inside Habbie's mind

• jobs are not protected against running concurrently (design issue)
• intervals are fixed (design issue)
• people tend to schedule the same job at the same time on whole clusters of machines (typical usage issue)

The second and third issue are closely related in that both cause undesirable load spikes because many things happen at the same time, either because of intervals phasing up or because of similar jobs running on a bunch of machines at the same time, perhaps hammering the network.

A specific pet peeve is Mailman Reminder Day. Firstly, I just don’t see the point; if my address is on a mailing list and that list is practically dead, I just don’t care. Secondly, it means every first of the month I have tens of reminders that I just delete. Some of these lists are busy — for those, the reminder is a minor nuisance. But many lists are extremely quiet (think software release announcements) and for some of these lists, the reminders are over 50% of the total mail volume. It’s so wasteful. Also, I can’t help but think that all those reminders being sent out at the same time (well, divided over 24 hours because of time zone differences) cannot be good for the mail ecosystem as a whole.

For issues two and three, Colm MacCárthaigh wrote a few great posts detailing why cron is bad, and showcasing one potential solution to the issues at hand. I suggest reading these posts fully, they are very insightful:

• Period Pain
• Period Pain part 2
• Period Pain 3

This one popped up on irc a few weeks ago; a google search suggests it's at least 6 months old. I have been unable to track the original source down.

The question is: if you input echo cat | sed statement into your Unix shell, what comes out? Can you predict it? If not, can you try it and then explain it?

find -L . -type l

Replace . with the directory you want to investigate, of course. Works with both GNU and (Free)BSD find. Rationale: -L makes find act on the (transitive closuse of the) target of a symlink - unless the target doesn’t exist, in which case find acts on the symlink itself. So, if the item to act on turns out to be a symlink, it has to be dangling.

Initially I had a more elaborate version based on the exact connect() code in the higher class, but this simpler version works just fine. Incidentally, what the xen xm commandline tool uses works identically ;)

class UHTTPConnection(httplib.HTTPConnection):
    """Subclass of Python library HTTPConnection that
       uses a unix-domain socket.
    """
 
    def __init__(self, path):
        httplib.HTTPConnection.__init__(self, 'localhost')
        self.path = path
 
    def connect(self):
        sock = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
        sock.connect(self.path)
        self.sock = sock

This small hack plus the fine PHP serialization classes from Scott Hurring are the basis of OpenPanel.coreclient, which allows for very easy provisioning and querying of OpenPanel/opencore data. We use it mostly for unit testing.