In
computer programming
Computer programming or coding is the composition of sequences of instructions, called computer program, programs, that computers can follow to perform tasks. It involves designing and implementing algorithms, step-by-step specifications of proc ...
, negative cache is a
cache
Cache, caching, or caché may refer to:
Science and technology
* Cache (computing), a technique used in computer storage for easier data access
* Cache (biology) or hoarding, a food storing behavior of animals
* Cache (archaeology), artifacts p ...
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
A software bug is a design defect ( bug) in computer software. A computer program with many or serious bugs may be described as ''buggy''.
The effects of a software bug range from minor (such as a misspelled word in the user interface) to sev ...
.
Examples
Consider a
web browser
A web browser, often shortened to browser, is an application for accessing websites. When a user requests a web page from a particular website, the browser retrieves its files from a web server and then displays the page on the user's scr ...
which attempts to load a page while the network is unavailable. The browser will receive an error code indicating the problem, and may display this error message to the user in place of the requested page. However, it is incorrect for the browser to place the error message in the page cache, as this would lead it to display the error again when the user tries to load the same page - even after the network is back up. The error message must not be cached under the page's
URL
A uniform resource locator (URL), colloquially known as an address on the Web, is a reference to a resource that specifies its location on a computer network and a mechanism for retrieving it. A URL is a specific type of Uniform Resource Identi ...
; 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
Troubleshooting is a form of problem solving, often applied to repair failed products or processes on a machine or a system. It is a logical, systematic search for the source of a problem in order to solve it, and make the product or process ope ...
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
The Domain Name System (DNS) is a hierarchical and distributed name service that provides a naming system for computers, services, and other resources on the Internet or other Internet Protocol (IP) networks. It associates various informatio ...
requires that caching nameservers remember negative responses
[RFC 2308] 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
An anti-pattern in software engineering, project management, and business processes is a common response to a recurring problem that is usually ineffective and risks being highly counterproductive. The term, coined in 1995 by computer programmer An ...
.
Negative cache still may recover if the cached records expires.
See also
*
Perl Design Patterns Book
References
Software bugs
Software anomalies
Cache (computing)
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