In-Mapper Combiner

Source: https://stackoverflow.com/questions/28198873/difference-between-combiner-and-in-mapper-combiner-in-mapreduce

You are probably already aware that a combiner is a process that runs locally on each Mapper machine to pre-aggregate data before it is shuffled across the network to the various cluster Reducers.

The in-mapper combiner takes this optimization a bit further: the aggregations do not even write to local disk: they occur in-memory in the Mapper itself