The (deprecated) Summingbird builder API builds up a single
MapReduce job using a SourceBuilder. After any number of calls to
flatMap, leftJoin, filter, merge, etc, the user calls
"groupAndSumTo", equivalent to "sumByKey" in the Producer
API. This call converts the SourceBuilder into a CompletedBuilder
and prevents and future flatMap operations.
The (deprecated) Summingbird builder API builds up a single MapReduce job using a SourceBuilder. After any number of calls to flatMap, leftJoin, filter, merge, etc, the user calls "groupAndSumTo", equivalent to "sumByKey" in the Producer API. This call converts the SourceBuilder into a CompletedBuilder and prevents and future flatMap operations.