simple pub/sub

bobicanprogram icanbob at gmail.com
Fri Jan 22 22:30:54 CET 2010


On Jan 21, 11:54 am, Steve Howell <showel... at yahoo.com> wrote:
> Hi, I'm looking for ideas on building a simple architecture that
> allows a bunch of independent Python processes to exchange data using
> files and perform calculations.
>
> One Python program would be collecting data from boat instruments on a
> serial port, then writing that info out to a file, and also
> transmitting data to instruments as needed (again through the serial
> port).  It would be the most complex program in terms of having to
> poll file descriptors, or something similar, but I want to limit its
> job to pure communication handling.
>
> Then, I would have a bunch of programs doing various calculations on
> the input stream.  It's totally acceptable if they just sleep a bit,
> wake up, see if there is any new data, and if so, do some
> calculations, so I will probably just use this:
>
> http://www.dabeaz.com/generators/follow.py
>
> Some of the "calculator" programs might be looking for trends in the
> data, and when a certain threshold is reached, they will need to
> notify the comm program of the new message.  To avoid collisions
> between all the different calculator programs, I am thinking the
> simplest thing is just have them each write to their own output file.
>
> So then the comm program has multiple data sources.  It wants to track
> to the serial port, but it also wants to monitor the output files from
> the various calculator processes.  I want to do this in a mostly
> nonblocking way, so I would probably poll on the file descriptors.
>
> The only problems are that select "...cannot be used on regular files
> to determine whether a file has grown since it was last read," and it
> only works on Unix for files.  (I'm using Unix myself, but one of the
> other programmers insists on Windows, and is resisting Python as
> well.)
>
> So a variation would be that the calculator programs talk to the comm
> program via sockets, which seems slightly on the heavy side, but it is
> certainly feasible.
>
> I know that something like twisted could probably solve my problem,
> but I'm also wondering if there is some solution that is a little more
> lightweight and batteries-included.
>
> At the heart of the matter, I just want the programs to be able to do
> the following tasks.
>
>   1) All programs will occasionally need to write a line of text to
> some data stream without having to lock it.  (I don't care if the
> write blocks.)
>   2) All programs will need to be able to do a non-blocking read of a
> line of text from a data stream.  (And there may be multiple
> consumers.)
>   3) The comm program will need to be able to poll the serial port and
> input data streams to see which ones are ready.
>
> Any thoughts?


You should definitely check out the SIMPL toolkit (http://
www.icanprogram.com/simpl).  It has some pretty mature Python hooks.
SIMPL has been used to construct data acquisition applications not
unlike the one you describe.

bob



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