A critical system is a system which must be highly reliable and retain this

reliability Reliability, reliable, or unreliable may refer to: Science, technology, and mathematics Computing * Data reliability (disambiguation), a property of some disk arrays in computer storage * High availability * Reliability (computer networking), a ...

as it evolves without incurring prohibitive costs. There are four types of critical systems:

safety critical A safety-critical system (SCS) or life-critical system is a system whose failure or malfunction may result in one (or more) of the following outcomes: * death or serious injury to people * loss or severe damage to equipment/property * environme ...

mission critical A mission critical factor of a system is any factor (component, equipment, personnel, process, procedure, software, etc.) that is essential to business operation or to an organization. Failure or disruption of mission critical factors will resu ...

, business critical and security critical. Description For such systems, trusted methods and techniques must be used for development. Consequently, critical systems are usually developed using well-tested techniques rather than newer techniques that have not been subject to extensive practical experience. Developers of critical systems are naturally conservative, preferring to use older techniques whose strengths and weaknesses are understood, rather than new techniques which may appear to be better, but whose long-term problems are unknown. Expensive software engineering techniques that are not cost-effective for non-critical systems may sometimes be used for critical systems development. For example, formal mathematical methods of software development have been successfully used for safety and security critical systems. One reason why these formal methods are used is that it helps reduce the amount of testing required. For critical systems, the costs of

verification Verify or verification may refer to: General * Verification and validation, in engineering or quality management systems, is the act of reviewing, inspecting or testing, in order to establish and document that a product, service or system meets ...

and validation are usually very high—more than 50% of the total system development costs.

Classification

A critical system is distinguished by the consequences associated with system or function failure. Likewise, critical systems are further distinguished between fail-operational and fail safe systems, according to the tolerance they must exhibit to failures: * Fail-operational — typically required to operate not only in nominal conditions (expected), but also in degraded situations when some parts are not working properly. For example, airplanes are fail-operational because they must be able to fly even if some components fail. * Fail-safe — must safely shut down in case of single or multiple failures. Trains are fail-safe systems because stopping a train is typically sufficient to put into safe state.

Safety critical

''Safety critical'' systems deal with scenarios that may lead to loss of life, serious personal injury, or damage to the natural environment. Examples of safety-critical systems are a control system for a chemical manufacturing plant, aircraft, the controller of an unmanned train metro system, a controller of a nuclear plant, etc.

Mission critical

''Mission critical'' systems are made to avoid inability to complete the overall system, project objectives or one of the goals for which the system was designed. Examples of mission-critical systems are a navigational system for a spacecraft, software controlling a

baggage handling system A baggage handling system (BHS) is a type of conveyor system installed in airports that transports checked luggage from ticket counters to areas where the bags can be loaded onto airplanes. A BHS also transports checked baggage coming from airplan ...

of an airport, etc.

Business critical

''Business critical'' systems are programmed to avoid significant tangible or intangible economic costs; e.g., loss of business or damage to reputation. This is often due to the interruption of service caused by the system being unusable. Examples of a business-critical systems are the customer accounting system in a bank, stock-trading system,

ERP system Enterprise resource planning (ERP) is the integrated management of main business processes, often in real time and mediated by software and technology. ERP is usually referred to as a category of Business management tools, business management ...

of a company, Internet search engine, etc.

Security critical

''Security critical'' systems deal with the loss of sensitive data through theft or accidental loss.

Notes