code: def backend_to_retrieve(self, namespace, stream):
      
      if namespace not in self.namespaces:
        raise NamespaceMissing('`{}` namespace is not configured'
                               .format(namespace))
      stream_prefix = self.get_matching_prefix(namespace, stream)
      read_backend = self.prefix_read_backends[namespace][stream_prefix]
      return (read_backend,
              self.prefix_confs[namespace][stream_prefix][read_backend])
docs: Return backend enabled for reading for `stream`.