[Twisted-Python] Control the number of parallel system calls

[Matt Goodall]

Daniel de la Cuesta wrote:
> Hi,
> 
> I am developing a video conversion server. To process the video
> conversion I use the method "getProcessOutput" to call "ffmpeg".
> 
> The user upload the video using a HTTP POST and each video must be
> converted to 8 or 10 output formats.
> 
> Currently, all the conversions for each video are processed in parallel,
> what I want to do is to control the number of parallel system calls to
> "ffmpeg". For example I only want 2 processes at the same time and when
> one of them finishes the next process start.
> 
> How can I do that?
> 
> Is there any function in Twisted that implements that?
> 
> Thank you


I wrote something similar to what you want some time ago, although I
don't think I ever actually used it in the end. See attached module.

I included a __main__ section in the module that hopefully demonstrates
its use. Note that it was written as a Service so it needs starting,
hence the strange delayed call to startService before the reactor is run.

Basically, the thing is a service that makes the normal getProcessOutput
and getProcessValue utilities available. However, the execution requests
are queued and the maximum number of processes allowed to run is limited.

Hope it helps.

- Matt
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