System Template

cache()

Allows the web page to specify whether stations on the network should cache this page.

See Also

Syntax

void = rb.page.cache(cache, private, expires)

Parameters

The 'cache' method takes 3 parameters:

Results

The 'cache' method returns no useful information.

Remarks

Browsers and intermediate devices can create and store local copies of content from web servers and then deliver this version to browsers without having to load the origin server.

Usually his behaviour is beneficial and can reduce load on the origin server and decrease page load times and latency of requests.

Where the content is dynamic though it's important that the browser always load data from the origin server. HTTP defines a set of headers that allow the origin server to instruct intermediate devices and browsers whether they may keep local copies of requests and for how long those copies may be used.

Whitebeam can generate these headers on behalf of applications. The rb:cachecontrol hedader and this method both allow the application to define caching policy for the current request.

Alternatively the application can request that Whitebeam generate no cache control headers and take control of them itself.

Calling rb.page.cache() with no parameters switches off Whitebeams internal cache header logic.

Alternatively parameters this call can tell Whitebeam what values to place in the headers.

There are two main uses of the Presentation Engine in server side applications:

1. to generate dynamic page content from the information available from the templates.
2. to construct client side pages from static server-side content - possibly by reformatting some existing data.

Examples of the latter include the processing of existing XML documents to produce HTML and the use of 'include' files to define a sites look and feel

The second of these uses generates basically static client side content that can take significant resource on the server end to create. and potentially use network bandwidth unnecessarily. It is far better in these cases to allow the intermediate devices and the browsers to cache these pages. By default the Presentation Engine instructs network devices to not cache the output from any Pattern Pages. This is always safe - but potentially inefficient.

This method provides a mechanism whereby a Web Author can enable page caching - on a per page basis. The page will be capable of being cached by calling this method with a parameter of 'true'.

A further refinement of this allows the page to specify that the content - although static - is logically only available to specific users. This means the output contains data targetted at a specific user. It is necessary to prevent such a page being cached and send to other users - while at the same time allowing that users page to cache the datat. You do this by setting the 'max-age' parameter.