[OpenLayers-Dev] Caching layerimages by browsers
tschaub at opengeo.org
Thu Mar 4 14:06:11 EST 2010
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
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
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.
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