NixOS, and declarative immutable systems, are a great fit for CI/CD pipelines.  With the entire system in code, ensuring and auditing reproducible environments becomes easy.  Applications can also be “nixified,” so both system and application are fully declarative and in version control. The NixOS system is mounted read-only, which makes for a good fit in immutable autoscaling groups.  Instance userdata may contain a valid NixOS configuration, which is assumed on boot, so as to implement any necessary post-bake changes.

“NixOS. The Purely Functional Linux Distribution. NixOS is a Linux distribution with a unique approach to package and configuration management. Built on top of the Nix package manager, it is completely declarative, makes upgrading systems reliable, and has many other advantages.”
— https://nixos.org/

There are four main parts to Nix and NixOS (Expressions, OS, Modules, and Tests). We will examine each one:

The Nix expression language
This is the “package manager” functionality of NixOS, which can be downloaded from here. Each open source project, including the Linux OS, has a “nix expression” that describes how it is to be built. This includes explicitly declaring each dependency. Each dependency is also declared with a nix expression, so the entire system is declarative. All Nix packages exist in the Nixpkgs Github. Here is the one for ElectricsSheep, a distributed screen saver for evolving artificial organisms:

The underlying mechanism of dependency and package management is a system of symbolic links. Packages are built and deployed in an immutable “nix store”. This read only location exists at /nix/store. In this location, the package name has a hash added, which is computed as a result of evaluating all build input dependencies. Therefore, we can have the same package available many times, each with a unique hash, and a unique version of dependencies. Symbolic links from /run/current-system/sw/bin/ to the hashed package name in nix store determine which package is called, as /run/current-system/sw/bin/ would be in the user’s $PATH.

Should any change to a packaging expression happen, all packages depending on it would rebuild. If a mistake is found, it is easy to revert the change to the dependency, and rebuild. Ensuring the reproducibility of system packages is a huge win, and this solution to “dependency hell” works very well in practice. The Nix package manager can be run on any Linux distribution, such as Fedora or Ubuntu, and also works on Darwin/OSX as well. Custom code can also be packaged with nix expressions, and so both the app and the OS are fully declarative and reproducible.
The NixOS Linux distribution
Nix packaging of open source software, including Linux kernel and boot processes, make up the NixOS Linux distribution. Official releases are available online. NixOS Channels are the method for specifying which version of NixOS is to be installed. NixOS is controlled by the /etc/nixos/configuration.nix file, which declaratively defines the NixOS environment, including defining which NixOS channel is to be used on the system. Whenever configuration.nix is updated, a nixos-rebuild switch can be executed, which switches to the new configuration immediately, as well as adding a new “generation” to Grub/EFI, so the new version can be booted. Here is an example configuration.nix:

As of NixOS 16.03, AWS EC2 Instance Metadata support is built in. However, instead of the usual Cloudinit directives, the NixOS instance expects the Userdata to be a valid configuration.nix. Upon boot, the system will “switch” to what is defined in the provided configuration.nix via EC2 Userdata. This allows for immutable declarative instances in AWS AutoScaling Groups.
NixOS Configuration Management Modules
NixOS modules define how services, via SystemD, are to be configured and run. Modules are written so that parameters can be set which correspond how the systemd process is to be run.
This is the Buildbot NixOS Module, which writes out buildbot configuration, based on module parameters, and then ensures the service is running:

NixOS Tests
NixOS tests are a mechanism to ensure NixOS expressions and modules are working as expected.  Virtual machine(s) are spun up so as to perform the declared tests. Currently VM’s spun up for testing use the QEMU hypervisor, although NixOS tests are moving to libvirt, ideally supporting autodetection of available system virtualization technologies. As an example, here is the buildbot continuous integration server test:

After downloading and building all dependencies, the test will perform a build that starts a QEMU/KVM virtual machine containing the nix system.  It is also possible to bring up this test system interactively to facilitate debugging. The virtual machine mounts the Nix store of the host which makes vm creation very fast, as no disk image needs to be created. These tests can then be implemented in a continuous integration environment such as buildbot or hydra.
In addition to declaratively expressing and testing system packages, applications can be nixified in the same way.  Application tests can then be written in the same manner, so both system and application can go thru a continuous integration pipeline. In a Docker microservices environment, where applications are defined as immutable containers, NixOS is the perfect host node OS, running the Docker daemon and Nomad, Kube, etc.
NixOS is in fast active development. Many users are also NixOS contributors, so most nix packaging of open source projects stay up-to-date. Unstable and release channels are available. Installation is very well documented online. The ability to easily “switch” between configuration versions, or “generations,” which include their own grub/efi boot entry, makes for a great workstation distro.  The declarative reproducibility of a long term stable release, with cloudinit userdata support, makes for a great server distribution.
Thanks for reading,
@hackoflamb

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