Details
Major Description
Reducer generates <K, List(V)> for reduce(). In some cases such as SecondarySort, although current V and next V share the same K, their actual corresponding Ks are different. For example, in SecondarySort, map() outputs
Key Value
<1, 3> 3
<1, 1> 1
<2, 5> 5
<1, 8> 8
After partition by Key.getFirst(), sort and group by Key.getFirst(),
reducer gets:
Key Value
-----Group 1-----
<1, 1> 1
<1, 3> 3
<1, 8> 8
-----Group 2-----
<2, 5> 5
reduce() receives:
Key List<Value>
<1, 1> List<1, 3, 8>
<2, 5> List<5>
When invoking V.next(), we can get next V (e.g, 3). But we do not have API to get its corresponding Key (e.g, <1, 3>). We can only get the first Key (e.g., <1,1>).
If we let user be able to get latest key, SecondarySort does not need to emit value in map(). So that the network traffic is better.
Another example is Join. If we can get latest Key, we do not need to put table label in both key and value.
