[QGIS-Developer] QGIS Server plugins and thread safety

[David Marteau]

Hi,

IMHO, The multi process approach has to be preferred over  multi-threading

Some thought about it:

Forking sever instance for each request is imho a bad idea as it is not scalable and defeat resource  control.

I think that it is not a problem  by itself to consider the server engine as single worker process:  there is numerous cases where it is much better handled using appropriates tools:

- Use supervisor with fcgi prefork to handle a pool of server (not sure is is appropriate here)
- Embed the server in a asynchronous framework (like tornado, or python 3 asyncio) and use multiprocessing to handle your server workers.
- Embed as wsgi  application and use gunicorn mumérous options to handle requests queues and the number workers.
…


David. 


IMHO this can be handled in much better way with other tools:

> Le 19 nov. 2017 à 10:34, Alessandro Pasotti <apasotti at gmail.com> a écrit :
> 
> Hi,
> 
> mi recent experiments with multi-threaded python wrappers for QGIS server showed a critical flaw in my original implementation of the server plugins.
> 
> First, I want to stress that this is not a problem in FCGI server implementation but only if the server is used directly from python in a multi threaded server implementation.
> 
> The problem is that the server interface that exposes the request handler is a static property of the interface, that is changed on every request.
> This means that there is a race condition in setting/accessing the handler from a plugin filter, and that a plugin filter might access to the handler for a wrong request.
> 
> The solution is to pass the request handler (or the request and/or response objects depending on the filter) to the plugin filters and leave the interface for static properties only.
> 
> This would be a big API change for the server interface and filters and I'm not sure we have time/resources to do that now.
> 
> As an alternative we could simply document the issue and change the testing code to create a new server instance to server every request (to be verified, and with consitent performance degradation).
> 
> What's your opinion about this issue?
> 
> BTW,  tomorrow I'll file a ticket for this problem.
> 
> -- 
> Alessandro Pasotti
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