GAE loses shared resources

[Jacob Brookover <jb...@cast.org>]

Hello all,

I¹m running Wicket on Google App Engine.  Things have been going fairly
well, but I¹ve encountered a new issue.  GAE, at any moment, can wipe and
restart your application.  In doing so, you lose everything including
anything you¹ve added to SharedResources.  Unfortunately, this can happen in
the middle of a page load as illustrated here:

-Page request comes in
-Nifty MarkupFilter reads initial markup and creates some SharedResources
(for a variety of image states based on the filename in the markup)
-MarkupFilter sets the URL in the markup based on the URL provided by the
mounted SharedResource (e.g. ³img/logo.png² becomes
³/resources/INDIRA/img/logo_d.png² because I wanted the disabled version).
-Markup is returned to the browser
-GAE restarts the application, losing the added SharedResource mapping
-Browser requests the resource according to the mounted URL
-Wicket cannot find resource, missing image

My current hack idea is to add any shared resource to the GAE MemCache (URL
maps to File).  That works, but now, I just need to know where Wicket
actually looks up the file so I can add a fallback to check the GAE MemCache
if the file is not found.  My understanding is that the mounted
SharedResource path resolves to an actual path in the ServletContext, but I
can¹t figure out where that is happening.

I initially thought that I would just override SharedResources.get(), but I
don¹t think that is called when simply handling requests.  I also found
references to WebRequestCodingStrategy.RESOURCES_PATH_PREFIX, but none of
that seemed to help.  Finally, I dove deep into WicketFilter,
WebRequest/Response, SharedResourceRequestTarget etc but got completely
lost.  I assume it¹s in there someplace, though.

Any help would be appreciated.

Thanks!

Jake

Re: GAE loses shared resources

[jbrookover <jb...@cast.org>]

I'm going to bump this with some more information...

I see that Application's getSharedResources() is final, as is
SharedResources's get() method.  So, that doesn't seem like it would work. 
SharedResourceRequestTarget is referenced in several places directly (not
using a factory), so that's probably not ideal.  Also, there would be a lot
of code duplication in it's get() method.  So, still looking for a solution
to this issue.  Seems pretty critical for extending an application in the
cloud.

Thanks!

Jake


jbrookover wrote:
> 
> Hello all,
> 
> I¹m running Wicket on Google App Engine.  Things have been going fairly
> well, but I¹ve encountered a new issue.  GAE, at any moment, can wipe and
> restart your application.  In doing so, you lose everything including
> anything you¹ve added to SharedResources.  Unfortunately, this can happen
> in
> the middle of a page load as illustrated here:
> 
> -Page request comes in
> -Nifty MarkupFilter reads initial markup and creates some SharedResources
> (for a variety of image states based on the filename in the markup)
> -MarkupFilter sets the URL in the markup based on the URL provided by the
> mounted SharedResource (e.g. ³img/logo.png² becomes
> ³/resources/INDIRA/img/logo_d.png² because I wanted the disabled version).
> -Markup is returned to the browser
> -GAE restarts the application, losing the added SharedResource mapping
> -Browser requests the resource according to the mounted URL
> -Wicket cannot find resource, missing image
> 
> My current hack idea is to add any shared resource to the GAE MemCache
> (URL
> maps to File).  That works, but now, I just need to know where Wicket
> actually looks up the file so I can add a fallback to check the GAE
> MemCache
> if the file is not found.  My understanding is that the mounted
> SharedResource path resolves to an actual path in the ServletContext, but
> I
> can¹t figure out where that is happening.
> 
> I initially thought that I would just override SharedResources.get(), but
> I
> don¹t think that is called when simply handling requests.  I also found
> references to WebRequestCodingStrategy.RESOURCES_PATH_PREFIX, but none of
> that seemed to help.  Finally, I dove deep into WicketFilter,
> WebRequest/Response, SharedResourceRequestTarget etc but got completely
> lost.  I assume it¹s in there someplace, though.
> 
> Any help would be appreciated.
> 
> Thanks!
> 
> Jake
> 
> 

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