Determine which best describes when the reduce method is first called in a MapReduce job?

• A. Reducers start copying intermediate key-value pairs from each Mapper as soon as it has completed. The reduce method is called only after all intermediate data has been copied and sorted.
• B. Reducers start copying intermediate key-value pairs from each Mapper as soon as it has completed. The programmer can configure in the job what percentage of the intermediate data should arrive before the reduce method begins.
• C. Reducers start copying intermediate key-value pairs from each Mapper as soon as it has completed. The reduce method is called as soon as the intermediate key-value pairs start to arrive.
• D. Reduce methods and map methods all start at the beginning of a job, in order to provide optimal performance for map-only or reduce-only jobs.

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You are developing a MapReduce job for sales reporting. The mapper will process input keys representing the year (IntWritable) and input values representing product indentifies (Text).
Indentify what determines the data types used by the Mapper for a given job.

• A. The InputFormat used by the job determines the mapper's input key and value types.
• B. The key and value types specified in the JobConf.setMapInputKeyClass and JobConf.setMapInputValuesClass methods
• C. The data types specified in HADOOP_MAP_DATATYPES environment variable
• D. The mapper-specification.xml file submitted with the job determine the mapper's input key and value types.

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In the reducer, the MapReduce API provides you with an iterator over Writable values. What does calling the next () method return?

• A. It returns a reference to a Writable object. The API leaves unspecified whether this is a reused object or a new object.
• B. It returns a reference to the same Writable object if the next value is the same as the previous value, or a new Writable object otherwise.
• C. It returns a reference to a different Writable object time.
• D. It returns a reference to the same Writable object each time, but populated with different data.
• E. It returns a reference to a Writable object from an object pool.

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Given a directory of files with the following structure: line number, tab character, string:
Example:
1abialkjfjkaoasdfjksdlkjhqweroij
2kadfjhuwqounahagtnbvaswslmnbfgy
3kjfteiomndscxeqalkzhtopedkfsikj
You want to send each line as one record to your Mapper. Which InputFormat should you use to complete the line: conf.setInputFormat (____.class) ; ?

• A. KeyValueFileInputFormat
• B. SequenceFileAsTextInputFormat
• C. SequenceFileInputFormat
• D. BDBInputFormat

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You want to count the number of occurrences for each unique word in the supplied input data. You've decided to implement this by having your mapper tokenize each word and emit a literal value 1, and then have your reducer increment a counter for each literal 1 it
receives. After successful implementing this, it occurs to you that you could optimize this by specifying a combiner. Will you be able to reuse your existing Reduces as your combiner in this case and why or why not?

• A. No, because the sum operation in the reducer is incompatible with the operation of a Combiner.
• B. No, because the Reducer and Combiner are separate interfaces.
• C. Yes, because Java is a polymorphic object-oriented language and thus reducer code can be reused as a combiner.
• D. Yes, because the sum operation is both associative and commutative and the input and output types to the reduce method match.
• E. No, because the Combiner is incompatible with a mapper which doesn't use the same data type for both the key and value.

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