zaqar/marconi/storage/mongodb/utils.py

# Copyright (c) 2013 Red Hat, Inc.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#    http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
# implied.
# See the License for the specific language governing permissions and
# limitations under the License.

import collections
import random
import re

from bson import errors as berrors
from bson import objectid

from marconi.common import exceptions
from marconi.openstack.common import timeutils
from marconi.storage import exceptions as storage_exceptions


DUP_MARKER_REGEX = re.compile(r"\$queue_marker\s+dup key: { : [^:]+: (\d)+")


def dup_marker_from_error(error_message):
    """Extracts the duplicate marker from a MongoDB error string.

    :param error_message: raw error message string returned
        by mongod on a duplicate key error.

    :raises: marconi.common.exceptions.PatternNotFound
    :returns: extracted marker as an integer
    """
    match = DUP_MARKER_REGEX.search(error_message)
    if match is None:
        description = (_("Error message could not be parsed: %s") %
                       error_message)
        raise exceptions.PatternNotFound(description)

    return int(match.groups()[0])


def cached_gen(iterable):
    """Converts the iterable into a caching generator.

    Returns a proxy that yields each item of iterable, while at
    the same time caching those items in a deque.

    :param iterable: an iterable to wrap in a caching generator

    :returns: (proxy(iterable), cached_items)
    """
    cached_items = collections.deque()

    def generator(iterable):
        for item in iterable:
            cached_items.append(item)
            yield item

    return (generator(iterable), cached_items)


def calculate_backoff(attempt, max_attempts, max_sleep, max_jitter=0):
    """Calculates backoff time, in seconds, when retrying an operation.

    This function calculates a simple linear backoff time with
    optional jitter, useful for retrying a request under high
    concurrency.

    The result may be passed directly into time.sleep() in order to
    mitigate stampeding herd syndrome and introduce backpressure towards
    the clients, slowing them down.

    :param attempt: current value of the attempt counter (zero-based)
    :param max_attempts: maximum number of attempts that will be tried
    :param max_sleep: maximum sleep value to apply before jitter, assumed
        to be seconds. Fractional seconds are supported to 1 ms
        granularity.
    :param max_jitter: maximum jitter value to add to the baseline sleep
        time. Actual value will be chosen randomly.

    :raises: ValueError
    :returns: float representing the number of seconds to sleep, within
        the interval [0, max_sleep), determined linearly according to
        the ratio attempt / max_attempts, with optional jitter.
    """
    if max_attempts < 0:
        raise ValueError("max_attempts must be >= 0")

    if max_sleep < 0:
        raise ValueError("max_sleep must be >= 0")

    if max_jitter < 0:
        raise ValueError("max_jitter must be >= 0")

    if not (0 <= attempt < max_attempts):
        raise ValueError("attempt value is out of range")

    ratio = float(attempt) / float(max_attempts)
    backoff_sec = ratio * max_sleep
    jitter_sec = random.random() * max_jitter

    return backoff_sec + jitter_sec


def to_oid(obj):
    """Creates a new ObjectId based on the input.

    Raises MalformedID when TypeError or berrors.InvalidId
    is raised by the ObjectID class.

    :param obj: Anything that can be passed as an
        input to `objectid.ObjectId`

    :raises: MalformedID
    """
    try:
        return objectid.ObjectId(obj)
    except (TypeError, berrors.InvalidId):
        msg = _("Wrong id %s") % obj
        raise storage_exceptions.MalformedID(msg)


def oid_utc(oid):
    """Converts an ObjectId to a non-tz-aware datetime."""
    try:
        return timeutils.normalize_time(oid.generation_time)
    except AttributeError:
        raise TypeError(_("Expected ObjectId and got %s") % type(oid))


class HookedCursor(object):

    def __init__(self, cursor, denormalizer):
        self.cursor = cursor
        self.denormalizer = denormalizer

    def __getattr__(self, attr):
        return getattr(self.cursor, attr)

    def __iter__(self):
        return self

    def __len__(self):
        return self.cursor.count(True)

    def next(self):
        item = self.cursor.next()
        return self.denormalizer(item)