Question about combiners

[#1]

Is a combiner always defined in terms of a reducer or is there another way to code the combiner class?

Thanks in advance to all who answer.

[#2 · Accepted solution]

Like the definitive guide says "Hadoop allows the user to specify a
combiner function to be run on the map output, and the combiner function’s
output forms the input to the reduce function. "
​. Frequently the code in the reducer and combiner is similar but doesn't
have to be. ​


Your question is unclear.
​ Can you elaborate a bit?
​

[#3]

Thanks, your answer was fine.