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MapReduce Tutorial : Custom sorting and grouping comparators.

Sorting comparator

The keys are sorted before processed by a reducer, using a
Raw comparator.

Grouping comparator

In a reduce, it is guaranteed that keys are processed in ascending order. Sometimes it would be useful if the values associated with one key could also be processed in ascending order.

Step 28: Custom data types. Overview Step 30: Custom input formats.

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