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Subject: Difference between combiner and aggregator
From: jamal sasha <jamalshasha@gmail.com>
To: "user@hadoop.apache.org" <user@hadoop.apache.org>
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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?

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<div dir=3D"ltr">Hi,<div style>=A0I am trying to understand the difference =
between combiner and aggregator.</div><div style><br></div><div style>Based=
 on my readings:</div><div style>Wordcount example (mapper)</div><div style=
>
<br></div><div style>aggregator</div><div style>class Mapper</div><div styl=
e>=A0 method MAP</div><div style>=A0 H &lt;-- Associative array</div><div s=
tyle>=A0 for all term t in document:</div><div style>=A0 =A0 =A0 H{t} =3D H=
{t} + 1</div>
<div style>=A0 for all term t ele H do</div><div style>=A0 =A0 =A0 EMIT(ter=
m t, count H{t})</div><div style><br></div><div style><br></div><div style>=
combiner:</div><div style><div>class Mapper</div><div style>=A0method INITI=
ALIZE</div>
<div>=A0 H &lt;-- Associative array<br></div><div>=A0 method MAP</div><div>=
=A0 for all term t in document:</div><div>=A0 =A0 =A0 H{t} =3D H{t} + 1</di=
v><div style>=A0method CLOSE</div><div>=A0 for all term t ele H do</div><di=
v>=A0 =A0 =A0 EMIT(term t, count H{t})</div>
<div><br></div><div style>So, second method is how combiner is implemented.=
</div><div style>But 1 seems much simpler?</div><div style>What are the gai=
ns I get using combiner instead of local aggregations?</div><div><br></div>
</div></div>

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