Assuming default settings, which best describes the order of data provided to a reducer's reduce method:

Question

Options

• AThe keys given to a reducer aren't in a predictable order, but the values associated with those keys
• BBoth the keys and values passed to a reducer always appear in sorted order.
• CNeither keys nor values are in any predictable order.
• DThe keys given to a reducer are in sorted order but the values associated with each key are in no

• A
  2% (1)
• B
  15% (6)
• C
  5% (2)
• D
  78% (32)

Explanation

Reducer has 3 primary phases: The Reducer copies the sorted output from each Mapper using HTTP across the network. The framework merge sorts Reducer inputs by keys (since different Mappers may have output The shuffle and sort phases occur simultaneously i.e. while outputs are being fetched they are To achieve a secondary sort on the values returned by the value iterator, the application should extend the key with the secondary key and define a grouping comparator. The keys will be sorted using the entire key, but will be grouped using the grouping comparator to decide which keys and values are sent in the same call to reduce. In this phase the reduce(Object, Iterable, Context) method is called for each <key, (collection of values)> in the sorted inputs. The output of the reduce task is typically written to a RecordWriter via TaskInputOutputContext.write(Object, Object). The output of the Reducer is not re-sorted.

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