def set_core_connections_per_host(self, host_distance, core_connections):
        """
        Sets the minimum number of connections per Session that will be opened
        for each host with :class:`~.HostDistance` equal to `host_distance`.
        The default is 2 for :attr:`~HostDistance.LOCAL` and 1 for
        :attr:`~HostDistance.REMOTE`.

        Protocol version 1 and 2 are limited in the number of concurrent
        requests they can send per connection. The driver implements connection
        pooling to support higher levels of concurrency.

        If :attr:`~.Cluster.protocol_version` is set to 3 or higher, this
        is not supported (there is always one connection per host, unless
        the host is remote and :attr:`connect_to_remote_hosts` is :const:`False`)
        and using this will result in an :exc:`~.UnsupporteOperation`.
        """
        if self.protocol_version >= 3:
            raise UnsupportedOperation(
                "Cluster.set_core_connections_per_host() only has an effect "
                "when using protocol_version 1 or 2.")
        old = self._core_connections_per_host[host_distance]
        self._core_connections_per_host[host_distance] = core_connections
        if core_connections < old:
            self._ensure_core_connections()
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