Xuefu Zhang created SPARK-3693:
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Summary: Cached Hadoop RDD always return rows with the same value
Key: SPARK-3693
URL: https://issues.apache.org/jira/browse/SPARK-3693
Project: Spark
Issue Type: Bug
Components: Spark Core
Affects Versions: 1.2.0
Reporter: Xuefu Zhang
While trying RDD caching, it's found that caching a Hadoop RDD causes data
correctness issues. The following code snippet demonstrates the usage:
{code}
public final class Test {
private static final Pattern SPACE = Pattern.compile(" ");
public static void main(String[] args) throws Exception {
SparkConf sparkConf = new SparkConf().setAppName("Test");
JavaSparkContext ctx = new JavaSparkContext(sparkConf);
...
JavaPairRDD<BytesWritable, BytesWritable> input =
ctx.hadoopRDD(jobConf, CombineHiveInputClass.class,
WritableComparable.class, Writable.class);
input = input.cache();
input.foreach(new VoidFunction<Tuple2<BytesWritable, BytesWritable>>() {
@Override
public void call(Tuple2<BytesWritable, BytesWritable> row) throws
Exception {
if (row._1() != null) {
System.out.println("Key: " + row._1());
}
if (row._2() != null) {
System.out.println("Value: " + row._2());
}
}
});
ctx.stop();
}
}
{code}
In this case, row._2() always gives the same value. If we disable caching by
removing input.cache(), the program gives the expected rows.
Further analysis shows that MemoryStore (see
https://github.com/apache/spark/blob/master/core/src/main/scala/org/apache/spark/storage/MemoryStore.scala#L236)
is storing the references to (key, value) pairs returned by
HadoopRDD.getNext() (See
https://github.com/apache/spark/blob/master/core/src/main/scala/org/apache/spark/rdd/HadoopRDD.scala#L220),
but this method always returns the same (key, value) object references, except
each getNext() call updates values inside these objects. When there are no more
records (key, value) objects are filled with empty strings (no values) in
CombineFileRecordReader. As all pairs in MemoryStore.vector refer to the same
key, value object pairs, all values become NULL.
Probably MemoryStore should instead store a copy of <key, value> pair rather
than keeping a reference to it.
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