HADOOP-12730. Hadoop streaming -mapper and -reducer options are wrongly documented as required. Contributed by Kengo Seki.

hadoop-common-project/hadoop-common/CHANGES.txt

@@ -1642,6 +1642,9 @@ Release 2.8.0 - UNRELEASED
     HADOOP-12659. Incorrect usage of config parameters in token manager of
     KMS. (Mingliang Liu via xyao)
 
+    HADOOP-12730. Hadoop streaming -mapper and -reducer options are wrongly
+    documented as required. (Kengo Seki via aajisaka)
+
 Release 2.7.3 - UNRELEASED
 
   INCOMPATIBLE CHANGES

hadoop-tools/hadoop-streaming/src/site/markdown/HadoopStreaming.md.vm

@@ -96,8 +96,8 @@ The Hadoop streaming command options are listed here:
 |:---- |:---- |:---- |
 | -input directoryname or filename | Required | Input location for mapper |
 | -output directoryname | Required | Output location for reducer |
-| -mapper executable or JavaClassName | Required | Mapper executable |
-| -reducer executable or JavaClassName | Required | Reducer executable |
+| -mapper executable or JavaClassName | Optional | Mapper executable. If not specified, IdentityMapper is used as the default |
+| -reducer executable or JavaClassName | Optional | Reducer executable. If not specified, IdentityReducer is used as the default |
 | -file filename | Optional | Make the mapper, reducer, or combiner executable available locally on the compute nodes |
 | -inputformat JavaClassName | Optional | Class you supply should return key/value pairs of Text class. If not specified, TextInputFormat is used as the default |
 | -outputformat JavaClassName | Optional | Class you supply should take key/value pairs of Text class. If not specified, TextOutputformat is used as the default |