Josh Rosen created SPARK-29419:
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Summary: Seq.toDS / spark.createDataset(Seq) is not thread-safe
Key: SPARK-29419
URL: https://issues.apache.org/jira/browse/SPARK-29419
Project: Spark
Issue Type: Improvement
Components: SQL
Affects Versions: 2.4.0, 3.0.0
Reporter: Josh Rosen
Assignee: Josh Rosen
The {{Seq.toDS}} and {{spark.createDataset(Seq)}} code is not thread-safe: if
the caller-supplied {{Encoder}} is used in multiple threads then
{{createDataset}}'s usage of the encoder may lead to incorrect answers because
the Encoder's internal mutable state will be updated by from multiple threads.
Here is an example demonstrating the problem:
{code:java}
import org.apache.spark.sql._
val enc = implicitly[Encoder[(Int, Int)]]
val datasets = (1 to 100).par.map { _ =>
val pairs = (1 to 100).map(x => (x, x))
spark.createDataset(pairs)(enc)
}
datasets.reduce(_ union _).collect().foreach {
pair => require(pair._1 == pair._2, s"Pair elements are mismatched: $pair")
}{code}
Due to the thread-safety issue, the above example results in the creation of
corrupted records where different input records' fields are co-mingled.
This bug is similar to SPARK-22355, a related problem in {{Dataset.collect()}}
(fixed in Spark 2.2.1+).
Fortunately, this has a simple one-line fix (copy the encoder); I'll submit a
patch for this shortly.
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