Threaded Design Question

[Mark T]

<half.italian at gmail.com> wrote in message 
news:1186683909.797328.68770 at i13g2000prf.googlegroups.com...
> Hi all!  I'm implementing one of my first multithreaded apps, and have
> gotten to a point where I think I'm going off track from a standard
> idiom.  Wondering if anyone can point me in the right direction.
>
> The script will run as a daemon and watch a given directory for new
> files.  Once it determines that a file has finished moving into the
> watch folder, it will kick off a process on one of the files.  Several
> of these could be running at any given time up to a max number of
> threads.
>
> Here's how I have it designed so far.  The main thread starts a
> Watch(threading.Thread) class that loops and searches a directory for
> files.  It has been passed a Queue.Queue() object (watch_queue), and
> as it finds new files in the watch folder, it adds the file name to
> the queue.
>
> The main thread then grabs an item off the watch_queue, and kicks off
> processing on that file using another class Worker(threading.thread).
>
> My problem is with communicating between the threads as to which files
> are currently processing, or are already present in the watch_queue so
> that the Watch thread does not continuously add unneeded files to the
> watch_queue to be processed.  For example...Watch() finds a file to be
> processed and adds it to the queue.  The main thread sees the file on
> the queue and pops it off and begins processing.  Now the file has
> been removed from the watch_queue, and Watch() thread has no way of
> knowing that the other Worker() thread is processing it, and shouldn't
> pick it up again.  So it will see the file as new and add it to the
> queue again.  PS.. The file is deleted from the watch folder after it
> has finished processing, so that's how i'll know which files to
> process in the long term.
>
> I made definite progress by creating two queues...watch_queue and
> processing_queue, and then used lists within the classes to store the
> state of which files are processing/watched.
>
> I think I could pull it off, but it has got very confusing quickly,
> trying to keep each thread's list and the queue always in sync with
> one another.  The easiset solution I can see is if my threads could
> read an item from the queue without removing it from the queue and
> only remove it when I tell it to.  Then the Watch() thread could then
> just follow what items are on the watch_queue to know what files to
> add, and then the Worker() thread could intentionally remove the item
> from the watch_queue once it has finished processing it.
>
> Now that I'm writing this out, I see a solution by over-riding or
> wrapping Queue.Queue().get() to give me the behavior I mention above.
>
> I've noticed .join() and .task_done(), but I'm not sure of how to use
> them properly.  Any suggestions would be greatly appreciated.
>
> ~Sean
>

Just rename the file.  We've used that technique in a similar application at 
my work for years where a service looks for files of a particular extension 
to appear in a directory.  When the service sees a file, in renames it to a 
different extension and spins off a thread to process the contents.

-Mark T.