1. Critical systems design
Atliko: Nerijus Jusas

2. Three types of critical system
Safety-critical systems.
Mission-critical systems.
Business-critical systems.

3. General guidance for developing critical system
Avoid complexity in the design.
Use deterministic design techniques.
Separate critical from non-critical portions of the system.

4. Dependability

5. Other dimensions of dependability
Repairability: reflects the extent to which the system can be repaired in the event of a failure;
Maintainability: reflects the extent to which the system can be adapted to new : reflects the extent to which the system can be adapted to new requirements;
Error tolerance: reflects the extent to which user input errors can be avoided and tolerated;

6. Cost of dependability

7. Reliability
Failure function, is a probability that the system will survive beyond a specified time t.
Failure rate (λ(t)) expresses the number of failures in a given time period.

8. Failure rate

9. Fault – system failure concept
Design reliability
Fault – system failure concept

10. Design reliability (2)
The first step in a safety analysis process is to determine and identify the hazards of the system and to evaluate their severity and probability/likelihood, which expresses risk:
If the total hazard resulting from system operation consists of n disjunctive hazards then total risk of the system can be calculated:

11. Fault – error – failure chain
Redundancy
Because electronics can fail suddenly and without warning, redundant and fault-tolerant systems are traditionally used for safety-critical functions, such as in aerospace.
Fault – error – failure chain

12. Redundancy in computer system
There are two types of redundancy in critical computer systems:
Active (‘hot’)
Passive (‘cold’)

13. N-modular redundancy

14. Critical systems software
Reliability of software can be hard to determine;
Software does not wear out or break down;
It may have a large number of states that cannot be fully tested;

15. Characteristic of critical software developing
Degree of Control: The degree of control that the software exercises over safety-critical functions in the system.
Complexity: The complexity of the software system. Greater complexity increases the chances of errors.
Timing criticality: The timing criticality of hazardous control actions.

16. Five rules for creating critical software
1. Communicate; 2. Have and follow good software engineering practices and procedures; 3. Perform safety and development analyses; 4. Incorporate appropriate software development methodologies, techniques &design features; 5. Caveat emptor;

17. Communication in software developing

18. V-Model

19. Conclusion
A critical system is a system where failure can lead to high economic loss, physical damage or threats to life;
The dependability in a system reflects the user’s trust in that system;
Reliability and availability are generally seen as necessary but not sufficient conditions for safety;
The more and more software are used in critical system, so it must be designed and created as goo as possible;

20. Questions Types of critical systems ?
What is demonstrated by fault function ?
What type of redundancy are used in computer system ?
What characteristics of critical software developing ?
Five rules for creating critical software ?