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Document the nix-cache-info file format used by binary caches, including the StoreDir, WantMassQuery, and Priority fields, their behavior, and links to related store options.
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Serving a Nix store via HTTP
You can easily share the Nix store of a machine via HTTP. This allows other machines to fetch store paths from that machine to speed up installations. It uses the same binary cache mechanism that Nix usually uses to fetch pre-built binaries from https://cache.nixos.org.
The daemon that handles binary cache requests via HTTP, nix-serve, is
not part of the Nix distribution, but you can install it from Nixpkgs:
$ nix-env --install --attr nixpkgs.nix-serve
You can then start the server, listening for HTTP connections on whatever port you like:
$ nix-serve -p 8080
To check whether it works, try fetching the nix-cache-info file on the client:
$ curl http://avalon:8080/nix-cache-info
StoreDir: /nix/store
WantMassQuery: 1
Priority: 30
When writing to a binary cache (e.g., with nix copy), Nix creates nix-cache-info automatically if it doesn't exist.
On the client side, you can tell Nix to use your binary cache using
--substituters, e.g.:
$ nix-env --install --attr nixpkgs.firefox --substituters http://avalon:8080/
The option substituters tells Nix to use this binary cache in
addition to your default caches, such as https://cache.nixos.org.
Thus, for any path in the closure of Firefox, Nix will first check if
the path is available on the server avalon or another binary caches.
If not, it will fall back to building from source.
You can also tell Nix to always use your binary cache by adding a line
to the nix.conf configuration file like this:
substituters = http://avalon:8080/ https://cache.nixos.org/