peer_strategy.py

# Copyright (c) 2016 Uber Technologies, Inc.
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# furnished to do so, subject to the following conditions:
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# The above copyright notice and this permission notice shall be included in
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from __future__ import absolute_import

import sys


class RankCalculator(object):
    """RankCalculator calculates the rank of a peer."""

    def get_rank(self, peer):
        raise NotImplementedError()


class PreferIncomingCalculator(RankCalculator):

    # TIERS lists three ranges for three different kinds of peers.
    # 0: ephemeral peers or unconnected peers
    # 1: peers with only outgoing connections
    # 2: peers with incoming connections
    TIERS = [sys.maxsize, sys.maxsize / 2, 0]

    def get_rank(self, peer):
        """Calculate the peer rank based on connections.

        If the peer has no incoming connections, it will have largest rank.
        In our peer selection strategy, the largest number has least priority
        in the heap.

        If the peer has incoming connections, we will return number of outbound
        pending requests and responses.

        :param peer: instance of `tchannel.tornado.peer.Peer`
        :return: rank of the peer
        """
        if not peer.connections:
            return self.TIERS[0]

        if not peer.has_incoming_connections:
            return self.TIERS[1] + peer.total_outbound_pendings

        return self.TIERS[2] + peer.total_outbound_pendings