tvl-depot/blog/content/english/lets-learn-nix-determinism-vs-reproducibility.md

title	date	draft
Lets Learn Nix: Reproducibility	2020-03-17T12:06:47Z	true

I am dedicating this page to defining and disambiguating some terminology. I think it is important to use these terms precisely, so it may be worthwhile to memorize these definitions and ensure that you are clarifying the discourse rather than muddying it.

Terms

• repeatable build:
• reproducible build:
• deterministic build:
• pure function:
• impure function:
• idempotent function:

TODO(wpcarro): Consistently and deliberately use reproducible and deterministic.

Repeatable vs. Reproducible

Is NixOS reproducible? Visit @grhmc's website, r13y.com, to find out.

At the time of this writing, 1519 of 1568 (i.e. 96.9%) of the paths in the nixos.iso_minimal.x86_64-linux installation image are reproducible.

What hinders reproducibility?

Timestamps.

If package A encodes a timestamp into its build artifact, then we can demonstrate that package A is not reproducible simply by building it at two different times and doing a byte-for-byte comparison of the build artifacts.

Does Nix protect developers against non-determinism

Yes. But not entirely. How?

Deterministic Nix derivation

{ pkgs ? import <nixpkgs> {}, ... }:

with pkgs;

stdenv.mkDerivation {
  name = "reproducible";
  phases = [ "buildPhase" ];
  buildPhase = "echo reproducible >$out";
}

Non-deterministic Nix derivation

We can introduce some non-determinism into our build using the date function.

# file: /tmp/test.nix
{ pkgs ? import <nixpkgs> {}, ... }:

with pkgs;

stdenv.mkDerivation {
  name = "non-reproducible";
  phases = [ "buildPhase" ];
  buildPhase = "date >$out";
}

Then run...

$ nix-build /tmp/test.nix
$ nix-build /tmp/test.nix --check --keep-failed

How do you test reproducibility?

We can use cmp to compare files byte-for-byte. The following comparison should fail:

$ echo foo >/tmp/a
$ echo bar >/tmp/b
$ cmp --silent /tmp/{a,b}
$ echo $?

And the following comparison should succeed:

$ echo hello >/tmp/a
$ echo hello >/tmp/b
$ cmp --silent /tmp/{a,b}
$ echo $?

Reproducible vs. deterministic

Reproducible builds are deterministic builds and deterministic build

Deterministic, Reproducible, Pure, Idempotent, oh my

• A pure function has no side-effects.

• An idempotent function can be executed more than once with the same arguments without altering the side-effects.

• A deterministic function ensures that

Deterministic vs. Reproducible

I can check if a build is reproducible using these tools.