Nix vs. Docker: An In-Depth Comparison

In software development, reproducibility and ease of deployment are critical. Nix and Docker are two powerful sets of tools that address these needs in fundamentally different ways. This post is a detailed guide into the differences between Docker vs Nix and the advantages of each. 

Why Is Reproducibility Important?

Setting up a complex development environment is a time-consuming process. It can take hours and sometimes even days. Replicating the development environment across different systems with different configurations and getting it right can be a real headache. That's why you need reproducible development environments that you can precisely recreate without having to worry about the underlying system. This problem has been recognized and resolved in all kinds of ways since the 1990s. Both Nix and Docker solve it, but they approach it quite differently. 

Nix

Nix is a package manager that uses a dynamically typed, declarative-functional programming language. This language treats packages as values, ensuring that each package functions deterministically based on its inputs, leading to highly reproducible builds. Nix supports all Linux distributions and MacOS, and it offers access to the largest open-source package collection in the world. 

How Does the Nix Package Manager Work?

Nix does not install packages the way traditional package managers do, by executing a command like "apt install" or "brew install." Instead of using an imperative command, Nix uses a declarative approach. A derivation in Nix is computed from a file that contains the instructions for building the package. For a derivation to be considered valid, the file must include the following information: 

• Reference to the dependencies of the package to be built
• Instructions on how to build the package
• Meta information about the package, such as the maintainers

When a package is built successfully, a path will be created with the build result in the following format, where "hash" is created from the derivation data: /nix/store/<hash>-<name>-<version>. See the set of Nix expressions that generate the software derivation "GNU hello" in the example below: 

{ lib, stdenv, fetchurl }:

stdenv.mkDerivation rec {

  pname = "hello";

  version = "2.10";

  src = fetchurl {

    url = "mirror://gnu/hello/${pname}-${version}.tar.gz";

    sha256 = "0ssi1wpaf7plaswqqjwigppsg5fyh99vdlb9kzl7c9lng89ndq1i";

  };

  doCheck = true;

  meta = with lib; {

    description = "A program that produces a familiar, friendly greeting";

    longDescription = ''

      GNU Hello is a program that prints "Hello, world!" when you run it.

      It is fully customizable.

    '';

    homepage = "https://www.gnu.org/software/hello/manual/";

    changelog = "https://git.savannah.gnu.org/cgit/hello.git/plain/NEWS?h=v${version}";

    license = licenses.gpl3Plus;

    maintainers = [ maintainers.eelco ];

    platforms = platforms.all;

  };

}

This code ensures that the same derivation will always build the same package. 

Advantages of Using Nix

Reproducible Setup

A reproducible setup is the clearest advantage of using Nix to manage packages and dependencies. If two people build a package with the same derivation, both will have the same final result. Even if there's a difference in the version of some dependency, the path generated by the derivation will change, making it clear that there has been a change. 

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And it's even easier when we use flakes. Using flakes, we can declare packages directly from GitHub in addition to packages from the official Nix repository. Nix also creates a file flake.lock that contains the exact version of the package. This way you can reproduce a configuration more faithfully. 

Multiple Versions of the Same Package Simultaneously

Since each package is installed in its own path, there are no collisions. This is very attractive for developers, as it can be compared with virtual environments used by Python or any other program. 

Furthermore, as each built version of the package generates a new path, you always have the option of returning to an old version as long as it has not been deleted by the garbage-collector. 

Atomic Upgrades and Rollbacks

Another big advantage of Nix is that changes to the system are atomic.  Nix does not overwrite configuration changes but always creates new ones, allowing you to roll back at any time. This means that updates will either succeed completely or leave the system unchanged. This makes upgrades and rollbacks incredibly safe. 

Multi-User Environment Management

Nix allows multiple users to coexist on a single system with their own isolated environments, preventing conflicts and ensuring reproducibility for each user. 

Challenges

Steep learning curve: Nix's functional approach and declarative language can be difficult for new users, which makes it difficult to onboard new developers. 

Smaller community: While growing, the Nix community is still smaller than Docker's, which means there are comparatively fewer resources available. 

Docker

Docker is the most popular containerization platform. A containerization platform makes it possible to isolate applications and their dependencies in autonomous environments called containers. Each container operates autonomously, containing everything an application needs to run, including code or binary, libraries, and configuration files. These containers are lightweight, portable, and compatible with different platforms. Unlike traditional virtualization, where you run virtual machines with their full operating systems, Docker shares the host system's kernel, allowing for a more efficient use of resources. 

How Does Docker Work?

Docker's operation relies on the Linux kernel and kernel features, such as cgroups and namespaces. These are the functions that separate processes so that they can run independently. This is because the purpose of containers is to run multiple processes and applications separately. That's what makes it possible to optimize infrastructure usage without reducing the level of security that separate systems enjoy. 

All container tools like Docker come with an image-based deployment model. This model simplifies sharing an application or set of services across multiple environments. 

Additionally, Docker helps automate the deployment of applications within a container environment. With these various tools, users gain full access to applications and are able to accelerate deployment, control versions, and assign them. 

Main Advantages of Docker

Portability

Docker containers are incredibly portable, running seamlessly on different operating systems and with different cloud providers. For example, containerized applications can be easily transferred from on-premises systems to cloud environments. 

Isolation

Docker does more than allow you to develop applications that are easy to assemble, maintain, and move. Its containers also isolate applications from the host system and from each other, enhancing security and preventing conflicts. 

Massive Ecosystem

Docker boasts a vast ecosystem of tools, images, and services, making it easy to get started and find solutions for various use cases. 

Challenges

Reproducibility at runtime: Although Docker provides reproducibility at runtime, it doesn't guarantee reproducibility of the build process. This results in different builds of the same Dockerfile in slightly different container images. 

Overhead: Containers can introduce some overhead compared to running applications directly on the host system. 

Final Words: Nix vs Docker

If you need system-level reproducibility, Nix is a better option as its functional approach extends to the entire system configuration. But if you prioritize portability and want to deploy applications across diverse environments quickly and easily, Docker is a better option. 

Nix and Docker are not direct competitors. Rather, they are complementary tools. You can use Nix to build reproducible Docker images and benefit from the strengths of both tools. This way, you can get the reproducibility of Nix during the build process and the portability and ease of deployment that Docker offers. 

‍This post was written by Talha Khalid. Talha is a full-stack developer and data scientist who loves to make the cold and hard topics exciting and easy to understand.