20 lines
1.6 KiB
Markdown
20 lines
1.6 KiB
Markdown
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## Wait what if you're building remotely?
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If you pass `--build-host` to `nixos-rebuild`
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Building a NixOS configuration is secretly two steps: evaluation and realisation.
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Evaluation turns your nix code into a derivation: a file with build instructions
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(environment variables, a build script, and arguments), a list of other derivations its needs before it can build,
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and the outputs it will create when you run it.
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Evaluation is always done on the local machine (where you run `nixos-rebuild`), because the build host might have different channels (for non-flake builds) or no access to the source of some inputs (for flake builds)[^flake-eval].
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Realisation (you can think of it as building) takes a derivation and its tree of dependencies then creates the output paths by running each
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build script in its own sandbox (or downloading the result from a trusted substituter, often [cache.nixos.org](https://cache.nixos.org/)). Most of the hard work happens here.
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In order to get the derivations from the local machine to the build host,
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`nixos-rebuild` uses `nix copy --derivation --to`,
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which works just like `nix-copy-closure` but copies derivations instead of the entire closure.
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Than the hard work can happen on the destination without needing to copy all the nix source code and dependencies.
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[^flake-eval]: It is actually possible to evaluate flakes on the remote machine, but this isn't supported. The `nix flake archive` command, which copies a flake and all of its inputs to the nix store, can copy to another machine with the `--to` argument. Building this way works but I haven't bothered writing a patch.
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