code: public SitesConfigurationBuilder addInUseBackupSite(String site) {
        Set<String> sites = attributes.attribute(IN_USE_BACKUP_SITES).get();
        sites.add(site);
        attributes.attribute(IN_USE_BACKUP_SITES).set(sites);
        return this;
     }
docs: Defines the site names, from the list of sites names defined within 'backups' element, to
  which this cache backups its data.