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What is Map Reduce
MapReduce is a programming model designed for
processing large volumes of data in parallel by dividing
the work into a set of independent tasks.
MapReduce programs are written in a particular style
inf luenced by functional programming constructs,
specifically idioms for processing lists of data.
Map and Reduce
Conceptually, MapReduce programs transform lists of
input data elements into lists of output data elements.
A MapReduce program will do this twice, using two
different list processing idioms: map, and reduce.
A mapreduce job is the a unit of work that the client
wants to be performed it consist of the input data and
theMapReduce Program and configuration info .
Hadoop runs the job by dividing it into tasks ,of which
there are two type :map tasks and the reduce tasks
How much data is
at a time
The Combiner is a "mini-reduce" process which
operates only on data generated by one machine.
Word count is a prime example for where a Combiner
is useful. The Word Count program in listings 1--3
emits a (word, 1) pair for every instance of every word it
sees. So if the same document contains the word "cat"
3 times, the pair ("cat", 1) is emitted three times; all of
these are then sent to the Reducer.
One of the primary reasons to use Hadoop to run your jobs
is due to its high degree of fault tolerance.
The primary way that Hadoop achieves fault tolerance is
through restarting tasks. Individual task nodes
(TaskTrackers) are in constant communication with the
head node of the system, called the JobTracker. If a
TaskTracker fails to communicate with the JobTracker for
a period of time (by default, 1 minute), the
JobTracker will assume that the TaskTracker in
question has crashed. The JobTracker knows which map
and reduce tasks were assigned to each
Hadoop provides an API to MapReduce that allows you to write your
map and reduce functions in languages other than Java. Hadoop
Streaming uses Unix standard streams as the interface between
Hadoop and your program, so you can use any language that can
read standard input and write to standard output to write your
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MapReduce basic Concepts like what is mapper and what is reducer