backoff.py

# -*- coding: utf-8 -*-

"""
The MIT License (MIT)

Copyright (c) 2015-2020 Rapptz

Permission is hereby granted, free of charge, to any person obtaining a
copy of this software and associated documentation files (the "Software"),
to deal in the Software without restriction, including without limitation
the rights to use, copy, modify, merge, publish, distribute, sublicense,
and/or sell copies of the Software, and to permit persons to whom the
Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
DEALINGS IN THE SOFTWARE.
"""

import time
import random


class ExponentialBackoff:
    """An implementation of the exponential backoff algorithm

    Provides a convenient interface to implement an exponential backoff
    for reconnecting or retrying transmissions in a distributed network.

    Once instantiated, the delay method will return the next interval to
    wait for when retrying a connection or transmission.  The maximum
    delay increases exponentially with each retry up to a maximum of
    2^10 * base, and is reset if no more attempts are needed in a period
    of 2^11 * base seconds.

    Parameters
    ----------
    base: :class:`int`
        The base delay in seconds.  The first retry-delay will be up to
        this many seconds.
    integral: :class:`bool`
        Set to True if whole periods of base is desirable, otherwise any
        number in between may be returned.
    """

    def __init__(self, base=1, *, integral=False):
        self._base = base

        self._exp = 0
        self._max = 10
        self._reset_time = base * 2**11
        self._last_invocation = time.monotonic()

        # Use our own random instance to avoid messing with global one
        rand = random.Random()
        rand.seed()

        self._randfunc = rand.randrange if integral else rand.uniform

    def delay(self):
        """Compute the next delay

        Returns the next delay to wait according to the exponential
        backoff algorithm.  This is a value between 0 and base * 2^exp
        where exponent starts off at 1 and is incremented at every
        invocation of this method up to a maximum of 10.

        If a period of more than base * 2^11 has passed since the last
        retry, the exponent is reset to 1.
        """
        invocation = time.monotonic()
        interval = invocation - self._last_invocation
        self._last_invocation = invocation

        if interval > self._reset_time:
            self._exp = 0

        self._exp = min(self._exp + 1, self._max)
        return self._randfunc(0, self._base * 2**self._exp)