These are all the options that are expected to exist for this contract to be respected.

A database that can be backed up will provide a databasebackup option.

What this option defines is, from the user perspective - that is you - an implementation detail but it will at least define how to create a database dump, the user to backup with and how to restore from a database dump.

Here is an example module defining such a databasebackup option, which defines the user to backup with (user) and how to backup (backupCmd) and restore (restoreCmd) the database:

{
  options = {
    myservice.databasebackup = mkOption {
      type = shb.contracts.databasebackup.mkRequester = {
        user = "myservice";
        backupCmd = ''
          ${pkgs.postgresql}/bin/pg_dumpall | ${pkgs.gzip}/bin/gzip --rsyncable
        '';
        restoreCmd = ''
          ${pkgs.gzip}/bin/gunzip | ${pkgs.postgresql}/bin/psql postgres
        '';
      };
    };
  };
};

Now, on the other side we have a service that uses this databasebackup option and actually backs up files. This service is a provider of this contract and will provide a result option.

Let’s assume such a module is available under the databaseBackupService option and that one can create multiple backup instances under databaseBackupService.instances. Then, to actually backup the myservice service, one would write:

databaseBackupService.instances.myservice = {
  request = myservice.databasebackup.request;
  
  settings = {
    enable = true;

    repository = {
      path = "/srv/backup/myservice";
    };

    # ... Other options specific to backupservice like scheduling.
  };
};

It is advised to backup files to different location, to improve redundancy. Thanks to using contracts, this can be made easily either with the same databaseBackupService:

databaseBackupService.instances.myservice_2 = {
  request = myservice.databasebackup.request;
  
  settings = {
    enable = true;
  
    repository = {
      path = "<remote path>";
    };
  };
};

Or with another module databaseBackupService_2!