Why inconsistent results using subtraction in reduce?

Question

Given the following:

val rdd = List(1,2,3)

I assumed that rdd.reduce((x,y) => (x - y)) would return -4 (i.e. <

Answer 1

As aforementioned by @TzachZohar the function must satisfy the two properties so that the parallel computation is sound; by collecting the rdd, reduce relaxes the properties required in the function, and so it produces the result from a sequential (non parallel) computation, namely,

val rdd = sc.parallelize(1 to 3)

rdd.collect.reduce((x,y) => (x-y))
Int = -4