Can you move this now, and select the backend once only in the server and then use that throughout?

Something like this:
this.backend = null;
if (settings.backend.type === 'cassandra') {
   this.backend = new CassandraBackend(name, options, function(err) {
       // error handling
       // call remaining setup
  });
} else if ( settings.backend.type === 'mock' ) {
      this.backend = new MockBackend(name, options, function(err) {
       // error handling
       // call remaining setup
  });
}