Reproducibility

Reproducibility is a software design concept, where an operation for the same inputs yields the same output. This is an important property for Nix, because hashes are based on the inputs of a build, and thus it must be guaranteed that the same inputs yield in the same build output.

{ stdenv, fetchGit }:
stdenv.mkDerivation {
  name = "my-package";
  src = fetchgit {
    url = "https://github.com/DeterminateSystems/riff";
    sha256 = "sha256-7mnx7J0AacL2P2mNuNNB+kKE7VR8nniVG+PSrwpZixE=";
  };
}

The above code consists of two derivations: fetchgit and mkDerivation.

The reproducibility of mkDerivation is guaranteed because the output of fetchgit is pinned to a particular hash. And because mkDerivation itself isn't allowed to touch the network, and is hermetically sealed from the host, no impurities in the build can exist.

The output of fetchgit is guaranteed because its output is fixed to a particular hash. While it can touch the network, it will fail to build if the output from the fetch doesn't match the user-provided output hash.

You can see how the reproducibility of Nix builds are layered, like a packaging onion.