Derivations: Difference between revisions

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== Motivation ==

While the need to build software, package libraries and execute build processes is clear to anyone using any operating system, the natural question that may arise is ''Why go out of our way dealing with this complicated process when I can just run a few terminal commands?'' For most distributions, the answer is ''they don't do things this way''. Most Linux distributions, and most operating systems for that matter, are designed to be change over time; the same build process will yield different results each time it's invoked. For example, remember trying to build a package twice; the first time you build it the installation will be successful, but the second time it's built you might get an error about the paths it's trying to write to already exist. Build processes in most Linux distributions are ''stateful'', the context in which they're run might change as you're using that system.

While the need to build software, package libraries and execute build processes is clear to anyone using any operating system, the natural question that may arise is ''Why go out of our way dealing with this complicated process when I can just run a few terminal commands?'' For most distributions, the answer is ''they don't do things this way''. Most Linux distributions, and most operating systems for that matter, are designed to change over time; the same build process will yield different results each time it's invoked. For example, remember trying to build a package twice; the first time you build it the installation will be successful, but the second time it's built you might get an error about the paths it's trying to write to already exist. Build processes in most Linux distributions are ''stateful'', the context in which they're run might change as you're using that system.

However, the Nix ecosystem is ''fundamentally'' different in this regard; when you build a derivation, a unique path in the [[Nix store]] is assigned, and all possible outputs (including filesystem operations) produced by it will be persisted under that path. No other derivation can modify those files; the result of the derivation is '''uniquely determined''' by its input configuration, and subsequent reruns will produce ''exactly'' the same outputs, under different Nix store paths. Any potential issues regarding being able to reproduce a build process are addressed ''by design'', if a derivation was successful once, it will always build successfully as long as its inputs don't change.

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 == Motivation ==
-While the need to build software, package libraries and execute build processes is clear to anyone using any operating system, the natural question that may arise is ''Why go out of our way dealing with this complicated process when I can just run a few terminal commands?'' For most distributions, the answer is ''they don't do things this way''. Most Linux distributions, and most operating systems for that matter, are designed to be change over time; the same build process will yield different results each time it's invoked. For example, remember trying to build a package twice; the first time you build it the installation will be successful, but the second time it's built you might get an error about the paths it's trying to write to already exist. Build processes in most Linux distributions are ''stateful'', the context in which they're run might change as you're using that system.
+While the need to build software, package libraries and execute build processes is clear to anyone using any operating system, the natural question that may arise is ''Why go out of our way dealing with this complicated process when I can just run a few terminal commands?'' For most distributions, the answer is ''they don't do things this way''. Most Linux distributions, and most operating systems for that matter, are designed to change over time; the same build process will yield different results each time it's invoked. For example, remember trying to build a package twice; the first time you build it the installation will be successful, but the second time it's built you might get an error about the paths it's trying to write to already exist. Build processes in most Linux distributions are ''stateful'', the context in which they're run might change as you're using that system.
 However, the Nix ecosystem is ''fundamentally'' different in this regard; when you build a derivation, a unique path in the [[Nix store]] is assigned, and all possible outputs (including filesystem operations) produced by it will be persisted under that path. No other derivation can modify those files; the result of the derivation is '''uniquely determined''' by its input configuration, and subsequent reruns will produce ''exactly'' the same outputs, under different Nix store paths. Any potential issues regarding being able to reproduce a build process are addressed ''by design'', if a derivation was successful once, it will always build successfully as long as its inputs don't change.