Consider this scenario: Your company has recently accepted a contract with another company. During the contract period, hundreds of your company's employees will access a special Web site created by your partner company. The Web site contains a lot of documentation and other information that your employees will use during the contract period. To make the best use of Web site and to speed that Web site's information along to your employees, you make sure your cache policy is working in the most effective way, and as users begin accessing the Web site, your ISA Server arrays begin caching the data from the special Web site in order to serve it to your users more quickly.

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.

Negative caching refers to ISA Server's ability to serve Web pages and Web objects from the ISA Server cache whose Time-to-Live (TTL) value has already expired. If you recall, cached items are held in the cache for a certain period of time before they are updated from the actual Web site where they came from. The TTL period depends on your cache policy and other factors, such as caching load, popularity of the object, etc. Just before the TTL expires, ISA Server can actively attempt to refresh the item from the Web site, or it can passively do so when a user requests the item from the Internet.

Curt Simmons is the Author of 'Microsoft Internet Security and Acceleration Server 2000 Study Guide : Exam 70-227 ( Certification Study Guides)'

Amazon.com (June 2001)

CHAPTER 10 of this book available on ISAserver.org! CLICK HERE!!!!

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.

In the second half of the window, you see the two "If Web site of expired object cannot be reached" radio button option. The first radio button enables you to tell ISA Server not to return expired objects from the cache. In this case, the ISA Server will return an error page to the user. The second option to allow ISA Server to use negative caching so that the expired object is returned to the user while ISA Server is attempting to refresh the object from the Internet.

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.