Hi Folks:
I don't know what to file this under but here goes:
Santa repeatedly sleeps until wakened by either all of his nine
reindeer, back from their holidays, or by a group of three of his ten
elves. If awakened by the reindeer, he harnesses each of them to his
sleigh, delivers toys with them and finally unharnesses them (allowing
them to go off on holiday). If awakened by a group of elves, he shows
each of the group into his study, consults with them on toy R&D and
finally shows them each out (allowing them to go back to work). Santa
should give priority to the reindeer in the case that there is both a
group of elves and a group of reindeer waiting
I recently modified the stackless.py library to support yet another new
concurrency feature: join patterns. Well I think of my version as a sawed-off
version of join patterns. Join patterns are a synchronization and message
passing construct. The closest thing to a join pattern in Twisted is a deferred
list. Stackless has nothing of the sort.
The Santa Claus Problem is a notorious problem in concurrency
(http://www.youtube.com/watch?v=pqO6tKN2lc4). I have come up with a relatively
short solution. I think one would be hard pressed to come up with a shorter
solution in Python. My desire was to compete with Simon Peyton Jones Haskell
version in "Beautiful Code." The full solution is here:
http://andrewfr.wordpress.com/2011/11/30/the-santa-claus-problem-with-join-patterns.
The nucleus is:
def santa(reindeer, elves):
joinObject = stackless.join().addPattern([ch for _, ch, _ in reindeer]).\
addPattern([ch for _, ch, _ in elves],3)
reindeerPattern, elfPattern = joinObject.patterns
while True:
pattern = joinObject.join()
if reindeerPattern.ready():
print "*** REINDEER GET PRIORITY ***"
reindeerPattern.join()
pattern = reindeerPattern
if pattern is reindeerPattern:
harness(reindeerPattern)
deliveryToys(reindeerPattern)
unharness(reindeerPattern)
elif pattern is elfPattern:
consultWithSanta(elfPattern)
stopTwisted()
In this solution I use Twisted to control the timer (I use the blockOn
technique that Christopher Armstrong first used).
def tick(seconds):
tickCh = stackless.channel()
reactor.callLater(seconds, tickCh.send, None)
tickCh.receive()
However I could just as easily make reindeer and elves web services or RestFull
interfaces. I am still working on the Join Pattern API. Also it has also been
a goal of mine to write a stacklessreactor to better integrate Stackless with
the Twisted Reactor.
Cheers,
Andrew
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