Question: Can a mapreduce program be configured such that only reducer is executed not mapper[even though there is a map function defined in the program]. Can this be achieved just by changing job configuration.

I want to implement incremental computation in mapreduce(with input as append only files). For example

For wordcount,

If wordcount is already executed on a file, after which some more data is appended to the input file.

If again wordcount is executed on the updated input file, I want to execute wordcount only on the new data and combine the old results with this. **For this combining of outputs I want to execute reducer alone separately**.

Answer: 
No, this is not possible. Hadoop requires you to do a `map`, while the `reduce` is optional.

If you want to do a group-by, you can try to use `Apache Tez` and configure a DAG that will the same what you want to archieve (might be still hacky, because you will need to use the internal data format).
yes You can!, use this code as mapper(python\_version)

`import sys
for i in sys.stdin:
 print i`

this will do the trick, because mapper is must one. so just print the contents of the input with a dummy mapper

I guess that helped!