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+---
+title: "Lets Learn Nix: Reproducibility"
+date: 2020-03-17T12:06:47Z
+draft: 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][who-grhmc]'s website,
+[r13y.com](https://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
+
+```nix
+{ pkgs ? import <nixpkgs> {}, ... }:
+
+with pkgs;
+
+stdenv.mkDerivation {
+  name = "reproducible";
+  phases = [ "buildPhase" ];
+  buildPhase = "echo reproducible >$out";
+}
+```
+
+## Non-deterministic Nix derivation
+
+We can introduce some non-determinism into our build using the `date` function.
+
+```nix
+# file: /tmp/test.nix
+{ pkgs ? import <nixpkgs> {}, ... }:
+
+with pkgs;
+
+stdenv.mkDerivation {
+  name = "non-reproducible";
+  phases = [ "buildPhase" ];
+  buildPhase = "date >$out";
+}
+```
+
+Then run...
+
+```shell
+$ 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:
+
+```shell
+$ echo foo >/tmp/a
+$ echo bar >/tmp/b
+$ cmp --silent /tmp/{a,b}
+$ echo $?
+```
+
+And the following comparison should succeed:
+
+```shell
+$ 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][wtf-repro-tools].
+
+[wtf-repro-tools]: https://reproducible-builds.org/tools/
+[who-grhmc]: https://twitter.com/grhmc