When first introduced to Web content caching, many network administrators assume that the object cache on an proxy server is basically the same as a browser's cache, which all users access when they click the Back button. The logical extension from this assumption is the fear that iChain Proxy Services will serve stale content that doesn't accurately reflect the fresh content on the origin Web server.
Actually, most time-sensitive Web content is flagged by Webmasters in such a way that it cannot become stale unless a caching system ignores the Webmaster's settings. The proxy server honors all flags that affect cache freshness, including Time to Expire, Don't Cache, and Must Revalidate directives.
In addition, the proxy server can be fine-tuned for cache freshness in the following ways:
Accelerated checking of objects that have longer-than-desirable Time to Expire headers.
Delayed checking of objects that have shorter-than-desirable Time to Expire headers.
Checking objects for freshness that do not include Time to Expire headers.
This is administered at Configure > Tuning > Cache Freshness. For more information on configuring the iChain Proxy Services for cache freshness, see Section 4.9.1, Managing Cache Freshness.
Cache freshness is a primary concern of most appliance administrators. The following sections briefly explain how your appliance ensures fresh content for network users and the options you have for adjusting this appliance feature.
Although the following explanation is an over-simplification, it lays the foundation for the specific examples that follow this section.
An iChain Proxy Server has timers that it applies to every cached object.
Each time an object is cached or revalidated, the appliance starts a timer for that object. As long as the timer is running, the appliance vends the object from cache. After the time has expired and when the appliance receives a request for the object, it will issue an IF-MODIFIED-SINCE request to the origin Web server.
If the object has changed, the iChain Proxy Server retrieves the updated object into cache and serves it to the requesting browser before restarting the timer.
If the object has not changed, the iChain Proxy Server vends the object from cache and resets the timer, and the countdown for vending the object from cache begins again.
If a browser forces a refresh of the object, the iChain Proxy Server honors the browser request, retrieves and caches the object regardless of whether it has changed, and restarts the timer.
More than 80% of all Web objects have either no Time to Expire directives or they are set to stay cached for as long as weeks or even months.
Because many of these objects actually change fairly frequently, the appliance has two timers for ensuring their freshness. You can configure these timers in the Cache Freshness dialog box, administered at Configure > Tuning > Cache Freshness.
HTTP Maximum: This timer overrides an object's Time to Expire settings if it is longer than the timer's value.
The default timer value is six hours. This means that iChain Proxy Services does not vend an object that has been in cache longer than six hours without first checking whether it should be refreshed.
HTTP Default: The iChain Proxy Server applies this timer to objects that don't have Time to Expire settings.
The default timer value is two hours. This means that the iChain Proxy Server does not vend an object that has no Time to Expire setting that has been in cache longer than two hours without first checking whether it should be refreshed.
Most Webmasters ensure that their time-sensitive objects have appropriate Time to Expire directives. Late-breaking news stories and photographs, for example, might stay in cache for only a few minutes before expiring.
By default, the proxy server simply honors the Webmasters' instructions and revalidates the objects in cache as directed.
However, some appliance installations, such as those connected through a modem, might need to limit how often these objects are refreshed. The appliance has a third timer for this purpose, also accessible in the Cache Freshness dialog box.
HTTP Minimum: This timer sets the minimum number of hours or minutes the proxy server serves HTTP data from cache before revalidating it against content on the origin Web server. No requested object is revalidated sooner than specified by this value.
The default value for this timer is 0, meaning that the proxy server honors the Time to Expire directive for each object (assuming, of course, it is not longer than the HTTP Maximum timer).
If the timer is set to a value other than 0, it then overrides any object's Time to Expire directive that is shorter than the value set.
The default timer settings explained in the previous sections are tuned for most appliance installations. However, you might have special requirements that need the default settings to be adjusted.
Perhaps you are accelerating content that doesn't contain Time to Expire directives but changes frequently and needs to be refreshed more often than every two hours. You can adjust the HTTP Default timer in the Cache Freshness dialog box so that iChain Proxy Services refreshes the objects more frequently.
Perhaps you have severe Internet bandwidth restrictions and an environment with users who don't require object freshness checks every six hours. You can adjust the HTTP Maximum timer in the Cache Freshness dialog box to a different setting that meets your requirements and conserves bandwidth.
If you choose to adjust the timer values, avoid settings that result in objects refreshing more often than is necessary. Otherwise you could negate the bandwidth and response-time benefits of having the appliance on your network.