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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.
Consider a web browser 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; 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.
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 he/she 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.
The Hypertext Transfer Protocol (HTTP) is an application layer protocol in the Internet protocol suite model for distributed, collaborative, hypermedia information systems. HTTP is the foundation of data communication for the World Wide Web, where hypertext documents include hyperlinks to other resources that the user can easily access, for example by a mouse click or by tapping the screen in a web browser.
Time to live (TTL) or hop limit is a mechanism which limits the lifespan or lifetime of data in a computer or network. TTL may be implemented as a counter or timestamp attached to or embedded in the data. Once the prescribed event count or timespan has elapsed, data is discarded or revalidated. In computer networking, TTL prevents a data packet from circulating indefinitely. In computing applications, TTL is commonly used to improve the performance and manage the caching of data.
A web server is computer software and underlying hardware that accepts requests via HTTP, the network protocol created to distribute web pages, or its secure variant HTTPS. A user agent, commonly a web browser or web crawler, initiates communication by making a request for a specific resource using HTTP, and the server responds with the content of that resource or an error message. The server can also accept and store resources sent from the user agent if configured to do so.
The HTTP 404, 404 Not Found, 404, 404 Error, Page Not Found or File Not Found error message is a Hypertext Transfer Protocol (HTTP) standard response code, in computer network communications, to indicate that the browser was able to communicate with a given server, but the server could not find what was requested. The error may also be used when a server does not wish to disclose whether it has the requested information.
In computing, load balancing refers to the process of distributing a set of tasks over a set of resources, with the aim of making their overall processing more efficient. Load balancing can optimize the response time and avoid unevenly overloading some compute nodes while other compute nodes are left idle.
In computer networking, a proxy server is a server application that acts as an intermediary between a client requesting a resource and the server providing that resource.
An anti-pattern 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 Andrew Koenig, was inspired by the book Design Patterns, which highlights a number of design patterns in software development that its authors considered to be highly reliable and effective.
In computing, a crash, or system crash, occurs when a computer program such as a software application or an operating system stops functioning properly and exits. The program responsible may appear to hang until a crash reporting service reports the crash and any details relating to it. If the program is a critical part of the operating system, the entire system may crash or hang, often resulting in a kernel panic or fatal system error.
Representational state transfer (REST) is a software architectural style that was created to guide the design and development of the architecture for the World Wide Web. REST defines a set of constraints for how the architecture of an Internet-scale distributed hypermedia system, such as the Web, should behave. The REST architectural style emphasises the scalability of interactions between components, uniform interfaces, independent deployment of components, and the creation of a layered architecture to facilitate caching components to reduce user-perceived latency, enforce security, and encapsulate legacy systems.
A page fault is a type of exception raised by computer hardware when a running program accesses a memory page that is not currently mapped by the memory management unit (MMU) into the virtual address space of a process. Logically, the page may be accessible to the process, but requires a mapping to be added to the process page tables, and may additionally require the actual page contents to be loaded from a backing store such as a disk. The processor's MMU detects the page fault, while the exception handling software that handles page faults is generally a part of the operating system kernel. When handling a page fault, the operating system tries to make the required page accessible at the location in physical memory or terminates the program in cases of an illegal memory access.
In software architecture, publish–subscribe is a messaging pattern where senders of messages, called publishers, do not program the messages to be sent directly to specific receivers, called subscribers, but instead categorize published messages into classes without knowledge of which subscribers, if any, there may be. Similarly, subscribers express interest in one or more classes and only receive messages that are of interest, without knowledge of which publishers, if any, there are.
The usage share of web browsers is the proportion, often expressed as a percentage, of visitors to a group of web sites that use a particular web browser.
"Abort, Retry, Fail?" is an error message found in DOS operating systems, which prompts the end-user for a course of action to follow. Although welcomed by some, the message also has been cited as an example of poor usability in computer user interfaces.
An error message is information displayed when an unforeseen problem occurs, usually on a computer or other device. On modern operating systems with graphical, error messages are often displayed using dialog boxes. Error messages are used when user intervention is required, to indicate that a desired operation has failed, or to relay important warnings. Error messages are seen widely throughout computing, and are part of every operating system or computer hardware device. Proper design of error messages is an important topic in usability and other fields of human–computer interaction.
The Xbox 360 video game console is subject to a number of technical problems and failures that can render it unusable. However, many of the issues can be identified by a series of glowing red lights flashing on the face of the console; the three flashing red lights being the most infamous. There are also other issues that arise with the console, such as discs becoming scratched in the drive and "bricking" of consoles due to dashboard updates. Since its release on November 22, 2005, many articles have appeared in the media portraying the Xbox 360's failure rates, with the latest estimate by warranty provider SquareTrade to be 23.7% in 2009, and currently the highest estimate being 54.2% by a Game Informer survey.
DNS hijacking, DNS poisoning, or DNS redirection is the practice of subverting the resolution of Domain Name System (DNS) queries. This can be achieved by malware that overrides a computer's TCP/IP configuration to point at a rogue DNS server under the control of an attacker, or through modifying the behaviour of a trusted DNS server so that it does not comply with internet standards.
A single-page application (SPA) is a web application or website that interacts with the user by dynamically rewriting the current web page with new data from the web server, instead of the default method of a web browser loading entire new pages. The goal is faster transitions that make the website feel more like a native app.
In computer science, robustness is the ability of a computer system to cope with errors during execution and cope with erroneous input. Robustness can encompass many areas of computer science, such as robust programming, robust machine learning, and Robust Security Network. Formal techniques, such as fuzz testing, are essential to showing robustness since this type of testing involves invalid or unexpected inputs. Alternatively, fault injection can be used to test robustness. Various commercial products perform robustness testing of software analysis.
Negative testing is a method of testing an application or system that ensures that the plot of the application is according to the requirements and can handle the unwanted input and user behavior. Invalid data is inserted to compare the output against the given input. Negative testing is also known as failure testing or error path testing. When performing negative testing exceptions are expected. This shows that the application is able to handle improper user behavior. Users input values that do not work in the system to test its ability to handle incorrect values or system failure.
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