The primary function of computer systems is to ensure high-availability applications, whereby availability is defined as the ratio of the period of service availability and the total duration of the monitoring. In order to increase the overall availability of the system or the application, redundancy of the computer equipment is necessary, i.e. the distribution of the system functionality to multiple components. This way, the system remains functional even in case of failure of any individual component.
Depending on the importance of individual applications and tolerable period of their unavailability, there are several levels of availability:
The described ensuring of availability represents only a computer level and, in general terms, ensuring the availability of applications in the narrow sense of the word. You can read more about this topic in the description of the data center activities.