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{yen}

Forwards

The keys of a sink, other than reduce, of, at and out, give elements to other sinks. They run at the receiver, with the receiver’s args. You already know one: out gives to the flake (see Flake outputs).

In this example a host has a guest. The kinds are in the example code. These are the sinks:

modules/sinks.nix
# A sink's other keys give elements to other sinks. They run at the receiver, with its args.
{ yen, ... }:
let
  inherit (yen.lib.reduce) merge;
in
{
  yen.kinds.host.sinks = {
    ports.config =
      { ports, ... }:
      {
        firewall.allowedTCPPorts = ports;
      }; # (1) ports feed config
    config = {
      reduce = merge;
      out =
        { host, config, ... }:
        {
          config.${host.name} = config;
        };
    };
  };
  yen.kinds.guest.sinks = {
    ports.config =
      { ports, ... }:
      {
        firewall.allowedTCPPorts = ports;
      }; # (2)
    config = {
      reduce = merge;
      config =
        { guest, config, ... }:
        {
          vms.${guest.name} = config;
        }; # (3) its own name: a forward up, to the host
    };
  };
}
  • At a host, (1) makes ports feed config: aspects give port numbers, and the host turns them into firewall config.
  • At a guest, (2) does the same.
  • At a guest, (3) is a key of config with its own name. A sink cannot feed itself, so this forward goes strictly up: to the nearest config sink above the guest, the host’s.

Aspects give the elements:

modules/aspects.nix
{ yen, ... }:
{
  yen.aspects.ssh = {
    at = yen.kinds.host;
    ports = 22; # (1)
  };
  yen.aspects.runner = {
    at = yen.guest.ci;
    ports = 8080;
    config.runner.enable = true; # (2)
  };
}

(1) gives a port at every host. (2) is a config element at the guest ci. The guest’s config arrives inside the host’s config, under vms.ci:

result
{
  server = {
    firewall = {
      allowedTCPPorts = [
        22
      ];
    };
    vms = {
      ci = {
        firewall = {
          allowedTCPPorts = [
            8080
          ];
        };
        runner = {
          enable = true;
        };
      };
    };
  };
}
flowchart BT
  n6c4c773b6d["fleet main"]
  n04467a35ae["host server"]
  n99b25160dc["guest ci"]
  n99b25160dc -->|"config"| n04467a35ae
  n04467a35ae -->|"out"| n6c4c773b6d
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