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"""Logical Clocks and Synchronization."""
from __future__ import absolute_import, unicode_literals
from threading import Lock
from itertools import islice
from operator import itemgetter
from .five import python_2_unicode_compatible, zip
__all__ = ('LamportClock', 'timetuple')
R_CLOCK = '_lamport(clock={0}, timestamp={1}, id={2} {3!r})'
@python_2_unicode_compatible
class timetuple(tuple):
"""Tuple of event clock information.
Can be used as part of a heap to keep events ordered.
Arguments:
clock (int): Event clock value.
timestamp (float): Event UNIX timestamp value.
id (str): Event host id (e.g. ``hostname:pid``).
obj (Any): Optional obj to associate with this event.
"""
__slots__ = ()
def __new__(cls, clock, timestamp, id, obj=None):
return tuple.__new__(cls, (clock, timestamp, id, obj))
def __repr__(self):
return R_CLOCK.format(*self)
def __getnewargs__(self):
return tuple(self)
def __lt__(self, other):
# 0: clock 1: timestamp 3: process id
try:
A, B = self[0], other[0]
# uses logical clock value first
if A and B: # use logical clock if available
if A == B: # equal clocks use lower process id
return self[2] < other[2]
return A < B
return self[1] < other[1] # ... or use timestamp
except IndexError:
return NotImplemented
def __gt__(self, other):
return other < self
def __le__(self, other):
return not other < self
def __ge__(self, other):
return not self < other
clock = property(itemgetter(0))
timestamp = property(itemgetter(1))
id = property(itemgetter(2))
obj = property(itemgetter(3))
@python_2_unicode_compatible
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.
See Also:
* `Lamport timestamps`_
* `Lamports distributed mutex`_
.. _`Lamport Timestamps`: https://en.wikipedia.org/wiki/Lamport_timestamps
.. _`Lamports distributed mutex`: https://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, Lock=Lock):
self.value = initial_value
self.mutex = Lock()
def adjust(self, other):
with self.mutex:
value = self.value = max(self.value, other) + 1
return value
def forward(self):
with self.mutex:
self.value += 1
return self.value
def sort_heap(self, h):
"""Sort heap of events.
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.
Will return the latest event.
"""
if h[0][0] == h[1][0]:
same = []
for PN in zip(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]
# clock values unique, return first item
return h[0]
def __str__(self):
return str(self.value)
def __repr__(self):
return '<LamportClock: {0.value}>'.format(self)
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