title: How to use local aggregation methods in MapReduce programs such as in-mapper combiner?
body: <p>I want to implement a word length program that categorize words in 4 categories on large corpus by using local aggregation methods but I don't have deepest knowledge about how these methods work. Because I am so new in MapReduce field. For example what is the sharpest differences between combiner and in-mapper combiner? In addition I should add a combiner and in-mapper combiner to my code and should measure the differences between them. But I don't have any idea where I should start, if someone help me, I appreciate it.</p>