nix-freeze-tree

Posted on January 25, 2020 by Jack Kelly
Tags: nix, haskell, coding

I really like using Nix to build and manage my computers. Declarative machine definitions are incredibly powerful, and NixOS actually achieves what tools like Ansible and Chef have tried to reach for.

This web site is served from a NixOS machine, and is a collection of static files which get copied across by nix copy when it sends the built NixOS expression to the server. Until recently, my built website lived in a single derivation, which meant that any time I changed anything, a full rebuild of the site would get sent up my terrible internet connection.

I have written a tool to split up the big derivation called nix-freeze-tree (Hackage). It lets me split the website derivation down to individual files, and symlink them together back into the tree I want. The individual derivations are fixed-output derivations, which means that Nix knows the hash of the files before it builds the derivation, and can check the hash of the files to copy and skip files that haven’t changed.

If you’re reading this, then I’ve successfully wired nix-freeze-tree into my site build pipeline and deployed it.

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