PURELY FUNCTIONAL PACKAGE MANAGEMENT WITH NIX
Eric Rasmussen / @theerasmas
January 19, 2015

WHO AM I
OSS contributor
haskell/python/javascript developer
works on a hybrid dev/ops team
prior LUGOD speaker (not about Linux)

PACKAGING: THE GOOD PARTS
installs in one click/command
automatic dependency resolution

there's just one problem

PACKAGING: THE BAD PARTS
dependency hell
obscure errors
high maintenance costs

TL;DR PACKAGING IS HARD

PACKAGE MANAGERS

CHOICES (OS)
dpkg/apt
rpm/yum
pacman
homebrew
mac ports
various app stores

CHOICES (PL)
easy_install
pip
go get
maven
npm
rubygems
sbt
cabal
package.el
cpan
pear
pecl

DO WE REALLY NEED ANOTHER?

(via http://xkcd.com/927/)

INTRODUCING NIX
new model for package management
introduced in Eelco Dolstra's PhD Thesis (2006)
based on functional programming principles

WHAT NIX OFFERS
minimal and portable
declarative
reproducible builds
deterministic

FUNCTIONAL PURITY
Function takes inputs and produces output
Ex: Addition takes two numbers and makes a new one
40 + 2 = 42

FUNCTIONAL PURITY
Most programming languages don't enforce this!
40 + 2 =
=
=
=
=

new log file with debug output
database calls
HTTP service calls...
42, maybe?

NIX PACKAGES ARE PURE
Input: other packages, configuration options
Output: a package

EXAMPLE: GCC

/nix/store/r8vvq9kq18pz08v249h8my6r9vs7s0n3-gcc-4.3
inside the prefix: bin, lib, share, ... directories
r8vvq9kq18pz08v249h8my6r9vs7s0n3 is a hash of function
inputs

PURITY IN NIX
no global install directories (/usr, /bin)
/nix/store is immutable (mounted read-only)
nix expressions cannot write to files

IMPURITY IN NIX
packages can make network calls (curl/git clone/etc)
race conditions in parallel builds
stdenv on OS X depends on globals

EXAMPLE: NGINX
{ stdenv, fetchurl, fetchgit, openssl, zlib, pcre, libxml2, libxslt, expat }:
stdenv.mkDerivation rec {
name = "nginx-${version}";
version = "1.4.4";
src = fetchurl {
url = "http://nginx.org/download/nginx-${version}.tar.gz";
sha256 = "1f82845mpgmhvm151fhn2cnqjggw9w7cvsqbva9rb320wmc9m63w";
};
buildInputs = [ openssl zlib pcre libxml2 libxslt ];
configureFlags = [ "--with-http_spdy_module" ];
postInstall = "mv $out/sbin $out/bin";
meta = with stdenv.lib; {
description = "A reverse proxy and lightweight webserver";
maintainers = [ maintainers.iElectric ];
platforms = platforms.all;
license = licenses.bsd2;
};

BINARY PACKAGES
binary packages are built in hydra build farms
purity lets us substitute pre-built packages based on the hash
major speedups when installing on common platforms

ADD NIX TO YOUR WORKFLOW IN 2 DAYS
Day 1: install a package
Day 2: myEnvFun

DAY 1: INSTALLATION
$ curl -L http://git.io/nix-install.sh | bash
$ source ~/.nix-profile/etc/profile.d/nix.sh
$ nix-env -i nginx

DAY 2: CONFIG.NIX / MYENVFUN
Note: fun is for "functional" (having fun is optional)
# ~/.nixpkgs/config.nix
{
packageOverrides = pkgs : with pkgs; {
pyred2 = pkgs.myEnvFun {
name = "pyred2";
buildInputs = [ python27Full redis ];
};
pyred3 = pkgs.myEnvFun {
name = "pyred3";
buildInputs = [ python3 redis ];
};
};
}

Using myEnvFun
$ nix-env -i env-pyred2
$ load-env-pyred2
env-pyred2 loaded
pyred2:[eric@nixos:~]$ python
python
python2.7
python2
python2.7-config

python2-config
python-config

EASY TO UNINSTALL IF NEEDED
$ rm -rf /nix
$ rm -rf ~/nix-profile/

INTERMISSION

NIXOS
Declarative config at the system level
Nix as package manager
Nix expressions to configure the OS

NIXOS
stateless config management
NixOS modules for services

CONFIGURATION.NIX
{ config, pkgs, ... }: with pkgs;
{
networking.firewall.allowedTCPPorts = [ 8000 ];
services.postgresql = {
enable = true;
package = pkgs.postgresql93;
authentication = pkgs.lib.mkOverride 10 ''
local postgres root ident
host myuser myuser 127.0.0.1/32 password
local all all ident
'';
initialScript = "bootstrap_or_something.sql";
};
environment.systemPackages = [ emacs24-nox git tmux ghc.ghc783 ];
}
}

ENFORCING GOOD HABITS
Harder to make one-off hacks
Config and build changes must be codified
Example: add hosts to /etc/hosts
# configuration.nix
# will extend /etc/hosts
networking.extraHosts = ''
some_ip some_host
some_ip2 some_host2
'';

IS NIXOS FOR ME?
maybe!
requires learning nix/writing packages
great IRC support but few docs/tutorials

IS NIX FOR ME?
try it out!
won't interfere with existing packages

REFERENCES
NixOS.org
Nix Package Manager Manual
NixOS Manual
Domen Kožar's 2014 Fosdem talk