Hadoop reducer string manipulation doesn't work?

The basic flow of data through the system is.

The basic flow of data through the system is: Input -> Map -> Reduce -> output. As a performance optimization the combiner has been added to allow a computer (one of the many in the hadoop cluster) to do a partial aggregation of the data before it is transmitted to the system where the actual reducer is run. In the word count example it is fine to start with these values : 1 1 1 1 1 1 1 1 1 1 combine them into 3 4 2 1 and the reduce them into the final result 10 So the combiner is essentially a performance optimization.

If you do not specify a combiner it will not change the information going through (i.e. It's an "identity reducer"). So you can only use the SAME class as both the combiner and reducer if the dataset remains valid that way.In your case: that is not true --> your data is now invalid.

You do: conf. SetCombinerClass(SimpleReduce. Class); conf.

SetReducerClass(SimpleReduce. Class); So this makes the output of your mapper go through your reducer twice. The first one adds: "start" & "end" The second one adds "start" & "end" again.

Simple solution: // conf. SetCombinerClass(SimpleReduce. Class); conf.

SetReducerClass(SimpleReduce. Class); HTH.

I had a problem wherein the reducer won't get all the data sent by the mapper. The reducer would only get upto the specific portion output. Collect will emit.

For Eg. For the Input Data: 12345 abc@bbc. Com|m|1975 12346 [email protected]|m|1981 if I say output.

Collect(key,mail_id); Then it will not get the next two fields - sex and year of birth. // conf. SetCombinerClass(SimpleReduce.

Class); Solved the problem.

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