f(BlA):“Pairs” (a,*)→32 Reducer holds this value in memory (a,b)→3 (a,b)→3/32 (a,b2)→12 (a,b2)→12/32 (a,b3)→7 (a,b3)→7/32 (a,b4)→1 (a,b4)→1/32 For this to work: Must emit extra(a,*for every b in mapper Must make sure all a's get sent to same reducer (use partitioner) Must make sure (a,*comes first(define sort order) Must hold state in reducer across different key-value pairsf(B|A): “Pairs” ◼ For this to work: ◼ Must emit extra (a, *) for every bn in mapper ◼ Must make sure all a’s get sent to same reducer (use partitioner) ◼ Must make sure (a, *) comes first (define sort order) ◼ Must hold state in reducer across different key-value pairs (a, b1 ) → 3 (a, b2 ) → 12 (a, b3 ) → 7 (a, b4 ) → 1 … (a, *) → 32 (a, b1 ) → 3 / 32 (a, b2 ) → 12 / 32 (a, b3 ) → 7 / 32 (a, b4 ) → 1 / 32 … Reducer holds this value in memory