[Qgis-developer] [QGIS Server] How to share QgsPostgresSharedData between multiple QGIS instances

[Alessandro Pasotti]

On Thu, Dec 22, 2016 at 4:10 PM, Matthias Kuhn <matthias at opengis.ch> wrote:

> Hi Alessandro
>
> >
> > In Lyon we also talked about how to share caches between different
> > server processes, do you think that the shared memory segment would be
> > appropriate in that case?
>
> What are the caches used for?
>

mainly per-project capabilities.

With QGIS 3 refactoring the QgsProject class will probably hold all the
information we need instead of parsing the project XML like we are doing
now, I think that we'll need to cache the QgsProject instance itself if we
want scalability by spawning multiple server processes without the need to
re-parse the project file, this in case we need to serve multiple projects
of course.

I was more oriented at a memcached/redis backend that would scale well if
the server instances are running on different containers (in that case the
shared memory will no work).



> I wouldn't exclude a shared-memory segment per-se but I also cannot say
> it's THE solution. I'm not sure if there's a general answer to it.
>
> Whatever it is, it needs to be properly protected against concurrent
> access and possible deadlocks.
>
>
I guess that in case of an external caching backend this would not be a
problem.



> > For a cache it does not matter if it's not persistent.
> >
> > The idea would be to have the server honour the 12 factors patterns.
>
> I'm not familiar with it, but as far as I can tell, all requirements to
> a cache are listed on the process page (https://12factor.net/processes)
> which looks quite sensible to me.
>
> Regards
> Matthias
>



-- 
Alessandro Pasotti
w3:   www.itopen.it
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