2 ms·

Could you possibly clarify what you mean by "skip those first steps?" Which steps are you referring to?

• iso8859-1 3y ago
  Presumably, the Magic Nix Cache speeds up your build by caching the output of the "first steps" (really leaves in a tree of inputs). Otherwise, how would it work? So by steps, I am referring to the steps that your Nix derivation consists of.
  • biggestlou 3y ago
    When Nix "realises" a derivation, it realises the entire dependency tree (here, "realises" means either building or fetching, depending on whether a dependency is already in the Nix store). For every single derivation (dependency) in the tree, Nix first calculates what the store path for that dependency would be and then uses that to determine if it's already stored in the Nix store. So it would look at, say, a glibc dependency in a derivation and determine that the Nix store path would be /nix/store/7kn2mkg0g49lfflkdip7i39q3zsck4pc-glibc. If that's already in the Nix store then it doesn't need to build that. And it applies this logic throughout the entire dependency tree. In some cases, the entire dependency tree has already been built and written to the Nix store, in which Nix knows that it doesn't have to build anything. So Nix's caching logic doesn't apply only to the "first steps" of a Nix build (or realisation, to be more specific); it applies to all steps.
    • grhmc 3y ago
      Check out https://zero-to-nix.com/concepts/closures https://zero-to-nix.com/concepts/closures and https://zero-to-nix.com/concepts/realisation https://zero-to-nix.com/concepts/realisation, which I think add more color to this (and also is largely authored by biggestlou here.)