Mapreduce shuffle and sort

Question

For each mapreduce, is shuffle and sort operation performed before or its happened after the completion of all mappers?

Answer 1

The mapreduce task happens in the following order

1. Mapping
2. Shuffle
3. Sort
4. Reduce

Shuffle can happen while mappers are generating data since it is only a data transfer. But the sort and reduce can only start once all the mappers are done.