Joins are commonplace in Big Data processing. They occur on the value of a join key and on a data type in the datasets that participate in a join. In this book, we will refrain from explaining the different join semantics such as inner joins, outer joins, and cross joins, and focus on inner join processing using MapReduce and the optimizations involved in it.
In MapReduce, joins can be done in either the Map task or the Reduce task. The former is called a Map-side join and the latter is called a Reduce-side join.
Reduce-side joins are meant for more general purposes and do not impose too many conditions on the datasets that participate in the join. However, the shuffle step is very heavy on resources.
The basic idea involves tagging each record with a data source tag and extracting the join key in the Map tasks. The Reduce task receives all the records with the same join key and does the actual join. If one of the datasets participating in the join is very...