Critical System, phase-||
3.Availability and reliability in SE:
System availability and reliability:
These are closely related features that can be both Expressed as a numeric probability. The reliability of a system is likel As stated, the services of this system will be provided correctly. Availability
It is likely that a system is running up to supply the Services for customers when they request them.
Even though they are closely related, you cannot imagine that there will be reliable systems Always available and vice versa. For example, some systems may have a height
Availability but rarely needed. If consumers expect
The need for continuous service again is high. However, if the consequences of failure are minimal and the system can recover from them quickly
The same system may require less reliability after failures.
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An example of a system where availability is incredibly important is a telephone exchange switch. Users expect dial tones when they pick up the phone so the system needs high availability. However, if the system error causes a connection
To fail this, it is often recovered. Exchange switches usually include repair facilities
Reset the system and retry the connection attempt. This can be done very quickly,
And the phone user will not even know that a failure has occurred. Therefore, the availability of dependency rather than reliability for these systems is key.
The difference and difference in its features is that it is not available
Only the system depends on when we need to improve Abidin
System pricing is not available. So, if this system is a one-time activity every year, and
System B is a once-a-month activity, then obviously more reliable.
Assume that System A will resume after Cakati and will take three days
It takes 10 minutes for B to restart. Year-round system availability (120)
Minute time is better than System A (4303 minutes)
Down time).
System interest and things of interest run more clearly.
Another difficulty with these definitions is that they do not take into account
Severity of failure or consequences of unavailability. Naturally people are
Are more concerned about system failures that have serious consequences, and their impact on their system reliability is influenced by these For example, say
Initial failure in engine management software causes the engine to fail
It expires immediately after startup, but it runs properly after restart, which corrects the initial problem. It does not affect the normal operation of the car,
And many drivers will not think that repair is needed. On the contrary, most drivers
One would think that an engine that once operates at high speed cuts the c
Month (say) is both unpredictable and unsafe and must be repaired.A rigorous definition of credibility combines the implementation of a system with its specification. That is, the system behaves reliably if its behavior is consistent
As described in the specification. However, a common reason for distrust is that the system specification does not match the expectations of the system's users. Unfortunately, many explanations are incomplete or incorrect and this is it
Software engineers are left to explain how the system should behave. As they are
Not domain specialists, therefore, they cannot enforce the behavior that consumers use.
Expect Trust and availability:
It compromises system failure. it can be Failure to provide the Service, failure to provide the Service as described, or the provision of any Service in such a manner as to be unsafe or unsafe. Some of these are failures
The result of specific errors or failures in related systems, Telecommunication system. However, many failures are the result of misguidance System error behavior derived from the system. When discussing reliability, it is helpful to distinguish between error, error and failure of terms.The complementary approaches that are used to improve the reliability of a system are:
Error avoidance techniques are used to reduce the chances of errors and / or errors occurring before.
System errors. Examples of such techniques include the use of error-prone programming language structures such as use of points and static analysis.
Detecting program anomalies.2. Use of error detection and verification techniques
This increases the likelihood that errors will be detected and eliminated beforehand
The system is used. An example is testing and fixing the system
Error detection techniques.
Error tolerance techniques that ensure that no system errors occur System errors or make sure that system errors do not result in system failure.
Adding self-diagnosis facilities to a system and using redundant system modules is an example of error tolerance techniques.
Safety:
Safety - Caltech systems are systems where system processes are always present
Safe. That is, the system should never harm the people or the environment of the system
Even if the system fails. Examples of key security systems include aircraft control and monitoring systems, chemical and pharmaceutical process control systems.
Plants and automobile control systems.Hardware control of key security systems is easy to implement and analyze
More than software control, however, we now develop systems of complexity that they
Cannot be controlled solely by hardware. Because some software control is necessary
The need to manage a large number of sensors and actuators with complex control rules. An example of such complexity is found in modern, aerodynamic
Unstable military aircraft require constant adjustment of their software.
Make sure their flight levels are not accidental.
Safety software falls into two categories:
1. Basic, security-related software This is software that embeds itself as a controller in the system. This type of software malfunction can cause a hardware malfunction, resulting in human injury or environmental damage. I concentrate
This kind of software.
2. Important Security Related Software This is software that can have indirect results
Examples of such systems of injury are computer-assisted engineering designs
Systems whose defects can result in a design error in the entity's existence Designed If this designed system malfunction, this error can hurt people. Medical is another example of a critical security system
There is a database of medicines available to patients. Improper drug intake may be administered as a result of defects in this system.
System reliability and system security are related but separate
Attributes. Indeed. , An important security system should be reliable in that it should be kept to its specifications and operate without fail.Arnot Features: To provide consistent service even when errors occur.
However, fault tolerant systems are not necessarily secure. This software can still be malicious and cause system misconduct, resulting in an accident.
Except for the fact that we can never be 100% sure that software systems
Error-free and error-tolerant, there are many other reasons for software systems
Those who are trusted are not necessarily safe: 1. The specification may be incomplete in which it does not specify the desired behavior
Of the system in some critical situations. A high percentage of system errors (Nakaju & Komi, 1991 L Lutz, 1993) are the result of specification
In a study of errors in embedded systems over design flaws, Lotus concludes:
... Problems with requirements are the main cause of security software errors that persist until integration and system testing.2-General Chat Chat Lounge A hardware error can cause the system to behave unexpectedly
The way and the software can present with unexpected environments. When
Components are near failure They can behave individually and generate signals that are out of range by software.
3-The. System operators can generate inputs that are not individually incorrect
Which, in some cases, can cause a system malfunction. An excellent example of this is when a mechanic instructs on utility management software.
Aircraft to increase aging. The software executed the mechanic's instructions completely. Unfortunately, the plane was on the ground at that time
Unless the aircraft was in the air, the system should have refused the command.