{
  config,
  inputs,
  lib,
  pkgs,
  this,
  ...
}:
let
  cfg = config.nixfiles.modules.postgresql;
in
{
  options.nixfiles.modules.postgresql = {
    enable = lib.mkEnableOption "PostgreSQL";

    package = lib.mkOption {
      type = lib.types.package;
      default = pkgs.postgresql_15;
      description = "PostgreSQL package to use.";
    };

    extraPostStart = lib.mkOption {
      type = with lib.types; listOf str;
      default = [ ];
      description = ''
        Additional post-startup commands.

        This could be used to provide a crude interface to grant permissions and
        such.
      '';
    };
  };

  config = lib.mkIf cfg.enable {
    assertions = [
      {
        assertion = lib.any (x: x == "en_GB.UTF-8/UTF-8") config.i18n.supportedLocales;
        message = "The locale must be available";
      }
    ];

    ark.directories = [ config.services.postgresql.dataDir ];

    services = {
      postgresql = {
        enable = true;

        inherit (cfg) package;

        # In hindsight, it was a poor choice to use ICU as a locale provider.
        # Now each time ICU version is bumped, I need to carefully upgrade each
        # database to match the version.
        initdbArgs = [
          "--encoding=UTF8"
          "--locale-provider=icu"
          "--icu-locale=en_GB@collation=posix"
          "--locale=en_GB.UTF-8"
          "--lc-collate=C"
          "--lc-ctype=C"
        ];

        # This crutch is here because some services cannot work via a UNIX
        # socket connection and I can't be bothered to configure proper
        # authentication.
        authentication = ''
          local all all trust
        '';
      };

      prometheus.exporters.postgres = {
        enable = true;
        listenAddress = "127.0.0.1";
        port = 9187;
      };
    };

    systemd.services = {
      postgresql.postStart =
        lib.optionalString (cfg.extraPostStart != [ ]) cfg.extraPostStart |> lib.concatLines;

      alloy.reloadTriggers = [ config.environment.etc."alloy/postgres.alloy".source ];
    };

    environment = {
      etc."alloy/postgres.alloy".text = with config.services.prometheus.exporters.postgres; ''
        prometheus.scrape "postgres" {
          targets = [
            {
              __address__ = "${listenAddress}:${toString port}",
              instance    = "${config.networking.hostName}",
            },
          ]
          forward_to = [prometheus.relabel.default.receiver]
        }
      '';

      variables.PSQLRC = toString (
        pkgs.writeText "psqlrc" ''
          \set QUIET 1

          \timing
          \x auto
          \pset null '[NULL]'
          \set PROMPT1 '%[%033[1m%]%M %n@%/%R%[%033[0m%]% λ '
          \set PROMPT2 '    … > '
          \set VERBOSITY verbose
          \set HISTCONTROL ignoredups
          \set HISTFILE /dev/null

          \unset QUIET
        ''
      );
    };

    topology = {
      nodes.${this.hostname}.services.postgresql = {
        name = "PostgreSQL";
        icon = "${inputs.homelab-svg-assets}/assets/postgresql.svg";
        details.listen.text = "127.0.0.1:5432";
      };
    };
  };
}