1).The canonical example application of MapReduce is a process to count
the appearances of
each different word in a set of documents:
void map(String name, String document):
// name: document name
// document: document contents
for each word w in document:
EmitIntermediate(w, 1);
void reduce(String word, Iterator partialCounts):
// key: a word
// values: a list of aggregated partial counts
int result = 0;
for each v in partialCounts:
result += ParseInt(v);
Emit(result);
Here, each document is split in words, and each word is counted initially
with a "1" value by
the Map function, using the word as the result key. The framework puts
together all the pairs
with the same key and feeds them to the same call to Reduce, thus this
function just needs to
sum all of its input values to find the total appearances of that word.
