{
  pkgs,
  lib,
  config,
  mkInstanceServices,
  ...
}:
let
  snapshotType = lib.types.submodule {
    options = {
      paths = lib.mkOption {
        type = lib.types.listOf lib.types.str;
        default = [ ];
        description = "snapshoted paths";
      };
      schedule = lib.mkOption {
        type = lib.types.str;
        default = "*-*-* 4:00:00";
        description = "Snapshot schedule";
      };
    };
  };

  instanceType = lib.types.submodule {
    options = {
      snapshots = lib.mkOption {
        type = snapshotType;
      };
    };
  };
in
{
  options.services.kopia.instances = lib.mkOption {
    type = lib.types.attrsOf instanceType;
  };

  config = lib.mkIf config.services.kopia.enable (
    let
      mkSnapshotService =
        # refactor with mkRepositoryArgs
        name: instance:
        lib.attrsets.nameValuePair "kopia-snapshot-${name}" {
          description = "Kopia S3 snapshot service";
          wants = [
            "kopia-repository-${name}.service"
          ];
          after = [ "kopia-repository-${name}.service" ];
          script = lib.strings.concatLines (
            lib.lists.forEach instance.snapshots.paths (x: ''
              ${pkgs.kopia}/bin/kopia snapshot create ${x} --description "Snapshot for ${name}"
            '')
          );
          serviceConfig = {
            Type = "simple";
            User = "${instance.user}";
            WorkingDirectory = "~";
            SetLoginEnvironment = true;
            # retry on failure
            Restart = "on-failure";
            # wait 30 seconds before restarting
            RestartSec = "30";
            # lower priority
            Nice = "-19";
            IOSchedulingClass = "idle";
          };
          unitConfig = {
            # limit the number of restarts to 5 in 1 day
            StartLimitInterval = "1d";
            StartLimitBurst = "5";
          };
        };
      mkSnapshotTimer =
        name: instance:
        lib.attrsets.nameValuePair "kopia-snapshot-${name}" {
          description = "Kopia S3 snapshot timer";
          wantedBy = [ "timers.target" ];
          timerConfig = {
            OnCalendar = instance.snapshots.schedule;
          };
        };
    in
    {
      # systemd service for repositories open
      systemd.services = mkInstanceServices config.services.kopia.instances mkSnapshotService;
      systemd.timers = mkInstanceServices config.services.kopia.instances mkSnapshotTimer;
    }
  );
}