{ config, lib, pkgs, ... }:
let
  cfg = config.services.prometheus;

  mkStaticTargets = targets: lib.singleton { inherit targets; };
  mkStaticTarget = target: mkStaticTargets (lib.singleton target);
in
{
  systemd.mounts = lib.singleton {
    after = [ "data.mount" ];
    before = [ "prometheus.service" ];
    wantedBy = [ "local-fs.target" ];
    what = "/data/prometheus";
    where = config.systemd.services.prometheus.serviceConfig.WorkingDirectory;
    type = "none";
    options = "bind";
  };

  services.prometheus = {
    enable = true;
    listenAddress = "127.0.0.1";
    globalConfig = {
      scrape_interval = "15s";
      evaluation_interval = "15s";
    };

    alertmanagers = [
      { static_configs = mkStaticTarget "${cfg.alertmanager.listenAddress}:${toString cfg.alertmanager.port}"; }
    ];
    alertmanager = {
      enable = true;
      listenAddress = "127.0.0.1";
      configuration = {
        global.resolve_timeout = "2m";

        route = {
          receiver = "matrix";
          group_by = [ "alertname" ];
          group_wait = "3m";
        };

        receivers = [
          {
            name = "matrix";
            webhook_configs = [
              # FIXME: Add correct URL
              { url = "http://matrix-alertmanager:3000/alerts"; }
            ];
          }
        ];
      };
    };

    scrapeConfigs = [
      {
        job_name = "prometheus";
        static_configs = mkStaticTarget "localhost:${toString cfg.port}";
      }
      {
        job_name = "node";
        static_configs = mkStaticTargets [
          "fuuko.vpn.sbruder.de:9100"
          "issei.vpn.sbruder.de:9100"
          "nunotaba.vpn.sbruder.de:9100"
          "sayuri.vpn.sbruder.de:9100"
          "vueko.vpn.sbruder.de:9100"
        ];
      }
    ];

    rules =
      let
        mkAlert = { name, expr, for ? "1m", description ? null }: {
          alert = name;
          inherit expr for;
          annotations = lib.optionalAttrs (description != null) { inherit description; };
        };
      in
      [
        (lib.generators.toYAML { } {
          groups = lib.singleton {
            name = "alert.rules";
            rules = map mkAlert [
              {
                name = "InstanceDown";
                expr = ''up{instance!~"(nunotaba|sayuri).vpn.sbruder.de:.*"} == 0'';
                description = "Instance {{ $labels.instance }} of job {{ $labels.job }} has been down for more than 1 minutes.";
              }
              {
                name = "SystemdUnitFailed";
                expr = ''node_systemd_unit_state{state="failed"} == 1'';
                description = "Systemd unit {{ $labels.name }} on {{ $labels.instance }} has state failed.";
              }
              {
                name = "NodeHighLoad";
                expr = ''sum by (instance) (node_load15) / count by (instance) (node_cpu_seconds_total{mode="system"}) > 2'';
                for = "15m";
                description = "This node is having a per-core load ≥ 2 for the last 15 minutes.";
              }
              {
                name = "NodeHighMemory";
                expr = ''(node_memory_MemTotal_bytes - node_memory_MemAvailable_bytes) / node_memory_MemTotal_bytes > 0.9'';
                for = "2m";
                description = "This node is using more than 90 % of available RAM.";
              }
              {
                name = "TP440ACPIBroken";
                expr = ''node_hwmon_temp_celsius{chip="thermal_thermal_zone0",instance="nunotaba.vpn.sbruder.de:9100",job="node",sensor="temp1"} == 48'';
                for = "10m";
                description = "Thinkpad T440’s ACPI temperature is broken. Its reported temperature is 48 °C for the last 10 minutes. That doesn’t seem right. Try suspending";
              }
            ];
          };
        })
      ];
  };

  krops.secrets.prometheus-htpasswd = {
    group = "nginx";
  };

  services.nginx.virtualHosts."prometheus.sbruder.de" = {
    enableACME = true;
    forceSSL = true;

    basicAuthFile = config.krops.secrets.prometheus-htpasswd.path;

    locations = {
      "/".proxyPass = "http://${cfg.listenAddress}:${toString cfg.port}";

      "/alertmanager/".proxyPass = "http://${cfg.alertmanager.listenAddress}:${toString cfg.alertmanager.port}/";
    };
  };
}