Negative cache

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In computer programming, negative cache is a cache that also stores "negative" responses, i.e. failures. This means that a program remembers the result indicating a failure even after the cause has been corrected. Usually negative cache is a design choice, but it can also be a software bug.

Examples

Consider a

; until the browser is able to successfully load the page, whenever the user tries to load the page, the browser must make a new attempt.

A frustrating aspect of negative caches is that the user may put a great effort into troubleshooting the problem, and then after determining and removing the root cause, the error still does not vanish.

There are cases where failure-like states must be cached. For instance, DNS requires that caching nameservers remember negative responses^[1] as well as positive ones. If an authoritative nameserver returns a negative response, indicating that a name does not exist, this is cached. The negative response may be perceived as a failure at the application level; however, to the nameserver caching it, it is not a failure. The cache times for negative and positive caching may be tuned independently.

Description

A negative cache is normally only desired if failure is very expensive and the error condition raises automatically without user's action. It creates a situation where the user is unable to isolate the cause of the failure: despite fixing everything they can think of, the program still refuses to work. When a failure is cached, the program should provide a clear indication of what must be done to clear the cache, in addition to a description of the cause of the error. In such conditions a negative cache is an example of a design anti-pattern.

Negative cache still may recover if the cached records expires.

See also

• Perl Design Patterns Book

References