Drowse: Simplifying Nix's Dynamic Derivations for Better Caching

In the evolving landscape of Nix package management, developers constantly seek ways to optimize build times and improve caching efficiency. A new project called drowse emerges as a thoughtful solution to one of Nix's persistent challenges: how to achieve fine-grained dependency caching without sacrificing evaluation performance or cluttering version control history.

The core problem that drowse addresses has long troubled Nix users working with language-specific package managers. Tools like crate2nix enable precise caching of dependencies, but they traditionally rely on one of two approaches, each with significant drawbacks. Import-from-derivation (IFD) serializes the evaluation process, dramatically increasing evaluation time. Alternatively, code generation creates files that bloat git histories and can still negatively impact performance due to the sheer size of some generated files.

Dynamic derivations, an experimental feature in Nix, promises to deliver the best of both worlds—fine-grained caching without IFD or code generation. However, working directly with dynamic derivations requires considerable boilerplate code, creating a barrier to adoption for many developers. This is where drowse steps in, aiming to abstract away the complexity while preserving the benefits.

At its essence, drowse provides three primary functions that wrap dynamic derivations in more approachable interfaces. The callPackage function offers behavior similar to Nix's standard pkgs.callPackage, but with the crucial difference that the contents of the source file are only evaluated at build time, not during Nix expression evaluation. This subtle shift enables significant performance improvements while maintaining the familiar development experience.

For Rust projects, the crate2nix function provides particular value. It mirrors the behavior of crate2nix's appliedCargoNix but eliminates the need for IFD, potentially accelerating evaluation times for Rust-based projects. The project even includes a template for quickly setting up a Rust project with both drowse and crate2nix, demonstrating its practical utility.

For developers who need more direct control, drowse offers mkDynamicDerivation, a lower-level wrapper around the standard mkDerivation specifically tailored for building dynamic derivations. This function provides the flexibility to implement custom build logic while still leveraging the underlying dynamic derivations mechanism.

The project's documentation thoughtfully acknowledges that dynamic derivations remain experimental and relatively undocumented in the Nix ecosystem. To help users overcome this learning curve, the author includes a curated list of resources, including tutorials from nix-ninja and a blog series by @fzakaria that explores dynamic derivations from both theoretical and practical perspectives.

From a broader perspective, drowse represents an important step in bridging the gap between cutting-edge Nix features and everyday developer needs. By reducing the friction around experimental features, tools like drowse help accelerate the entire ecosystem's evolution. The project's approach—providing familiar interfaces while leveraging newer capabilities—follows a pattern seen in other successful Nix tools that have made complex concepts more accessible.

Adoption of drowse does come with considerations, primarily the need to enable several experimental Nix features: ca-derivations, dynamic-derivations, and recursive-nix. This requirement underscores the experimental nature of the underlying technology but also signals that these features are likely to become more stable over time as community interest and use cases grow.

For developers managing complex projects with many dependencies, particularly those in ecosystems like Rust where crate2nix is already established, drowse offers a compelling path forward. It promises faster evaluations, cleaner git histories, and more granular caching without forcing developers to completely rework their existing workflows.

As the Nix ecosystem continues to evolve, tools like drowse demonstrate an important principle: innovation in package management isn't just about new features, but about making those features accessible and practical for the developers who will ultimately use them. By abstracting complexity while preserving power, drowse helps push the boundaries of what's possible with Nix's dependency management capabilities.