ANN: Axon.STM 1.0.1 (release) Minimalistic Software Transactional Memory (with examples)

Michael Sparks ms at
Mon Dec 24 12:56:10 CET 2007


I've received some great feedback since the initial beta release of
the minimalistic STM code I discussed and released 2 weeks ago. I've
incorporated the feedback, and created a couple of examples based on
the canonical dining philosophers example. (One based on normal python
threads, one based on Kamaelia)

It turns out that there was a potential race hazard during "using"
and "usevar" which I'd missed - many thanks to Richard Taylor for
pointing out this issue.

   * Improved locking. (fixed race hazards during copying for reading - the
     last release was noted as probably OK for CPython, but maybe not for
     Jython or IronPython. This version is more robust)
   * Added Dining Philosophers examples (threading & Axon threading)

Getting it
You can download this release version here:

Installing it
    tar zxf Axon.STM-1.0.1.tar.gz
    cd Axon.STM-1.0.1/
    sudo python install

What IS it?
Software Transactional Memory (STM) is a technique for allowing multiple
threads to share data in such a way that they know when something has gone
wrong. It's been used in databases (just called transactions there really)
for some time and is also very similar to version control. Indeed, you can
think of STM as being like variable level version control.

Note: Because this is NOT intended to be persistent, this is not an ACID
store because it doesn't support the D - durability across a crash. (after
all, we don't save the state to disk) (The other aspects atomicity,
consistency & isolation are supported though)

I've written this to allow a part of Kamaelia to share & manage a dictionary
of atomic values between threads simply, and as a result this code is also
going into mainline Kamaelia. (Specifically into Axon Kamaelia's core)

However STM is something that should hopefully be of use to others doing
concurrent things whether or not they're using kamaelia, hence this stand
alone release.

This stand alone release should not be used alongside mainline Axon yet.
(Well you can, as long as you reinstall your Axon over the top, but that's
icky :-)

Why is it useful?
[ please skip this (or correct me :) if you understand concurrency
  already :-) ]

Why do you need it? Well, in normal code, Global variables are generally
shunned because it can make your code a pain to work with and a pain to be
certain if it works properly. Even with linear code, you can have 2 bits of
code manipulating a structure in surprising ways - but the results are
repeatable. Not-properly-managed-shared-data is to threaded systems as
not-properly-managed-globals are to normal code. (This code is one way of
helping manage shared data)

Well, with code where you have multiple threads active, having shared data
is like an even nastier version of globals. Why? Well, when you have 2 (or
more) running in parallel, the results of breakage can become hard to
repeat as two pieces of code "race" to update values.

With STM you make it explicit what the values are you want to update, and
only once you're happy with the updates do you publish them back to the
shared storage. The neat thing is, if someone else changed things since you
last looked, you get told (your commit fails), and you have to redo the
work. This may sound like extra work (you have to be prepared to redo the
work), but it's nicer than your code breaking :-)

The way you get that message is the .commit raises a ConcurrentUpdate

Also, it's designed to work happily in code that requires non-blocking
usage - which means you may also get a "BusyRetry" exception under load. If
you do, you should as the exception suggests retry the action that you just
tried. (With or without restarting the transaction)

Apologies if that sounds too noddy :)

Docs for it

Using It

# Initialising a Store
from Axon.STM import Store

S = Store()

# Single values
greeting = S.usevar("hello")
print repr(greeting.value)
greeting.set("Hello World")

# Groups of values
D = S.using("account_one", "account_two", "myaccount")

D = S.using("account_one", "account_two", "myaccount")

Dining Philosophers

Pure python version:

Kamaelia version:

import time
import Axon
from Axon.STM import Store
import random

def all(aList, value):
    for i in aList:
        if value != i:
            return False
    return True

class Philosopher(Axon.ThreadedComponent.threadedcomponent):
    forks = ["fork.1", "fork.2"] # default for testing :-)
    def main(self): # start here :-)
        while 1:
            X = self.getforks()

    def getforks(self):
        gotforks = False
        while not gotforks:
                X =*self.forks)
                if all([ X[fork].value for fork in self.forks], None):
                    for fork in self.forks:
                        X[fork].value =
                    gotforks = True
            except Axon.STM.ConcurrentUpdate:
        print "Got forks!",, self.forks
        return X

    def releaseforks(self,X):
        print "releasing forks",
        for fork in self.forks:
            X[fork].value = None

S = Store()
N = 5
for i in range(1,N):
    Philosopher(store=S,forks=["fork.%d" % i ,"fork.%d" % (i+1)]).activate()

Philosopher(store=S,forks=["fork.%d" % N ,"fork.%d" % 1]).run()

Feedback is very welcome, preferably via email to the Kamaelia List
    * kamaelia-list at

Feedback especially regarding bugs and logical errors is particularly
welcome. (hopefully there aren't any - but it's always hard to spot your

Many thanks to Fuzzyman, Duncan Booth, John J Lee & Sylvain Hellegouarch for
feedback whilst I was prototyping this.

Further thanks go to Richard Taylor for detailed feedback and discussion
regarding locking and for pointing me at MASCOT which made me think of
doing the dining philosophers this way :-)

This will be merged onto the mainline of Kamaelia with some auxillary
functions , as another feather aimed at making concurrency easy to
work with :-)

Best Regards,

Michael Sparks, Kamaelia Project

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