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93 lines
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Markdown
93 lines
7.2 KiB
Markdown
(flakes)=
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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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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 offers a uniform [schema](https://nix.dev/manual/nix/2.18/command-ref/new-cli/nix3-flake.html#flake-format), where:
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- Other flakes can be 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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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 file`s 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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## Why are flakes controversial?
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{ref}`Flakes <flakes>` were originally 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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[experimental feature]: https://nix.dev/manual/nix/2.18/contributing/experimental-features.html
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The subject is considered controversial among Nix users and developers in terms of design, development processes, and community governance.
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In particular:
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- The RFC was closed without conclusion, and some design and implementation issues are not yet resolved.
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Examples include the notion of a global [flake registry], the [impossibility of parameterising flakes](https://github.com/NixOS/nix/issues/2861), and the [new command line interface and flakes being 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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- 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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- New Nix users were and still are encouraged by various individuals to adopt flakes despite there being no concrete plan or timeline for stabilisation.
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[flake registry]: https://nix.dev/manual/nix/2.18/command-ref/new-cli/nix3-registry.html
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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 [2022 community survey](https://discourse.nixos.org/t/2022-nix-survey-results/18983), more than half of the user base, a third of which were relative beginners, relied 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 enable flakes?
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You have to judge for yourself based on your needs.
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[Flakes](https://nix.dev/concepts/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.
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- The constraints imposed on flakes strengthen reproducibility by default, and enable various 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](https://nixos.wiki/wiki/Flakes#Flake_schema) helps with composing Nix projects from multiple sources in an orderly fashion.
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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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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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While flakes reduce complexity in some regards, they also introduce additional mechanisms and you will have to learn more about the system to fully understand how it works.
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There are downsides to relying on [experimental features](https://nix.dev/manual/nix/2.18/command-ref/conf-file.html#conf-experimental-features) in general:
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- Interfaces and behaviour 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 agreed-upon plan or timeline for stabilising flakes.
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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 a 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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