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diff --git a/doc/builders/fetchers.chapter.md b/doc/builders/fetchers.chapter.md index c99b46097e9..7bd1bbd6de0 100644 --- a/doc/builders/fetchers.chapter.md +++ b/doc/builders/fetchers.chapter.md @@ -1,12 +1,27 @@ # Fetchers {#chap-pkgs-fetchers} Building software with Nix often requires downloading source code and other files from the internet. -`nixpkgs` provides *fetchers* for different protocols and services. Fetchers are functions that simplify downloading files. +To this end, Nixpkgs provides *fetchers*: functions to obtain remote sources via various protocols and services. -## Caveats {#chap-pkgs-fetchers-caveats} +Nixpkgs fetchers differ from built-in fetchers such as [`builtins.fetchTarball`](https://nixos.org/manual/nix/stable/language/builtins.html#builtins-fetchTarball): +- A built-in fetcher will download and cache files at evaluation time and produce a [store path](https://nixos.org/manual/nix/stable/glossary#gloss-store-path). + A Nixpkgs fetcher will create a ([fixed-output](https://nixos.org/manual/nix/stable/glossary#gloss-fixed-output-derivation)) [derivation](https://nixos.org/manual/nix/stable/language/derivations), and files are downloaded at build time. +- Built-in fetchers will invalidate their cache after [`tarball-ttl`](https://nixos.org/manual/nix/stable/command-ref/conf-file#conf-tarball-ttl) expires, and will require network activity to check if the cache entry is up to date. + Nixpkgs fetchers only re-download if the specified hash changes or the store object is not otherwise available. +- Built-in fetchers do not use [substituters](https://nixos.org/manual/nix/stable/command-ref/conf-file#conf-substituters). + Derivations produced by Nixpkgs fetchers will use any configured binary cache transparently. + +This significantly reduces the time needed to evaluate the entirety of Nixpkgs, and allows [Hydra](https://nixos.org/hydra) to retain and re-distribute sources used by Nixpkgs in the [public binary cache](https://cache.nixos.org). +For these reasons, built-in fetchers are not allowed in Nixpkgs source code. + +The following table shows an overview of the differences: -Fetchers create [fixed output derivations](https://nixos.org/manual/nix/stable/#fixed-output-drvs) from downloaded files. -Nix can reuse the downloaded files via the hash of the resulting derivation. +| Fetchers | Download | Output | Cache | Re-download when | +|-|-|-|-|-| +| `builtins.fetch*` | evaluation time | store path | `/nix/store`, `~/.cache/nix` | `tarball-ttl` expires, cache miss in `~/.cache/nix`, output store object not in local store | +| `pkgs.fetch*` | build time | derivation | `/nix/store`, substituters | output store object not available | + +## Caveats {#chap-pkgs-fetchers-caveats} The fact that the hash belongs to the Nix derivation output and not the file itself can lead to confusion. For example, consider the following fetcher: |