Mark a connection as dead (failed) in the connection pool. If sniffing
        on failure is enabled this will initiate the sniffing process.

        :arg connection: instance of :class:`~elasticsearch.Connection` that failed

def mark_dead(self, connection):
        """
        Mark a connection as dead (failed) in the connection pool. If sniffing
        on failure is enabled this will initiate the sniffing process.

        :arg connection: instance of :class:`~elasticsearch.Connection` that failed
        """
        # mark as dead even when sniffing to avoid hitting this host during the sniff process
        self.connection_pool.mark_dead(connection)
        if self.sniff_on_connection_fail:
            self.sniff_hosts()