title: Partitioning! how does hadoop make it? Use a hash function? what is the default function?
body: <p>Partitioning is the process of determining which reducer instance will receive which intermediate keys and values. Each mapper must determine for all of its output (key, value) pairs which reducer will receive them. It is necessary that for any key, regardless of which mapper instance generated it, the destination partition is the same
  Problem: How does hadoop make it? Use a hash function? what is the default function? </p>