[SPARK-13339][DOCS] Clarify commutative / associative operator requirements for reduce, fold

Clarify that reduce functions need to be commutative, and fold functions do not

See https://github.com/apache/spark/pull/11091

Author: Sean Owen <sowen@cloudera.com>

Closes #11217 from srowen/SPARK-13339.

1 files changed, 2 insertions, 2 deletions

diff --git a/docs/streaming-programming-guide.md b/docs/streaming-programming-guide.md
index 677f5ff7be..4d1932bc8c 100644
--- a/docs/streaming-programming-guide.md
+++ b/docs/streaming-programming-guide.md
@@ -798,7 +798,7 @@ Some of the common ones are as follows.
   <td> <b>reduce</b>(<i>func</i>) </td>
   <td> Return a new DStream of single-element RDDs by aggregating the elements in each RDD of the
   source DStream using a function <i>func</i> (which takes two arguments and returns one).
-  The function should be associative so that it can be computed in parallel. </td>
+  The function should be associative and commutative so that it can be computed in parallel. </td>
 </tr>
 <tr>
   <td> <b>countByValue</b>() </td>
@@ -1072,7 +1072,7 @@ said two parameters - <i>windowLength</i> and <i>slideInterval</i>.
 <tr>
   <td> <b>reduceByWindow</b>(<i>func</i>, <i>windowLength</i>, <i>slideInterval</i>) </td>
   <td> Return a new single-element stream, created by aggregating elements in the stream over a
-  sliding interval using <i>func</i>. The function should be associative so that it can be computed
+  sliding interval using <i>func</i>. The function should be associative and commutative so that it can be computed
   correctly in parallel.
   </td>
 </tr>