The bforests in the periodic module have simple policies for automatic
tree rotation based on time. Each take one additional argument beyond
the usual bforest initialization: period.  This is effectively the
minimum amount of time a given value will be kept in the bforest.

For example, consider a periodic bforest with three trees and a period of three
hours.

As with normal bforests, periodic bforests come in four flavors:
Integer-Integer (IIBForest), Integer-Object (IOBForest), Object-Integer
(OIBForest), and Object-Object (OOBForest).  The examples here will deal
with them in the abstract: we will create classes from the imaginary and
representative BForest class, and generate keys from KeyGenerator and
values from ValueGenerator.  From the examples you should be able to
extrapolate usage of all four types.

We will also imagine that we control the time with a function called setNow.

    >>> import datetime
    >>> from zope.bforest import utils
    >>> setNow(datetime.datetime(2006, 9, 11, 22, 51, tzinfo=utils.UTC))
    >>> d = BForest(datetime.timedelta(hours=3), count=4)
    >>> d.last_rotation
    datetime.datetime(2006, 9, 11, 22, 51, tzinfo=<UTC>)

Now let's put in some keys and advance time.

    >>> first_hour = {KeyGenerator(): ValueGenerator()}
    >>> d.update(first_hour)
    >>> setNow(datetime.datetime.now(utils.UTC) + datetime.timedelta(minutes=50))
    >>> fifty_minutes_in_to_first_hour = {KeyGenerator(): ValueGenerator()}
    >>> d.update(fifty_minutes_in_to_first_hour)
    >>> setNow(datetime.datetime.now(utils.UTC) + datetime.timedelta(minutes=10))
    >>> second_hour = {KeyGenerator(): ValueGenerator()}
    >>> d.update(second_hour)
    >>> setNow(datetime.datetime.now(utils.UTC) + datetime.timedelta(hours=1))
    >>> third_hour = {KeyGenerator(): ValueGenerator()}
    >>> d.update(third_hour)
    >>> current = BForest._treemodule.BTree(first_hour.copy())
    >>> current.update(fifty_minutes_in_to_first_hour)
    >>> current.update(second_hour)
    >>> current.update(third_hour)
    >>> d == current
    True
    >>> setNow(datetime.datetime.now(utils.UTC) + datetime.timedelta(hours=1))
    >>> fourth_hour = {KeyGenerator(): ValueGenerator()}
    >>> d.update(fourth_hour)
    >>> current.update(fourth_hour)

So, now, three hours have passed since our first key.  Two hours and ten
minutes have passed since our second key.  Each bucket has an effective
period of (period/(bucket count - 1))--one hour, in our example.  We can't
discard the first bucket yet, because the second key hasn't been in for the
minimum of three hours.

    >>> d == current
    True

If we go to the next hour then we'll be able to discard the first
bucket, though.  The first key will have been in for four hours, and the
second key for three hours and 10 minutes.  As we said, the period
provided in the instantiation is intended to be the minimum length the
value is held.  Assuming that the rotation is triggered precisely every
bucket period (which will not be the case in actual use, typically), the
maximum length is ((bucket count/(bucket count -1)) * period).

    >>> setNow(datetime.datetime.now(utils.UTC) + datetime.timedelta(hours=1))
    >>> fifth_hour = {KeyGenerator(): ValueGenerator()}
    >>> current.update(fifth_hour)
    >>> d.update(fifth_hour)
    >>> d == current
    False
    >>> del current[first_hour.keys()[0]]
    >>> del current[fifty_minutes_in_to_first_hour.keys()[0]]
    >>> d == current
    True

Updating isn't the only way to rotate the trees though.  Basically
any non-clear mutating operation will cause a rotation: __setitem__,
update, __delitem__, pop, and popitem.  We've seen update; here are examples
of the rest.

    >>> setNow(datetime.datetime.now(utils.UTC) + datetime.timedelta(hours=1))
    >>> sixth_hour = {KeyGenerator(): ValueGenerator()}
    >>> d[sixth_hour.keys()[0]] = sixth_hour.values()[0] # __setitem__
    >>> current.update(sixth_hour)
    >>> del current[second_hour.keys()[0]]
    >>> d == current
    True
    >>> setNow(datetime.datetime.now(utils.UTC) + datetime.timedelta(hours=1))
    >>> del d[sixth_hour.keys()[0]] # __delitem__
    >>> current = BForest._treemodule.BTree(fourth_hour.copy())
    >>> current.update(fifth_hour)
    >>> d == current
    True
    >>> seventh_hour = {KeyGenerator(): ValueGenerator()}
    >>> d[seventh_hour.keys()[0]] = seventh_hour.values()[0]
    >>> setNow(datetime.datetime.now(utils.UTC) + datetime.timedelta(hours=1))
    >>> d.pop(seventh_hour.keys()[0]) == seventh_hour.values()[0] # pop
    True
    >>> d == BForest._treemodule.BTree(fifth_hour)
    True
    >>> eighth_hour = {KeyGenerator(): ValueGenerator()}
    >>> d[eighth_hour.keys()[0]] = eighth_hour.values()[0]
    >>> setNow(datetime.datetime.now(utils.UTC) + datetime.timedelta(hours=1))
    >>> d.rotateBucket()
    >>> setNow(datetime.datetime.now(utils.UTC) + datetime.timedelta(hours=1))
    >>> tenth_hour = {KeyGenerator(): ValueGenerator()}
    >>> d[tenth_hour.keys()[0]] = tenth_hour.values()[0]
    >>> setNow(datetime.datetime.now(utils.UTC) + datetime.timedelta(hours=1))
    >>> out = dict((d.popitem(),))
    >>> d == BForest._treemodule.BTree(tenth_hour)
    True
    >>> out == eighth_hour
    True

