The mapreduce algorithm has 4 main phases:

1. Map,

2. Combine,

3. Shuttle and sort

4. Phase output

Mappers simply execute on unsorted key/values pairs.They create the intermediate keys. Once these keys are ready the combiners pair the key/value pairs with the right key. The shuttle/sort is done by the framework their role being to group data and transfer it. Once completed, it will proceed for the output via the phase output process.