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authorValentin Gagarin <valentin.gagarin@tweag.io>2022-06-21 14:36:29 +0200
committerValentin Gagarin <valentin.gagarin@tweag.io>2022-08-04 12:37:47 +0200
commit93f721b0d26b6525d8eaa8a1606ebeb4b71a9b6a (patch)
tree3f0df7bbce36ffc998f6d314a0d1728d2fb75e64 /doc/manual/src/architecture/store
parent19d8a5d83942f13c3f532b4ac21cc91c7158c21a (diff)
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-# Derivation
-
-Derivations are recipes to create store objects.
-
-Derivations are the heart of Nix.
-Other system (like Git or IPFS) also store and transfer immutable data, but they don't concern themselves with *how* that data was created.
-This is where Nix comes in.
-
-Derivations produce data by running arbitrary commands, like Make or Ninja rules.
-Unlike those systems, derivations do not produce arbitrary files, but only specific store objects.
-They cannot modify the store in any way, other than creating those store objects.
-This rigid specification of what they do is what allows Nix's caching to be so simple and yet robust.
-
-Based on the above, we can conceptually break derivations down into 3 parts:
-
-1. What command will be run?
-
-2. What existing store objects are needed as inputs?
-
-3. What store objects will be produced as outputs?
-
-## What command will be run?
-
-The original core of Nix was very simple about this, in the mold of traditional Unix.
-Commands consist of 3 parts:
-
-1. Path to executable
-
-2. Arguments (Except for `argv[0]`, which is taken from the path in the usual way)
-
-3. Environment variables.
-
-## What existing store objects are needed as inputs?
-
-The previous sub-section begs the question "how can we be sure the path to the executable points to what we think it does?"
-It's a good questions!
-
-## What store objects will be produced as outputs?
-
-## Extra extensions
-
-### `__structuredAttrs`
-
-Historically speaking, most users of Nix made GNU Bash with a script the command run, regardless of what they were doing.
-Bash variable are automatically created from env vars, but bash also supports array and string-keyed map variables in addition to string variables.
-People also usually create derivations using language which also support these richer data types.
-It was thus desired a way to get this data from the language "planning" the derivation to language to bash, the language evaluated at "run time".
-
-`__structuredAttrs` does this by smuggling inside the core derivation format a map of named richer data.
-At run time, this becomes two things:
-
-1. A JSON file containing that map.
-2. A bash script setting those variables.
-
-The bash command can be passed a script which will "source" that Nix-created bash script, setting those variables with the richer data.
-The outer script can then do whatever it likes with those richer variables as input.
-
-However, since derivations can already contain arbitary input sources, the vast majority of `__structuredAttrs` can be handled by upper layers.
-We might consider implementing `__structuredAttrs` in higher layers in the future, and simplifying the store layer.