Github user massie commented on a diff in the pull request:
https://github.com/apache/spark/pull/7403#discussion_r38463750
--- Diff: core/src/main/scala/org/apache/spark/Dependency.scala ---
@@ -76,6 +78,15 @@ class ShuffleDependency[K, V, C](
override def rdd: RDD[Product2[K, V]] =
_rdd.asInstanceOf[RDD[Product2[K, V]]]
+ /**
+ * The key, value and combiner classes are serialized so that shuffle
manager
+ * implementation can use the information to build
+ */
+ val keyClassName: String = reflect.classTag[K].runtimeClass.getName
+ val valueClassName: String = reflect.classTag[V].runtimeClass.getName
+ // Note: It's possible that the combiner class tag is null, if the
combineByKey
+ // methods in PairRDDFunctions are used instead of
combineByKeyWithClassTag.
+ val combinerClassName: Option[String] =
Option(reflect.classTag[C]).map(_.runtimeClass.getName)
--- End diff --
If we make this `private[spark]`, that will prevent developers from
creating pluggable `ShuffleManager` implementations, see
`ShuffleManager.registerShuffle()`. The `ShuffleDependency` is annotated with
`@DeveloperApi` which should signal to developers that these attributes can
(and likely will) change over time, hopefully minimally.
Another approach would be to change the `registerShuffle` signature to be
something like, e.g.
```
def registerShuffle[K, V, C](
shuffleId: Int,
numMaps: Int,
dependency: ShuffleDependency[K, V, C],
keyClassName: String,
valueClassName: String,
combinerClassName: Option[String]): ShuffleHandle
```
..instead of passing that information in the dependency.
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