How often objects are refreshed

About the IMS query interval

NetCache verifies objects for consistency based on the expiration time, or TTL, of the object, as described in How long objects remain in the cache. In this way, if the Web server has not included an explicit expiration time in the object itself, NetCache intelligently calculates how long the object should remain in the cache before sending an If-Modified-Since (IMS) query that verifies with the Web server that the object is still current.

Modifying the interval between IMS queries

You can define IMS rules for NetCache to increase or decrease the IMS query interval for a class of objects in order to improve response time for those objects, because objects are verified with their origin Web servers less frequently. However, some objects delivered to users might be stale. You can monitor the average response times on the HTTP - General page under the Data tab.

Regardless of an object's lifetime setting or a NetCache TTL pattern applied to that object, NetCache verifies the consistency of an object whenever a defined IMS rule instructs NetCache to send an IMS query about that object. When the object is referenced by a user request, NetCache verifies the object if it has not been verified in the number of seconds specified in an IMS rule applied to that object.

The IMS rules table

The Header and Caching Behavior - IMS Rules tab page displays the IMS rules that determine the maximum time (in seconds) between object refreshes in this NetCache appliance.

No object can exceed this time between refreshes. Maximum age limits from the origin server that specify longer times are overridden by this setting.

This section lists the current IMS rules. IMS rules assign one or more forced refresh rates to objects based on time cached, source server, client, URL, or content type.

To view existing IMS rules in the Appliance Manager, select Setup tab > HTTP > Header and Caching Behavior. Click the IMS Rules tab to display the Current If-Modified-Since Rules table.This table lists the current If-Modified-Since rules.