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High availability (HA) is a characteristic of a system that aims to ensure an agreed level of operational performance, usually uptime, for a higher than normal period. [ 1 ] There is now more dependence on these systems as a result of modernization.
Five nines, commonly taken to mean "99.999%", may refer to: High availability of services, when a service is available for 99.999% of the time, or around 5 minutes of downtime per year; Nine (purity), a 99.999% pure substance; German 15 cm (5.9 in) artillery shells used in World War I
Availability is the probability that an item will be in an operable and committable state at the start of a mission when the mission is called for at a random time, and is generally defined as uptime divided by total time (uptime plus downtime).
Uptime is a measure of system reliability, expressed as the period of time a machine, typically a computer, has been continuously working and available. Uptime is the opposite of downtime . Htop adds an exclamation mark when uptime is longer than 100 days.
That's four nines or five nines of availability and uptime for their mission-critical line-of-business applications. And 9% of the respondents, so that's almost one out of 10 companies, say that they need greater than five nines of uptime. So what that means is, no downtime.
Continuous availability is an approach to computer system and application design that protects users against downtime, whatever the cause and ensures that users remain connected to their documents, data files and business applications.
Website monitoring can be done from both inside and outside of a corporate firewall.Traditional network management solutions focus on inside the firewall monitoring, whereas external performance monitoring will test and monitor performance issues across the Internet backbone and in some cases all the way to the end-user.
There is a difference between fault tolerance and systems that rarely have problems. For instance, the Western Electric crossbar systems had failure rates of two hours per forty years, and therefore were highly fault resistant. But when a fault did occur they still stopped operating completely, and therefore were not fault tolerant.