated using InputFormat.getSplits(jobConf, minPartitions) where minPartitions is only a hint of how many partitions one may want at minimum. As a hint it does not mean the number of partitions will be exactly the number given.

For SparkContext.textFile the input format class is https://hadoop.apache.org/docs/current/api/org/apache/hadoop/mapred/TextInputFormat.html[org.apache.hadoop.mapred.TextInputFormat].

The https://hadoop.apache.org/docs/current/api/org/apache/hadoop/mapred/FileInputFormat.html[javadoc of org.apache.hadoop.mapred.FileInputFormat] says:

FileInputFormat is the base class for all file-based InputFormats. This provides a generic implementation of getSplits(JobConf, int). Subclasses of FileInputFormat can also override the isSplitable(FileSystem, Path) method to ensure input-files are not split-up and are processed as a whole by Mappers.

TIP: You may find https://github.com/apache/hadoop/blob/trunk/hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core/src/main/java/org/apache/hadoop/mapred/FileInputFormat.java#L319[the sources of org.apache.hadoop.mapred.FileInputFormat.getSplits] enlightening.


Last update: 2020-10-06