What are flakes?#

What is usually referred to as “flakes” is:

Technically, a flake is a file system tree that contains a file named flake.nix in its root directory.

Flakes add the following behavior to Nix:

  1. A flake.nix file offers a uniform schema , where:

    • Other flakes can be referenced as dependencies providing Nix language code or other files.

    • The values produced by the Nix expressions in flake.nix are structured according to pre-defined use cases.

  2. References to other flakes can be specified using a dedicated URL-like syntax. A flake registry allows using symbolic identifiers for further brevity. References can be automatically locked to their current specific version and later updated programmatically.

  3. 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.

Nix handles flakes differently than regular Nix files in the following ways:

  • The flake.nix file is checked for schema validity.

    In particular, the metadata fields cannot be arbitrary Nix expressions. This is to prevent complex, possibly non-terminating computations while querying the metadata.

  • The entire flake directory is copied to Nix store before evaluation.

    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.

  • No external variables, parameters, or impure language values are allowed.

    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.

Why are flakes controversial?#

Originally proposed in RFC 49, flakes have been in development since 2019. Nix introduced the implementation as its first experimental feature in 2021.

The subject is considered controversial among Nix users and developers in terms of design, development processes, and community governance. In particular:

This led to a situation where the stable interface was only sparsely maintained for multiple years, and repeatedly suffered breakages due to ongoing development. Meanwhile, the new interface was adopted widely enough for evolving its design without negatively affecting users to become very challenging.

As of the 2022 community survey, more than half of the user base, a third of which were relative beginners, relied on experimental features. Nixpkgs as a contrasting example, while featuring a flake.nix for compatibility, does not depend on Nix experimental features in its code base.