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"""
kombu.clocks
============
Logical Clocks and Synchronization.
"""
from __future__ import absolute_import
from __future__ import with_statement
import threading
from itertools import islice, izip
__all__ = ['LamportClock']
class LamportClock(object):
"""Lamport's logical clock.
From Wikipedia:
A Lamport logical clock is a monotonically incrementing software counter
maintained in each process. It follows some simple rules:
* A process increments its counter before each event in that process;
* When a process sends a message, it includes its counter value with
the message;
* On receiving a message, the receiver process sets its counter to be
greater than the maximum of its own value and the received value
before it considers the message received.
Conceptually, this logical clock can be thought of as a clock that only
has meaning in relation to messages moving between processes. When a
process receives a message, it resynchronizes its logical clock with
the sender.
.. seealso::
* `Lamport timestamps`_
* `Lamports distributed mutex`_
.. _`Lamport Timestamps`: http://en.wikipedia.org/wiki/Lamport_timestamps
.. _`Lamports distributed mutex`: http://bit.ly/p99ybE
*Usage*
When sending a message use :meth:`forward` to increment the clock,
when receiving a message use :meth:`adjust` to sync with
the time stamp of the incoming message.
"""
#: The clocks current value.
value = 0
def __init__(self, initial_value=0):
self.value = initial_value
self.mutex = threading.Lock()
def adjust(self, other):
with self.mutex:
self.value = max(self.value, other) + 1
return self.value
def forward(self):
with self.mutex:
self.value += 1
return self.value
def sort(self, d):
return d[sorted(d)[0]]
def sort_heap(self, h):
"""List of tuples containing at least two elements, representing
an event, where the first element is the event's scalar clock value,
and the second element is the id of the process (usually
``"hostname:pid"``): ``sh([(clock, processid, ...?), (...)])``
The list must already be sorted, which is why we refer to it as a
heap.
The tuple will not be unpacked, so more than two elements can be
present. Returns the latest event.
"""
if h[0][0] == h[1][0]:
same = []
for i, PN in izip(h, islice(h, 1, None)):
if PN[0][0] != PN[1][0]:
break # Prev and Next's clocks differ
same.append(PN[0])
# return first item sorted by process id
return sorted(same, key=lambda event: event[1])[0]
# all clock values unique, return first item
return h[0]
def __str__(self):
return str(self.value)
def __repr__(self):
return '<LamportClock: %r>' % (self.value, )
|
