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Deadman timer for Google AppEngine

From: andrew cooke <andrew@...>

Date: Tue, 2 Aug 2011 20:49:35 -0400

class DeadmanTimer(Model):
    '''
    Each name is associated with a timestamp and a timeout period.  In normal
    use some process (whose continued existence is being monitored) will
    regularly call the `ping` method.  This will maintain the timestamp to
    within an accuracy of half the timeout period.  The `expired` method
    returns true if the timestamp is older than the timeout period.

    The uncertainty in the updating of the timestamp value allows us to
    provide an efficient (infrequent writing) implementation that accepts
    many pings within the given period.  So this is intended for use when
    the ping frequency is much higher than the checking for expiry.  For
    example, when a task is checked once an hour but will normally be running
    multiple times a second.
    '''

    timestamp = DateTimeProperty(required=True)
    timeout_period = IntegerProperty(required=True)

    def older_than(self, now, factor=1):
        age = now - self.timestamp
        secs = (age.microseconds + (age.seconds + age.days * 24 * 3600.0) *
        10**6) / 10**6
        return secs > factor * self.timeout_period

    @staticmethod
    def create(name, period):
        then = datetime.now() - timedelta(0, period+1)
        return DeadmanTimer.get_or_insert(name,
                        timeout_period=period, timestamp=then)

    @staticmethod
    def ping(name, default_period=60*60):
        logging.debug('ping %s' % name)
        timer = memcache.get('deadman:' + name)
        now = datetime.now()
        # if we may have expired then force a refresh
        if timer and timer.older_than(now, factor=0.5):
            timer = None
        if not timer:
            timer = DeadmanTimer.create(name, default_period)
            memcache.set('deadman:' + name, timer)
        if timer.older_than(now, factor=0.5):
            timer.timestamp = now
            timer.save()

    @staticmethod
    def expired(name, default_period=60*60):
        timer = DeadmanTimer.create(name, default_period)
        return timer.older_than(datetime.now())

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