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118 lines
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Markdown
118 lines
10 KiB
Markdown
(flakes-definition)=
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# Flakes
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What is usually referred to as "flakes" is:
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- A policy for managing dependencies between {term}`Nix expressions<Nix expression>`.
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- An [experimental feature] in Nix, implementing that policy and supporting functionality.
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[experimental feature]: https://nix.dev/manual/nix/2.18/contributing/experimental-features.html
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## What are flakes?
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Technically, a [flake](https://nix.dev/manual/nix/2.18/command-ref/new-cli/nix3-flake.html#description) is a file system tree that contains a file named `flake.nix` in its root directory.
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Flakes add the following behavior to Nix:
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1. A `flake.nix` file enforces a [schema], where:
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- Other flakes are referenced as dependencies providing {term}`Nix language` code or other files.
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- The values produced by the {term}`Nix expression`s in `flake.nix` are structured according to pre-defined use cases.
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[schema]: https://nix.dev/manual/nix/2.18/command-ref/new-cli/nix3-flake.html#flake-format
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1. References to other flakes can be specified using a dedicated [URL-like syntax](https://nix.dev/manual/nix/2.18/command-ref/new-cli/nix3-flake.html#flake-references).
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A [flake registry] allows using symbolic identifiers for further brevity.
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References can be automatically locked to their current specific version and later updated programmatically.
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[flake registry]: https://nix.dev/manual/nix/2.18/command-ref/new-cli/nix3-registry.html
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1. A [new command line interface], implemented as a separate experimental feature, leverages flakes by accepting flake references in order to build, run, or deploy software defined as a flake.
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[new command line interface]: https://nix.dev/manual/nix/2.18/command-ref/new-cli/nix.html
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Nix handles flakes differently than regular {term}`Nix files <Nix file>` in the following ways:
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- The `flake.nix` file is checked for schema validity.
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In particular, the metadata fields cannot be arbitrary Nix expressions.
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This is to prevent complex, possibly non-terminating computations while querying the metadata.
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- The entire flake directory is copied to Nix store before evaluation.
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This allows for effective evaluation caching, which is relevant for large expressions such as Nixpkgs, but also requires copying the entire flake directory again on each change.
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- No external variables, parameters, or impure language values are allowed.
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It means full reproducibility of a Nix expression, and, by extension, the resulting build instructions by default, but also prohibits parameterisation of results by consumers.
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(flakes-controversy)=
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## Why are flakes controversial?
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[Flakes](flakes-definition) were inspired by [Shea Levy's NixCon 2018 talk](https://www.youtube.com/watch?v=DHOLjsyXPtM), formally proposed in [RFC 49](https://github.com/NixOS/rfcs/pull/49), and have been in development since 2019.
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Nix introduced the implementation as its first [experimental feature] in 2021.
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The subject is considered controversial among Nix users and developers in terms of design, implementation quality, and decision making process.
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In particular:
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- The RFC was closed without conclusion, and some fundamental issues are not yet resolved.
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For example:
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- The notion of a [global flake registry](https://github.com/NixOS/flake-registry) saw [substantial criticism](https://github.com/NixOS/rfcs/pull/49#issuecomment-635635333) that was never addressed.
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While the source references of [registry entries can be pinned](https://nix.dev/manual/nix/2.18/command-ref/new-cli/nix3-registry-pin), local registry names in Nix expressions [introduce mutable system state](https://github.com/NixOS/nix/issues/7422) and are thus, in that regard, no improvement over channels as managed by [`nix-channel`](https://nix.dev/manual/nix/2.18/command-ref/nix-channel).
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- It is [impossible to parameterise flakes](https://github.com/NixOS/nix/issues/2861).
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This means that [flakes downgrade ease of use of the `system` parameter](https://github.com/NixOS/nix/issues/3843) of derivations, for producers and consumers.
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- the flakes proposal was criticised for [trying to solve too many problems at once](https://github.com/nixos/rfcs/pull/49#issuecomment-521998933) and [at the wrong abstraction layer](https://discourse.nixos.org/t/nixpkgs-cli-working-group-member-search/30517).
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Part of this is that [the new command line interface and flakes are closely tied to each other](https://discourse.nixos.org/t/2023-03-06-nix-team-meeting-minutes-38/26056#cli-stabilisation-announcement-draft-4).
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- As [predicted by RFC reviewers](https://github.com/NixOS/rfcs/pull/49#issuecomment-588990425), the original implementation introduced [regressions](https://discourse.nixos.org/t/nix-2-4-and-what-s-next/16257) in the [Nix 2.4 release](https://nix.dev/manual/nix/2.18/release-notes/rl-2.4.html), breaking some stable functionality without a [major version](https://semver.org/) increment.
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- Copying sources to the Nix store prior to evaluation adds a [significant performance penalty](https://github.com/NixOS/nix/issues/3121), especially for large repositories such as {term}`Nixpkgs`.
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Work to address this has been [in progress since May 2022](https://github.com/NixOS/nix/pull/6530), but risks introducing [its own set of issues](https://github.com/NixOS/nix/pull/6530#issuecomment-1850565931).
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- New Nix users were and still are encouraged by various individuals to adopt flakes despite there being no stability guarantees and no timeline to conclude the experiment.
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This led to a situation where the stable interface was only sparsely maintained for multiple years, and repeatedly suffered breakages due to ongoing development.
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Meanwhile, the new interface was adopted widely enough for evolving its design without negatively affecting users to become very challenging.
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As of the [2023](https://discourse.nixos.org/t/nix-community-survey-2023-results/33124) survey, 84% of the respondents rely on experimental features.
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{term}`Nixpkgs` as a contrasting example, while featuring a `flake.nix` for compatibility, does not depend on Nix experimental features in its code base.
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## Should I use flakes in my project?
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You have to judge for yourself based on your needs.
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[Flakes](flakes-definition) emphasize reproducible artifacts and convenience for their consumers, while classic Nix tools center around composable building blocks and customisation options for developers.
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Both paradigms have their own set of unique concepts and support tooling that have to be learned, with varying ease of use, implementation quality, and support status.
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At the moment, neither the stable nor the experimental interface is clearly superior to the other in all aspects.
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Flakes and the `nix` command suite bring multiple improvements that are relevant for both software users and package authors:
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- The new command-line interface, together with flakes, makes dealing with existing packages significantly more convenient in many cases.
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- The constraints imposed on flakes strengthen reproducibility by default, and enable some performance improvements when interacting with a large Nix package repository like {term}`Nixpkgs`.
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- Flake references allow for easier handling of version upgrades for existing packages or project dependencies.
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- The [flake schema][schema] helps with composing Nix projects from multiple sources in an orderly fashion.
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At the same time, flakes have [fundamental architectural issues](flakes-controversy) and a number of [problems with the implementation](https://github.com/NixOS/nix/issues?q=is%3Aissue+is%3Aopen+label%3Aflakes+sort%3Areactions-%2B1-desc), and there is no coordinated effort to resolve them systematically.
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There are also still many [open design questions around the `nix` command line interface](https://github.com/NixOS/nix/issues?q=is%3Aissue+is%3Aopen+label%3Anew-cli+sort%3Areactions-%2B1-desc), some of which are currently being worked on.
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While flakes reduce complexity in some regards, they also introduce some with additional mechanisms.
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You will have to learn more about the system to fully understand how it works.
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Other than that, and below the surface of the flake schema, Nix and the Nix language work exactly the same in both cases.
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In principle, the same level of reproducibility can be achieved with or without flakes.
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In particular, the process of adding software to {term}`Nixpkgs` or maintaining {term}`NixOS` modules and configurations is not affected by flakes at all.
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There is also no evidence that flakes could help solving the scalability challenges of either.
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Finally, there are downsides to relying on [experimental features][experimental feature] in general:
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- Interfaces and behavior of experimental features could still be changed by Nix developers.
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This may require you to adapt your code at some point in the future, which will be more effort when it has grown in complexity.
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[Currently there is no concrete timeline for stabilising flakes.](https://discourse.nixos.org/t/stabilising-the-new-nix-command-line-interface/35531#how-does-this-relate-to-flakes-3)
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In contrast, stable features in Nix can be considered stable indefinitely.
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- The [Nix maintainer team](https://nixos.org/community/teams/nix.html) focuses on fixing bugs and regressions in stable interfaces, supporting well-understood use cases, as well as improving the internal design and overall contributor experience in order to ease future development.
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Improvements to experimental features have low priority.
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- The [Nix documentation team](https://nixos.org/community/teams/documentation.html) focuses on improving documentation and learning materials for stable features and common principles.
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When using flakes, you will have to rely more heavily on user-to-user support, third-party documentation, and the source code.
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## Further reading
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- [Flakes aren't real and cannot hurt you: a guide to using Nix flakes the non-flake way](https://jade.fyi/blog/flakes-arent-real/) (Jade Lovelace, January 2024)
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- [Nix Flakes is an experiment that did too much at once...](https://samuel.dionne-riel.com/blog/2023/09/06/flakes-is-an-experiment-that-did-too-much-at-once.html) ([comments](https://discourse.nixos.org/t/nix-flakes-is-an-experiment-that-did-too-much-at-once/32707)) (Samuel Dionne-Riel, September 2023)
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- [Experimental does not mean unstable](https://determinate.systems/posts/experimental-does-not-mean-unstable) ([comments](https://discourse.nixos.org/t/experimental-does-not-mean-unstable-detsyss-perspective-on-nix-flakes/32703)) (Graham Christensen, September 2023)
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- [The Nix Hour: comparing flakes to traditional Nix](https://www.youtube.com/watch?v=atmoYyBAhF4) (Silvan Mosberger, November 2022)
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