| Andrew Cooke | Contents | Latest | RSS | Twitter | Previous | Next

C[omp]ute

Welcome to my blog, which was once a mailing list of the same name and is still generated by mail. Please reply via the "comment" links.

Always interested in offers/projects/new ideas. Eclectic experience in fields like: numerical computing; Python web; Java enterprise; functional languages; GPGPU; SQL databases; etc. Based in Santiago, Chile; telecommute worldwide. CV; email.

Personal Projects

Lepl parser for Python.

Colorless Green.

Photography around Santiago.

SVG experiment.

Professional Portfolio

Calibration of seismometers.

Data access via web services.

Cache rewrite.

Extending OpenSSH.

Last 100 entries

NSA Interceptng Gmail During Delivery; General IIR Filters; What's happening with Scala?; Interesting (But Largely Illegible) Typeface; Retiring Essentialism; Poorest in UK, Poorest in N Europe; I Want To Be A Redneck!; Reverse Racism; The Lost Art Of Nomography; IBM Data Center (Photo); Interesting Account Of Gamma Hack; The Most Interesting Audiophile In The World; How did the first world war actually end?; Ky - Restaurant Santiago; The Black Dork Lives!; The UN Requires Unaninmous Decisions; LPIR - Steganography in Practice; How I Am 6; Clear Explanation of Verizon / Level 3 / Netflix; Teenage Girls; Formalising NSA Attacks; Switching Brakes (Tektro Hydraulic); Naim NAP 100 (Power Amp); AKG 550 First Impressions; Facebook manipulates emotions (no really); Map Reduce "No Longer Used" At Google; Removing RAID metadata; New Bike (Good Bike Shop, Santiago Chile); Removing APE Tags in Linux; Compiling Python 3.0 With GCC 4.8; Maven is Amazing; Generating Docs from a GitHub Wiki; Modular Shelves; Bash Best Practices; Good Emergency Gasfiter (Santiago, Chile); Readings in Recent Architecture; Roger Casement; Integrated Information Theory (Or Not); Possibly undefined macro AC_ENABLE_SHARED; Update on Charges; Sunburst Visualisation; Spectral Embeddings (Distances -> Coordinates); Introduction to Causality; Filtering To Help Colour-Blindness; ASUS 1015E-DS02 Too; Ready Player One; Writing Clear, Fast Julia Code; List of LatAm Novels; Running (for women); Building a Jenkins Plugin and a Jar (for Command Line use); Headphone Test Recordings; Causal Consistency; The Quest for Randomness; Chat Wars; Real-life Financial Co Without ACID Database...; Flexible Muscle-Based Locomotion for Bipedal Creatures; SQL Performance Explained; The Little Manual of API Design; Multiple Word Sizes; CRC - Next Steps; FizzBuzz; Update on CRCs; Decent Links / Discussion Community; Automated Reasoning About LLVM Optimizations and Undefined Behavior; A Painless Guide To CRC Error Detection Algorithms; Tests in Julia; Dave Eggers: what's so funny about peace, love and Starship?; Cello - High Level C Programming; autoreconf needs tar; Will Self Goes To Heathrow; Top 5 BioInformatics Papers; Vasovagal Response; Good Food in Vina; Chilean Drug Criminals Use Subsitution Cipher; Adrenaline; Stiglitz on the Impact of Technology; Why Not; How I Am 5; Lenovo X240 OpenSuse 13.1; NSA and GCHQ - Psychological Trolls; Finite Fields in Julia (Defining Your Own Number Type); Julian Assange; Starting Qemu on OpenSuse; Noisy GAs/TMs; Venezuela; Reinstalling GRUB with EFI; Instructions For Disabling KDE Indexing; Evolving Speakers; Changing Salt Size in Simple Crypt 3.0.0; Logarithmic Map (Moved); More Info; Words Found in Voynich Manuscript; An Inventory Of 3D Space-Filling Curves; Foxes Using Magnetic Fields To Hunt; 5 Rounds RC5 No Rotation; JP Morgan and Madoff; Ori - Secure, Distributed File System; Physical Unclonable Functions (PUFs); Prejudice on Reddit; Recursion OK; Optimizing Julia Code

© 2006-2013 Andrew Cooke (site) / post authors (content).

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())

Comment on this post