Github user tdas commented on a diff in the pull request:
https://github.com/apache/spark/pull/8103#discussion_r49138016
--- Diff:
streaming/src/main/scala/org/apache/spark/streaming/api/java/JavaPairDStream.scala
---
@@ -336,7 +336,8 @@ class JavaPairDStream[K, V](val dstream: DStream[(K,
V)])(
* However, it is applicable to only "invertible reduce functions".
* Hash partitioning is used to generate the RDDs with Spark's default
number of partitions.
* @param reduceFunc associative reduce function
- * @param invReduceFunc inverse function
+ * @param invReduceFunc inverse function; such that for all x,
invertible y:
+ * `invReduceFunc(reduceFunc(x, y), y) = x`
--- End diff --
Why not
reduceFunc("x", "y") = "xy" ... y is always added to right
inverseReduceFunc("xy", "x") = "y" ... x is always removed from left
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