@classmethod
def create(cls, address):
    """ Create server instance unless already exists.

        Server instance is created for unique host / port pairs. For example,
        calling code below will produce two instances:

            >>> Server.create(('127.0.0.1', 8080))
            >>> Server.create(('127.0.0.1', 8081))

        The same service can be mounted under both servers.
        """
    host, port = address
    if cls._instances is None:
        cls._instances = dict()
    if host not in cls._instances:
        cls._instances[host] = dict()
    if port not in cls._instances[host]:
        cls._instances[host][port] = cls(address)
    return cls._instances[host][port]
