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content/2022-06-09-nix-registry.md

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+Title: Nix registry
+Author: Paul-Nicolas Madelaine
+Date: 2022-06-09
+Category: Blog
+
+*A quick note about a small trick I implemented in my NixOS configurations.*
+
+I started using [flakes](https://nixos.wiki/wiki/Flakes) a few months ago, and
+quickly began using them daily. Notably, I moved the NixOS configurations for
+all my machines in a single flake. But there has been a caveat in my workflow:
+using flakes for almost every project I compile made me end up with lots of
+different versions of Nixpkgs lying around. It became very annoying when
+compiling LaTeX documents: two LaTeX projects created a few hours apart could be
+using different versions of Nixpkgs, and thus different versions of TexLive.
+Downloading a few gigabytes every time I set up a LaTeX project was not very
+optimal. I began copying lock files between projects, but it only made me more
+frustrated.
+
+Nix uses a
+[registry](https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-registry.html)
+to resolve flake URIs. For instance, it resolves the special flake URI "nixpkgs"
+to "github:nixpkgs/nixos-unstable". We can configure the registry system-wide to
+map "nixpkgs" to a specific revision of the repository. Using the "rev"
+attribute of a flake input, we can even point it to the same revision as the one
+used by our flake-defined configuration.
+
+The module to configure the Nix registry looks like this:
+
+```
+{
+
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs";
+  };
+
+  outputs = { self, nixpkgs, }: {
+
+    nixosModule = {
+      nix.registry.nixpkgs = {
+        from = {
+          type = "indirect";
+          id = "nixpkgs";
+        };
+        to = {
+          type = "github";
+          owner = "nixos";
+          repo = "nixpkgs";
+          inherit (nixpkgs) rev;
+        };
+      };
+    };
+
+  };
+
+}
+```
+
+The Nix registry can also be configured at the user level, and this module can
+be used as a home-manager module.