On 04/15/2013 10:14 AM, Charles Hixson wrote:
On 04/14/2013 07:32 PM, Chris Rebert wrote:
On Apr 14, 2013 4:27 PM, "Charles Hixson" <charleshi...@earthlink.net
<mailto:charleshi...@earthlink.net>> wrote:
>
> What is the best approach to implementing actors that accept and
post messages (and have no other external contacts).
You might look at how some of the existing Python actor libraries are
implemented (perhaps one of these might even save you from
reinventing the wheel):
http://www.pykka.org/en/latest/
http://www.kamaelia.org/Docs/Axon/Axon.html
https://pypi.python.org/pypi/pulsar
Kinda old:
http://candygram.sourceforge.net/contents.html
http://osl.cs.uiuc.edu/parley/
Candygram looks interesting. I'd forgotten about it. The others look
either a bit limited (in different ways), or overly general, with the
costs that that brings. I'll need to study Candygram a bit more.
However, even Candygram seems to have a RAM centric model that I'd
need to work around. (Well, the mailbox synchronization must clearly
be RAM centric, but the storage shouldn't be.)
> So far what I've come up with is something like:
> actors = {}
> mailboxs = {}
>
> Stuff actors with actor instances, mailboxes with
multiprocessing.queue instances. (Actors and mailboxes will have
identical keys, which are id#, but it's got to be a dict rather than
a list, because too many are rolled out to disk.) And I'm planning
of having the actors running simultaneously and continually in a
threadpool that just loops through the actors that are assigned to
each thread of the pool.
<snip>
> It would, however, be better if the mailbox could be specific to
the threadpool instance, so less space would be wasted. Or if the
queues could dynamically resize. Or if there was a threadsafe dict.
Or... But I don't know that any of these are feasible. (I mean,
yes, I could write all the mail to a database, but is that a better
answer, or even a good one?)
My recollection is that the built-in collection types are threadsafe
at least to the limited extent that the operations exposed by their
APIs (e.g. dict.setdefault) are atomic.
Perhaps someone will be able to chime in with more details.
If list operations were threadsafe, why would multiprocessing.queue
have been created? I don't recall any claim that they were. Still,
I've found an assertion on StackOverflow that they are...at least for
simple assignment and reading. And the same would appear to be true
of dicts from that post. This *does* require that either the index be
constant, and the stored value be constant, or that the code section
be locked during the access. Fortunately I'm intending to have id#s
be unchangable, and the messages to be tuples, and thus constant.
OTOH, to use this approach I'll need to find some way to guarantee
that removing messages and posting messages don't occur at the same
time. So that still means I'll need to lock each access. The answer
that seems best is for each thread to have a mailbox that cells within
the thread post and read messages from. This will automatically deal
with internal to thread synchronization. Then I'll need a mailman
thread that...
This seems a promising approach, that avoids the problem of fixed
length queues, but I'll still need to do a lot of synchronization.
Still, it's a lot less, and each thread would be locked for shorter
amounts of time.
--
Charles Hixson
Currently it looks as if Pyro is the best option. It appears that
Python threads are very contentious, so I'll need to run in processes
rather than in threads, but I'll still need to transfer messages back
and forth. I'll probably use UnixSockets rather than IP, but this could
change...and using Pyro would make changing it easy. Still, pickle is
used in code transmission, and that makes IP a questionable choice.
--
Charles Hixson
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