Lazy HTTP caching

I have a website which is displayed to visitors via a kiosk. People can interact with it. However, since the website is not locally hosted, and uses an internet connection - the page loads are slow.

I would like to implement some kind of lazy caching mechanism such that as and when people browse the pages - the pages and the resources referenced by the pages get cached, so that subsequent loads of the same page are instant.

I considered using HTML5 offline caching - but it requires me to specify all the res开发者_开发知识库ources in the manifest file, and this is not feasible for me, as the website is pretty large.

Is there any other way to implement this? Perhaps using HTTP caching headers? I would also need some way to invalidate the cache at some point to "push" the new changes to the browser...

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The usual approach to handling problems like this is with HTTP caching headers, combined with smart construction of URLs for resources referenced by your pages.

The general idea is this: every resource loaded by your page (images, scripts, CSS files, etc.) should have a unique, versioned URL. For example, instead of loading /images/button.png, you'd load /images/button_v123.png and when you change that file its URL changes to /images/button_v124.png. Typically this is handled by URL rewriting over static file URLs, so that, for example, the web server knows that /images/button_v124.png should really load the /images/button.png file from the web server's file system. Creating the version numbers can be done by appending a build number, using a CRC of file contents, or many other ways.

Then you need to make sure that, wherever URLs are constructed in the parent page, they refer to the versioned URL. This obviously requires dynamic code used to construct all URLs, which can be accomplished either by adjusting the code used to generate your pages or by server-wide plugins which affect all text/html requests.

Then, you then set the Expires header for all resource requests (images, scripts, CSS files, etc.) to a date far in the future (e.g. 10 years from now). This effectively caches them forever. This means that all requests loaded by each of your pages will be always be fetched from cache; cache invalidation never happens, which is OK because when the underlying resource changes, the parent page will use a new URL to find it.

Finally, you need to figure out how you want to cache your "parent" pages. How you do this is a judgement call. You can use ETag/If-None-Match HTTP headers to check for a new version of the page every time, which will very quickly load the page from cache if the server reports that it hasn't changed. Or you can use Expires (and/or Max-Age) to reload the parent page from cache for a given period of time before checking the server.

If you want to do something even more sophisticated, you can always put a custom proxy server on the kiosk-- in that case you'd have total, centralized control over how caching is done.