I like to refer to an option’s value by name rather than hardcode a value in two places. For example,
services.foo = {
enable = true;
port = 9876;
};
networking.firewall.allowedTCPPorts = [ config.services.foo.port ];
I have a few different nixosConfigurations in a single flake.nix, and a few have services that talk to eachother. I was wondering if I could extend the idea of using config values by name across nixosConfigurations when host_b uses some of the services of host_a. For example, if services.foo is configured as above on host_a, then on host_b I might have something like:
services.bar.remote = "host_a:${nixosConfigurations.host_a.config.services.foo.port}";
I don’t have a really thorough understanding of how Nix actually builds the flake yet, or of things like recursive sets, but I can imagine this might not be feasible, or might not be possible at all. I guess the configs (or at least parts of them) of all the systems on which host_b depends would have to get built at the same time, and somehow this would need to be available to host_b’s configuration.nix.
Alternatively I could do a top-down approach and define foo’s port in a module that gets imported into both host_a and host_b, but I prefer the idea of setting the port number where the service originates and having that propagate elsewhere.