However, during a particularly busy time of work, the partner Web site suddenly goes down. Typically, downed Web servers would leave your users twiddling their thumbs because once the servers are down, the information users need is no longer available via the Web server. However, with ISA Server, your users do not have to find themselves in a dead zone of work because ISA Server can still provide the Web site information through negative caching.
Curt Simmons is the Author of 'Microsoft Internet Security and Acceleration Server 2000 Study Guide : Exam 70-227 ( Certification Study Guides)'
Curt Simmons is also the Author of 'Microsoft ISA Configuration and Administration '
Under a typical configuration, the cached object is no longer available on the TTL expires until it is refreshed from the Internet. However, you can change this behavior by using negative caching, which allows ISA Server to continue serving the expired object for a period of time. Obviously, this feature can be a great productivity tool since ISA Server can continue serving expired objects until it can refresh the objects from the Web, such as in the case of a down Web server, network communication problems, and other issues that might prevent refreshing. The down side, of course, is that the longer items remain the cache, the more stale they become. Depending on your business and the need fresh information, negative caching may not be a wise choice, but in many circumstances, it can serve a useful purpose.
You can easily enable and configure negative caching for both forward and reverse caching scenarios by accessing the Cache Configuration Properties window, Advanced tab, shown in the following figure.
By default, ISA Server returns the expired object to the client if less than 50 percent of the original TTL has passed since it expired, but no more than 60 minutes. So by default, you have a TTL percentage and a time in minutes that an expired object can be served. These values, of course, provide a default benchmark, and depending on your needs, you may wish to raise or lower them.
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Curt Simmons, MCSE, MCT, CTT, is a technology author and trainer from Dallas. Visit Curt on the Internet at http://curtsimmons.hypermart.net.