It is performed at a frame work level map emits key value pairs and the framework collects and groups all the values corresponding to a key from all the map tasks. Now the reducer takes the input as a key and a collection of values only. The reduce method signature defines it.
Sent from handheld, please excuse typos.
From: jamal sasha <[EMAIL PROTECTED]>
Date: Wed, 21 Nov 2012 14:50:51
To: [EMAIL PROTECTED]<[EMAIL PROTECTED]>
Reply-To: [EMAIL PROTECTED]
Subject: fundamental doubt
I guess i am asking alot of fundamental questions but i thank you guys for
taking out time to explain my doubts.
So i am able to write map reduce jobs but here is my mydoubt
As of now i am writing mappers which emit key and a value
This key value is then captured at reducer end and then i process the key
and value there.
Let's say i want to calculate the average...
Key2 value 2
Key 1 value 3
So the output is something like
Key1 average of value 1 and value 3
Key2 average 2 = value 2
Right now in reducer i have to create a dictionary with key as original
keys and value is a list.
Data = defaultdict(list) == // python usrr
But i thought that
Mapper takes in the key value pairs and outputs key: ( v1,v2....)and
Reducer takes in this key and list of values and returns
Key , new value..
So why is the input of reducer the simple output of mapper and not the list
of all the values to a particular key or did i understood something.
Am i making any sense ??