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<title>Declarative configuration with NixOS</title>
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# Declarative configuration with NixOS
## David Sferruzza
### 2018-07-01
---
# About me
- [@d_sferruzza](https://twitter.com/d_sferruzza)
- [github.com/dsferruzza](https://github.com/dsferruzza)
- PhD student in software engineering at *LS2N*
- Work at [Startup Palace](https://www.startup-palace.com)
 <!-- .element: class="stretch" -->
---
# Startup Palace
We **iteratively** design and develop web applications. <small>(among other things)</small>
Frequent reviews (by customers and team members) are an important part of the job.
> We need staging/preprod environments
 <!-- .element: class="stretch" -->
---
# Staging environments
They should be:
- quick to create
- quick to configure
- quick to remove
- cheap
- compatible with automatic deployment
 <!-- .element: class="stretch" -->
Note: This shoud be a **convenience**.
---
# About automatic deployment
We use [GitLab](https://about.gitlab.com/)
 <!-- .element: class="stretch" -->
> GitLab unifies chat, issues, code review,<br>CI and CD into a single UI
---
# About automatic deployment
GitLab CI is very flexible:
```yaml
my_deploy:
stage: deploy
image: org/my_docker_image
script:
- echo "I can do anything here"
- rsync ./my_projet $USER@$HOST:/var/www/
only:
- master
tags:
- docker
```
---
# Naive solution
We know [Debian](https://www.debian.org/). Let's use Debian!
And let's install **and** *configure*:
- Apache
- PHP
- PostgreSQL
- SSH
*by hand*!
 <!-- .element: class="stretch" -->
---
# Naive solution
It works but:
- it needs a lot of manual setup<br>→ admins need to be watchful and rigorous
- environments can diverge<br>→ it can quickly become a mess
- security/system updates can break stuff<br>→ you need to fix environments one by one
- it's soooo **boring** to use
> Boring means I will make mistakes
---
# Naive solution
We did it for a while.
*1 star. Would not recommend.*
 <!-- .element: class="stretch" -->
So, can we do better?
---
# Dockerfile
At that time, people were talking a lot about Docker. So I took a look.
The concept of the **Dockerfile** is interesting:
- you start from an existing image
- you *declare* how to change it
- you get a new image
```dockerfile
FROM debian:jessie
RUN apt-get update \
&& apt-get install --no-install-recommends -y \
openjdk-7-jdk
```
---
# Dockerfile
This is fine. But:
- you declare **how** to build an image<br>not **what** is the image you want to build
- this is too linear (layers of cache)
- it's hard to compose
- ...
 <!-- .element: class="stretch" -->
---
# Declarative vs. imperative
> **Imperative programming**: use statements that change a program's state
>
> **Declarative programming**: express the logic of a computation without describing its control flow
- imperative: shopping instructions
- declarative: shopping list
---
# The NixOS project
> <figure class="stretch"><img src="img/nixos.svg" alt=""></figure>
> <https://nixos.org/>
- Nix
- Nix Expression Language
- Nixpkgs
- NixOS (Linux distribution)
- ...
Note: Created by *Eelco Dolstra* for its PhD
---
# Nix
> The Purely Functional Package Manager
>
> <https://nixos.org/nix/>
Packages:
- are treated like values
- are built by functions that don't have side-effects
- never change after they have been built
Builds are **reproducible**.
---
# Nix store
The place where packages are stored. Usually in `/nix/store/`
Example:<br><small>`/nix/store/f4gxsj6pn4ygqadwyk2m6xg1ywhfwxg1-openssl-1.0.2h/`</small>
A package's directory name contains:
- its name
- its version
- a unique identifier that captures all its dependencies
Note: unique identifier = a cryptographic hash of the package’s build dependency graph
---
# The `which` package
```nix
{ stdenv, fetchurl }:
stdenv.mkDerivation rec {
name = "which-2.21";
src = fetchurl {
url = "mirror://gnu/which/${name}.tar.gz";
sha256 = "1bgafvy3ypbhhfznwjv1lxmd6mci3x1byilnnkc7gcr486wlb8pl";
};
meta = with stdenv.lib; {
homepage = http://ftp.gnu.org/gnu/which/;
platforms = platforms.all;
license = licenses.gpl3;
};
}
```
<!-- .element: class="smallcode" -->
```text
result/
├── bin
│ └── which
└── share
├── info
│ └── which.info
└── man
└── man1
└── which.1.gz
```
<!-- .element: class="smallcode" -->
---
# The `which` package
```nix
{ stdenv, fetchurl }:
stdenv.mkDerivation rec {
name = "which-2.21";
src = fetchurl {
url = "mirror://gnu/which/${name}.tar.gz";
sha256 = "1bgafvy3ypbhhfznwjv1lxmd6mci3x1byilnnkc7gcr486wlb8pl";
};
meta = with stdenv.lib; {
homepage = http://ftp.gnu.org/gnu/which/;
platforms = platforms.all;
license = licenses.gpl3;
};
}
```
<!-- .element: class="smallcode" -->
```text
$ ldd result/bin/which
linux-gate.so.1 (0xb778e000)
libc.so.6 => /nix/store/6nqfrgw8w5xmm0zarp8f51rvs9lvgq73-glibc-2.23/lib/libc.so.6 (0xb75ea000)
/nix/store/6nqfrgw8w5xmm0zarp8f51rvs9lvgq73-glibc-2.23/lib/ld-linux.so.2 (0xb7790000)
```
<!-- .element: class="smallcode" -->
Note: ldd - print shared library dependencies
---
# Nix
Nice features:
- multiple versions
- complete (and explicit) dependencies
- multi-user
- atomic upgrades and rollbacks
- transparent source/binary deployment
 <!-- .element: class="stretch" -->
Note: build & runtime dependencies
---
# User environments
 <!-- .element: class="stretch" -->
---
# Nix Expression Language
> It is a pure, lazy, functional language.
>
> It is **not** a general purpose language.
Types of expressions:
- strings, integers, paths, booleans, null
- functions/lambdas
- lists
- sets
- derivations (package build actions)
---
# Nix Expression Language
```nix
# This is a list of strings
[ "a" "b" ''c'' "var=${var}" ]
# This is a lambda
f = x: x * x
# This is a set
{ a = 5; b = f 2; }
# This is a recursive set
rec { a = b + 1; b = 5; }
```
---
# Nixpkgs
> A collection of packages for the Nix package manager.
>
> <https://github.com/NixOS/nixpkgs>
- has nearly 43,000 packages
- uses a permissive MIT/X11 license
- includes a [standard library](https://github.com/NixOS/nixpkgs/tree/master/lib) for Nix Expression Language
---
# The `which` package
```nix
{ stdenv, fetchurl }:
stdenv.mkDerivation rec {
name = "which-2.21";
src = fetchurl {
url = "mirror://gnu/which/${name}.tar.gz";
sha256 = "1bgafvy3ypbhhfznwjv1lxmd6mci3x1byilnnkc7gcr486wlb8pl";
};
meta = with stdenv.lib; {
homepage = http://ftp.gnu.org/gnu/which/;
platforms = platforms.all;
license = licenses.gpl3;
};
}
```
<!-- .element: class="smallcode" -->
---
# NixOS
The Purely Functional Linux Distribution
> Nix + Nixpkgs + systemd + Linux kernel
- declarative system configuration model
- reliable upgrades
- atomic upgrades
- rollbacks
- reproducible system configurations
- safe to test changes
- Nix goodness (multi-user package management, source-based model with binaries, consistency, ...)
Note: reliable = installing & upgrading is like from scratch
---
# NixOS
```nix
{ config, lib, pkgs, ... }:
{
boot.loader.grub.device = "/dev/sda";
fileSystems."/".device = "/dev/sda1";
networking.firewall = {
enable = true;
allowedTCPPorts = [ 80 ];
};
environment.systemPackages = with pkgs; [
wget
git
];
services = {
sshd.enable = true;
munin-node.enable = true;
munin-cron = {
enable = true;
hosts = ''
[${config.networking.hostName}]
address localhost
'';
};
};
}
```
<!-- .element: class="smallcode" -->
---
# NixOS
Realize the configuration:
```text
$ nixos-rebuild switch
```
Rollback to the previous configuration:
```text
$ nixos-rebuild switch --rollback
```
Only build configuration:
```text
$ nixos-rebuild build
```
Test configuration:
```text
$ nixos-rebuild test
$ nixos-rebuild build-vm
```
---
# NixOS
Build your own system config using:
- the `/etc/nixos/configuration.nix` file
- a functional programming language
- a module system
- packages and generic functions from Nixpkgs
 <!-- .element: class="stretch" -->
---
# Options
> A NixOS module can:
>
> - declare options
> - define options depending on other options' values
Built-in options: <https://nixos.org/nixos/options.html>
**Example**: I want a boolean option `myCustomBashAliases` that defines some Bash aliases when enabled.
---
# Options
In `/etc/nixos/myModule.nix`:
```nix
{ config, lib, pkgs, ... }:
{
options = {
myCustomBashAliases = lib.mkOption {
default = false;
description = "Enable my awesome Bash aliases.";
types = lib.types.bool;
};
};
config = lib.mkIf config.myCustomBashAliases {
programs.bash.shellAliases = {
ls = "ls --color=auto";
ll = "ls --color=auto -lh";
l = "ls --color=auto -lha";
};
};
}
```
<!-- .element: class="smallcode" -->
---
# Options usage
In `/etc/nixos/configuration.nix`:
```nix
{ config, pkgs, ... }:
{
imports =
[
# ...
./myModule.nix
];
myCustomBashAliases = true;
# ...
}
```
<!-- .element: class="smallcode" -->
This allows to build very powerful abstractions!
---
# Back to the problem
I want to be able to create staging environments:
- quickly
- easily
- in a reliable way
→ Let's write some NixOS modules!
 <!-- .element: class="stretch" -->
---
# Knights Of Nix
An abstraction over NixOS to manage staging environments for static and PHP web apps.
 <!-- .element: class="stretch" -->
Disclaimers:
- This is a **Proof of Concept**
- This is **not** a *Platform as a Service* (PaaS)
---
# Knights Of Nix
High-level definition of staging environments:
```nix
[
# Here is a fake app
rec {
appType = "php";
phpVersion = "70";
name = "test";
hostName = "${name}.mydomain.com";
userSha512Password = "...";
pgClearPassword = "...";
}
]
```
---
# Knights Of Nix
Nice features:
- multi-domain support (with redirections)
- HTTPS support (automatic certification generation with [Let's Encrypt](https://letsencrypt.org/))
- per-app user, with SSH access (I can auto-deploy from GitLab \\o/)
- per-app PHP version
- per-app Cron jobs
 <!-- .element: class="stretch" -->
---
# Bonus: nix-shell
Drop a Bash shell configured with the same environment that would be used to perform the build itself.
```text
$ nix-shell '<nixpkgs>' -A nodejs-5_x
these paths will be fetched (21.40 MiB download, 21.41 MiB unpacked):
/nix/store/mbv89fkb9av98alqfaay8c4n3czk82q9-node-v5.9.0.tar.gz
fetching path ‘/nix/store/mbv89fkb9av98alqfaay8c4n3czk82q9-node-v5.9.0.tar.gz’...
*** Downloading ‘https://cache.nixos.org/nar/0qzzksfad1g8kk3rfpdiyhdp9h5737nn70zywakarc2lf3qhg0g1.nar.xz’ (signed by ‘cache.nixos.org-1’) to ‘/nix/store/mbv89fkb9av98alqfaay8c4n3czk82q9-node-v5.9.0.tar.gz’...
% Total % Received % Xferd Average Speed Time Time Time Current
Dload Upload Total Spent Left Speed
100 21.4M 100 21.4M 0 0 1173k 0 0:00:18 0:00:18 --:--:-- 1374k
[nix-shell:~]$
```
<!-- .element: class="smallcode" -->
---
# Bonus: package customization
```nix
nixpkgs.config.packageOverrides = pkgs: rec {
simp_le = pkgs.simp_le.overrideDerivation (oldAttrs: {
patches = [
(pkgs.fetchpatch {
url = "https://github.com/kuba/simp_le/commit/" +
"4bc788fdd611c4118c3f86b5f546779723aca5a7.patch";
sha256 = "0036p11qn3plydv5s5z6i28r6ihy1ipjl0y8la0izpkiq273byfc";
})
];
});
};
```
<!-- .element: class="smallcode" -->
 <!-- .element: class="stretch" -->
Note: completely override package, not only the installed instance
---
# Conclusion
Nix & NixOS have very interesting properties that can make server management **easier** and **safer**.
You can use software practices to do sysadmin: modularity, DRY, versioning, abstractions, ...
> The NixOS project is still a bit young, but it grows and you should definitely take a look!
>
> → <https://nixos.org>
---
# References
- [NixOS manual](https://nixos.org/nixos/manual/)
- [NixOS options](http://nixos.org/nixos/options.html) & [NixOS packages](http://nixos.org/nixos/packages.html)
- [Nix manual](https://nixos.org/nix/manual/)
- [Nixpkgs source code](https://github.com/NixOS/nixpkgs)
- [Nix pills](https://lethalman.blogspot.fr/search/label/nixpills)
- [Nix/NixOS papers](https://nixos.org/~eelco/pubs/)
 <!-- .element: class="stretch" -->
---
# Questions?
 <!-- .element: class="stretch" -->
Twitter: [@d_sferruzza](https://twitter.com/d_sferruzza)
Slides on GitHub:
[dsferruzza/conf-declarative-configuration-with-nixos](http://github.com/dsferruzza/conf-declarative-configuration-with-nixos)
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