Active Caching on Downstream ISA Server Computers May Constantly Refresh Objects

When you are using Internet Security and Acceleration (ISA) Server in a chain, downstream ISA Server computers that have Active Caching enabled may appear to constantly refresh the same objects in (approximately) one minute intervals. In the Web Proxy log on the downstream ISA Server computer, the "Client agent (c-agent)" log field may have a "Mozilla/4.0 (compatible; MSIE 5.5; Windows NT 5.0) Active Cache Request" entry. If it is enabled, the "Object source (s-object-source)" is likely to show "VCache", while the "Result code (sc-status)" log field is "304" (where 304 translates to "Not Modified"). The "Object name (cs-uri)" log field shows that Active Caching keeps refreshing the same object in (approximately) one minute intervals.

This problem is most likely to occur if you have several independent downstream servers chaining to the same upstream standalone proxy server or array. Note that the upstream proxy server does not have to be an ISA Server computer to experience this problem. If the user requests follow a specific pattern, an object may be added to the Active Cache Update list on a downstream ISA Server computer. The downstream ISA Server computer, by using the Active Cache Update process, will then update that object before it expires. Depending on how and when the object was previously requested, when Active Caching tries to update the object, the upstream proxy may return the object from its own cache with an HTTP Age header. The HTTP Age header indicates to the downstream ISA Server computer the total sum (in seconds) of how long the object has been resident in each of the caches along the path from the origin Web server. As the HTTP Age header is used when the downstream ISA Server computer calculates the new Expiry time of the refreshed object, the Expiry time may practically stay the same, and under certain conditions, also cause the object to be eligible for an immediate refresh. Active Caching may then keep refreshing the same object until it expires in the cache on the upstream proxy server.

To work around this problem, disable Active Caching on downstream ISA Server computers.

Microsoft has confirmed that this is a problem in the Microsoft products that are listed at the beginning of this article.

For additional information about Active Caching and how it works, click the article number below to view the article in the Microsoft Knowledge Base: