megacli2prom.nix

# MegaCli to Prometheus text format exporter
#
# Scope: Gets data from MegaRAID compatible storage controllers and mogrifies
#        to Prometheus text format, saves to a temp file, to later be scraped
#        by the node exporter.
# 
# Usage: Import this to every server with a MegaRAID card that you want to
#        include in the central monitoring system
#
# See https://nixos.org/manual/nixos/stable/#module-services-prometheus-exporters

{ config, options, lib, ourpkgs, pkgs, ... }:

let
  cfg = config.services.private-storage.monitoring.megacli2prom;

in {
  options.services.private-storage.monitoring.megacli2prom = {
    enable = lib.mkEnableOption "MegaCli2Prom metrics gathering service";
    outFile = lib.mkOption {
      type = lib.types.str;
      description = "Where to store the temporary file for node exporter to scrape?";
      default = "/run/prometheus-node-exporter/megacli.prom";
    };
    interval = lib.mkOption {
      type = lib.types.str;
      description = ''
        How often to do it?
        See https://www.freedesktop.org/software/systemd/man/systemd.time.html#Calendar%20Events
      '';
      # Every five minutes.
      default = "*:0/5";
    };
  };

  config =
    lib.mkIf cfg.enable {
      environment.systemPackages = [ ourpkgs.megacli2prom pkgs.megacli ];
      systemd.services.megacli2prom = {
        enable = true;
        description = "MegaCli2Prom metrics gathering service";
        startAt = cfg.interval;
        path = [ pkgs.megacli ];
        # Save to a temp file and then move atomically so the
        # textfile collector won't read a partial file.
        # See https://github.com/prometheus/node_exporter#textfile-collector
        script = ''
          "${ourpkgs.megacli2prom}/bin/megacli2prom" > "${cfg.outFile}.tmp"
          mv "${cfg.outFile}.tmp" "${cfg.outFile}"
        '';
      };
  };
}