Mapserver caching and map generation

[Ed McNierney]

Emmanuel -

The client ALWAYS requests an image that's "not in the cache".  It's not
a cache.  It's a temporary directory where MapServer writes files.  If
five clients request the same image from MapServer five times in one
second, MapServer will ALWAYS create five different temporary files, and
will return five unique URLs to five different files - even though
they're all the same image.

	- Ed

Ed McNierney
President and Chief Mapmaker
TopoZone.com / Maps a la carte, Inc.
73 Princeton Street, Suite 305
North Chelmsford, MA  01863
ed at topozone.com
(978) 251-4242  

-----Original Message-----
From: Emmanuel BELO [mailto:emmanuel.belo at microgis.ch] 
Sent: Wednesday, March 01, 2006 4:15 PM
To: Ed McNierney
Cc: MAPSERVER-USERS at LISTS.UMN.EDU
Subject: Re: [UMN_MAPSERVER-USERS] Mapserver caching and map generation

To clarify my first question:

If a client requests an image that doesn't yet exists in the cache, the
application will launch the process to create the file (I suppose with a
pseudo-random filename) and give back the link. That's ok.
Now imagine you have 5 more requests for the same image (same query)
while the first one is still being processed.
Will the application detect that and queue these requests for a while to
give them the link created by the first once it has finished, or will it
just launch five other processes, just as if it was other queries,
resulting of the creation of 6 images that will be exactly the same
(result of the same query) ?
I tend to imagine that actually the second case will happen, but maybe
is there some kind of a "query caching mechanism" that may take care of
that and avoid duplicated work...

regards,

eb

Ed McNierney wrote:
> Emmanuel -
> 
> Each map request generates a unique image, and the URL for that image
is
> returned to the client browser requesting that image.  Since your Web
> server can process several HTTP requests at one time, there can be
> several MapServer instances all running at the same time.
> 
> There will be some interaction with the disk subsystem on which the
> temporary images are written, and from which they are read by the
client
> browsers.  Some disk systems can handle multiple simultaneous
read/write
> requests better than others.  Nothing will break - but some disk
> subsystems will take longer than others to respond when multiple reads
> and writes are happening.
> 
> In case you haven't discovered it yet, using MapServer as a WMS server
> or in the CGI "mode=map" mode, the output image is written directly to
> the client and no temporary files are written to disk.
> 
>      - Ed
> 
> Ed McNierney
> President and Chief Mapmaker
> TopoZone.com / Maps a la carte, Inc.
> 73 Princeton Street, Suite 305
> North Chelmsford, MA  01863
> ed at topozone.com
> (978) 251-4242  
> 
> -----Original Message-----
> From: UMN MapServer Users List [mailto:MAPSERVER-USERS at LISTS.UMN.EDU]
On
> Behalf Of Emmanuel BELO
> Sent: Wednesday, March 01, 2006 12:26 PM
> To: MAPSERVER-USERS at LISTS.UMN.EDU
> Subject: [UMN_MAPSERVER-USERS] Mapserver caching and map generation
> 
> Hello,
> 
> after cleaning the cache, mapserver generates the new images on
request.
> 
> What happens if 10 users ask for the same map during the generation of
> the first map? Are their request delayed or will 10 processes in
> parallel be initiated?
> 
> Best regards,
> 
> Emmanuel BELO
> emmanuel.belo at microgis.ch