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Regarding the code lines of the mapreduce index:

// Run a JanusGraph-Hadoop job to reindex
mgmt = graph.openManagement()
mr = new MapReduceIndexManagement(graph)
mr.updateIndex(mgmt.getGraphIndex("mixedExample"), SchemaAction.REINDEX).get()

As far as I know this runs on your local machine and all dependencies are present in the JanusGraph distribution. In other words, no need for an hadoop or spark cluster for this. 

Cheers,     Marc