How many times will the Reducer’s reduce method be invoked?

You have written a Mapper which invokes the following five calls to the OutputColletor.collect method:

output.collect (new Text (“Apple”), new Text (“Red”) ) ;

output.collect (new Text (“Banana”), new Text (“Yellow”) ) ;

output.collect (new Text (“Apple”), new Text (“Yellow”) ) ;

output.collect (new Text (“Cherry”), new Text (“Red”) ) ;

output.collect (new Text (“Apple”), new Text (“Green”) ) ;

How many times will the Reducer’s reduce method be invoked?
A . 6
B . 3
C . 1
D . 0
E . 5

View Answer

Answer: B

Explanation:

reduce() gets called once for each [key, (list of values)] pair.

To explain, let’s say you called:

out.collect(new Text("Car"),new Text("Subaru"); out.collect(new Text("Car"),new Text("Honda"); out.collect(new Text("Car"),new Text("Ford"); out.collect(new Text("Truck"),new Text("Dodge"); out.collect(new Text("Truck"),new Text("Chevy"); Then reduce() would be called twice with the pairs reduce(Car, <Subaru, Honda, Ford>) reduce(Truck, <Dodge, Chevy>)

Reference: Mapper output.collect()?