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Using Custom Cache Control Headers

In addition to fine-tuning cache freshness with the system's global HTTP timers as explained in Fine-Tuning Cache Freshness on Your Appliance, you can configure each proxy service to recognize custom headers in HTTP packets. Your Web server can then use these headers for transmitting caching instructions that only the configured appliance services will recognize and follow.


How Headers Work

Only the accelerator service containing the custom header definition follows the cache policies specified in the custom headers.

All other caches, including the non-configured appliance caches, requesting browsers, and external proxy caches (transparent caches, client accelerators, etc.) do not recognize the custom headers and follow only the cache policies specified by the standard cache control headers.

This means that you have the following options for configuring your Web server:


Implementing Custom Cache Control Headers

To implement custom cache control headers, you must do the following:

Custom Cache Control Headers override the following standard HTTP cache-control headers on the appliance but do not affect how browsers and external caches respond to them:

Cache-Control: no-store
Cache-Control: no-cache
Cache-Control: max-age= number
Cache-Control: private
Cache-Control: public
Pragma: no-cache
Expires: date

For example, you might do the following:

  1. While configuring a Web server accelerator service, you insert a string in the Custom Cache Control Header list with the value of FOOTTL.

    The appliance will now recognize FOOTTL as a custom cache control header on objects requested through the service you are configuring.

  2. You then configure the accelerated Web server to send FOOTTL: 600 in the headers of objects you want to be cached at the appliance.

    The appliance will recognize this header as overriding the standard HTTP cache-control headers listed above when objects are requested through the accelerator service you are configuring.

  3. Finally, you ensure that the Web server continues to send the following standard HTTP cache-control headers:
    • Cache-Control: Max-Age headers that cause browsers to cache objects for no longer than two minutes
    • Cache-Control: Private headers that cause external caches to not cache the objects.

When your Web server sends an object with the FOOTTL header in response to an appliance request made through the accelerator service, your appliance recognizes the custom header and caches the object for 10 minutes. Requesting browsers cache the object for only two minutes. External caches do not cache the object.

Thus, the appliance offloads a processing burden from the Web server by caching the frequently requested objects for 10 minutes (the value you specified in Step 2). Browsers, on the other hand, must always access the appliance to get the objects if their previous requests are older than two minutes. And the objects in the appliance's cache are kept fresh due to their relatively brief time-to-live value.



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