When you do map and reduce operations with functions that have no side effects (and your reduction function is associative), then changes in the ordering of operations don't affect the result of the overall map or reduce operation, any more than when a CPU reorders instructions to avoid pipeline stalls. Map and Reduce are a way of breaking your problem up in to pieces that have no inter-dependencies or shared mutable state, so that they can be executed in parallel without any of the pitfalls of concurrency.