[OpenLayers-Dev] Caching layerimages by browsers

Dipl. Inf. Carsten Eider eider at fh-bingen.de
Fri Mar 5 08:14:50 EST 2010

Hi Tim,

unfortunately I have no influence on the servers I get my images from.

THX Carsten

Am 04.03.2010 20:06, schrieb Tim Schaub:
> Hey-
> Dipl. Inf. Carsten Eider wrote:
>> Hi list,
>> while examining my WMS-Servers logs i was surpprised by the amount of
>> WMS-requests I found within.
>> Using Firefox and Firebug i recognized, that the tiles were not cached
>> by the browser, i.e.
>> each time they came into the viewport of the map they were re-requested!
>> Is there a way to use cached images by the browser instead of requesting
>> them time and again?
> Caching is best controlled by using the appropriate headers in your 
> server responses.
> If your server responds with an Expires header, we can't force the 
> browser to use a cached image (without essentially rewriting our own 
> cache) if the Expires date has been reached.
> If your server (or gateway cache) uses Etag or Last-Modified headers, 
> the browser will try to make requests with If-None-Match or 
> If-Modified-Since headers.  If the server responds with 304, Not 
> Modified, the browser will pull from its cache.
> Looking at the http://openlayers.org/dev/examples/example.html example, 
> you can see Etag headers on the static images (west-mini.png etc.).  The 
> GetMap responses from openlayers.org contain Expires headers 
> corresponding to the request time (so they immediately expire).  The 
> GetMap responses from www2.dmsolutions.ca don't contain any cache 
> related headers.
> When you refresh the page, requests for static images return 304, Not 
> Modified, and the browser pulls those from its cache.  Previous 
> responses for GetMap images make the browser think those should not be 
> cached, so they are requested with Cache-Control: max-age=0.  We can't 
> override the headers in this case, so if your server obeys, it will do 
> whatever it does to generate new GetMap responses.
> So, you can improve the caching situation by having your server (or 
> gateway cache) use appropriate headers on responses.  If you configure a 
> gateway cache like Varnish or Squid in front of your WMS, this should be 
> handled for you.  If you use a tile generator to pre-seed a tile cache 
> and serve those statically, you can configure your server to use 
> Last-Modified headers.  If you are confident that you won't be updating 
> static images, use future Expires headers to avoid the intermediate 
> request with If-None-Match or If-Modified-Since headers.
> The short story is that for image requests and effectively using the 
> browser's cache, OpenLayers isn't the place to make changes.
> Oh, and re-reading your question, I see you were talking about 
> panning/zooming instead of reloading the page.  The same advice applies, 
> with the additional note that your browser cache can eventually fill up. 
>   I think the default configuration for Firefox is 50MB.
> Hope that is some help.
> Tim
>> THX
>> Carsten
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Mit freundlichen Grüßen / Yours faithfully
Carsten Eider

Dipl. Inf. (FH)

Kompetenzzentrum für Innovative Informationssysteme

c/o Fachhochschule Bingen / University of applied sciences Bingen

Berlinstraße 109
55411 Bingen

Tel: +49 (0) 6721 / 409-179
Fax: +49 (0) 6721 / 994-251
email: eider at fh-bingen.de
Internet: iis.fh-bingen.de 

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