What is real time?

Perhaps more difficult to meet the stereotype "about computer" concept than real time. And yet, as practice shows, not only the majority of users, but most IT professionals have a very vague idea of what lies behind these very common term.

Real time - it is actually a sign that the system place a stringent requirement on the reaction rate (response time) to any external events. The term "real time", by the way, almost never used - talk about real-time systems or, more often about real-time. If the system is operating in real time, does not provide the required response time, then talk about her refusal.

Real-time systems tend to interact with processes in the real world (hence the "real" time), which usually do not depend on the actions of the system - on the contrary, by the way, the actions of both time and are dependent on external "stimuli". Events in the outside world can occur spontaneously, and the system operates in real time, should always provide a predictable response time to any of them. A typical task that runs in real time, a registration and processing of signals from external devices.

If we talk about the typical examples of systems that operate in real time, the first thing worth mentioning system controlling various technological processes - from production of any industrial products and finishing chemicals and even nuclear reactions. It is clear that for systems of a similar level of one of the main demands of their work is fault-tolerance - that is, the system's ability to carry out their work at virtually any change in external conditions. Failure to meet the challenge system in the required time could not only lead to financial losses, but even in casualties.

Accepted share real-time systems into two classes: system "hard" real-time systems "soft" real-time. The first class includes systems in which the excess of the maximum response time is tantamount to a denial of the system. The second also includes those systems for which short-term delays in the lead only to an acceptable degradation of the system.

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