Hi,
I am trying to understand the difference between combiner and aggregator.
Based on my readings:
Wordcount example (mapper)
aggregator
class Mapper
method MAP
H <-- Associative array
for all term t in document:
H{t} = H{t} + 1
for all term t ele H do
EMIT(term t, count H{t})
combiner:
class Mapper
method INITIALIZE
H <-- Associative array
method MAP
for all term t in document:
H{t} = H{t} + 1
method CLOSE
for all term t ele H do
EMIT(term t, count H{t})
So, second method is how combiner is implemented.
But 1 seems much simpler?
What are the gains I get using combiner instead of local aggregations?
