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Filename : balancing.py
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# MySQL Connector/Python - MySQL driver written in Python.
# Copyright (c) 2013, 2014, Oracle and/or its affiliates. All rights reserved.

# MySQL Connector/Python is licensed under the terms of the GPLv2
# <http://www.gnu.org/licenses/old-licenses/gpl-2.0.html>, like most
# MySQL Connectors. There are special exceptions to the terms and
# conditions of the GPLv2 as it is applied to this software, see the
# FOSS License Exception
# <http://www.mysql.com/about/legal/licensing/foss-exception.html>.
#
# This program is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program; if not, write to the Free Software
# Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA

"""Implementing load balancing"""


import decimal


def _calc_ratio(part, whole):
    """Calculate ratio

    Returns int
    """
    return int((part/whole*100).quantize(
        decimal.Decimal('1'), rounding=decimal.ROUND_HALF_DOWN))


class BaseScheduling(object):

    """Base class for all scheduling classes dealing with load balancing"""

    def __init__(self):
        """Initialize"""
        self._members = []
        self._ratios = []

    def set_members(self, *args):
        """Set members and ratios

        This methods sets the members using the arguments passed. Each
        argument must be a sequence where the second item is the weight.
        The first element is an identifier. For example:

            ('server1', 0.6), ('server2', 0.8)

        Setting members means that the load will be reset. If the members
        are the same as previously set, nothing will be reset or set.

        If no arguments were given the members will be set to an empty
        list.

        Raises ValueError when weight can't be converted to a Decimal.
        """
        raise NotImplementedError

    def get_next(self):
        """Returns the next member"""
        raise NotImplementedError

    @property
    def members(self):
        """Returns the members of this loadbalancer"""
        return self._members

    @property
    def ratios(self):
        """Returns the ratios for all members"""
        return self._ratios


class WeightedRoundRobin(BaseScheduling):

    """Class for doing Weighted Round Robin balancing"""

    def __init__(self, *args):
        """Initializing"""
        super(WeightedRoundRobin, self).__init__()
        self._load = []
        self._next_member = 0
        self._nr_members = 0

        if args:
            self.set_members(*args)

    @property
    def load(self):
        """Returns the current load"""
        return self._load

    def set_members(self, *args):
        if not args:
            # Reset members if nothing was given
            self._members = []
            return
        new_members = []
        for member in args:
            member = list(member)
            try:
                member[1] = decimal.Decimal(str(member[1]))
            except decimal.InvalidOperation:
                raise ValueError("Member '{member}' is invalid".format(
                    member=member))
            new_members.append(tuple(member))

        new_members.sort(key=lambda x: x[1], reverse=True)
        if self._members == new_members:
            return
        self._members = new_members
        self._nr_members = len(new_members)

        min_weight = min(i[1] for i in self._members)
        self._ratios = []
        for _, weight in self._members:
            self._ratios.append(int(weight/min_weight * 100))
        self.reset()

    def reset(self):
        """Reset the load"""
        self._next_member = 0
        self._load = [0] * self._nr_members

    def get_next(self):
        """Returns the next member"""
        if self._ratios == self._load:
            self.reset()

        # Figure out the member to return
        current = self._next_member

        while self._load[current] == self._ratios[current]:
            current = (current + 1) % self._nr_members

        # Update the load and set next member
        self._load[current] += 1
        self._next_member = (current + 1) % self._nr_members

        # Return current
        return self._members[current]

    def __repr__(self):
        return "{class_}(load={load}, ratios={ratios})".format(
            class_=self.__class__,
            load=self.load,
            ratios=self.ratios
        )

    def __eq__(self, other):
        return self._members == other.members