Dependability

Question 1

Dependability 4 Means

Answer 1

• Fault Prevention
• Fault Tolerance
• Fault Removal
• Fault Forecasting

Question 2

Dependability Attributes

Answer 2

Availability
Reliability
Safety
Confidentiality
Integrity
Maintainability

Question 3

Dependability Impairments

Answer 3

Faults
Errors
Failures

Question 4

Fault Types

Answer 4

transient fault

Permanent faults : remain int he system until they are repaired. broken wire or a software design error

Intermittent faults: are transient faults that occur from time to time. hardware component that is heat sensitive, it works for a time, stops working, cools down and then starts to work again, corrosion on connectors

Question 5

What are Errors

Answer 5

• wrong state in system+ / subsystem (wrong data element in memory register)
• error activated, when computation accesses the faulty data
• two types of software errors 1. Bohrbugs - can be reproduced by providing a specific input sequence, value domain
• Heisenbugs - can only be observed when input data and timing in relation to all other timing activities can be exactly reproduced

Question 6

Failure Modes

Answer 6

Constaint failure
Value failure
Fail silent
Fail sotp
Fail controlled

Question 7

2 Stages for fault Prevention

Answer 7

Fault avoidance and fault removal

Question 8

Fault tolerance Levels

Answer 8

1. Full Fault Tolerance - the system continues to operate in the presence of faults, albeit for a limited period, with no significant loss of functionality or performance
2. Graceful Degradation - the system continues to operate in the presence of errors, accepting a partial degradation of functionality or performance during recovery or repair
3. Fail safe - the system maintains its integrity while accepting a temporary halt in its operation

Question 9

How to detect fault (fault tolerance)

Answer 9

• requires fault detection and prior specification of intended behavior
• temporal failures should be decected by communication system
• logical failures have to be detected by the receiver of a message

Question 10

Never give up NGU strategy

Answer 10

stategy to deal with rare faults outside the fault hypothesis

Question 11

Differences between Fault Prevention and Fault Tolerance

Question 12

Nessessary conditions for the mutual exclusion

Answer 12

1. Only one process is allowed to enter the critical section
2. No assumption can be made about the speed and the amount of processors
3. No process is allowed to block another process outside the critical section
4. every process has only to wait a finite time to enter the critical section

Question 13

What are the 3 scheduling problems?

Answer 13

Race conditions
Starvation
Priority inversion

Question 14

4 Resource access protocols

Answer 14

• Non-Preemptive Protocol
• Highest Locker Priority
• Priority Inheritance Protocol
• Priority Ceiling Protocol

Question 15

[Priority inheritance protocol] direct blocking

Answer 15

occurs when higher priority processes tries to acquire a resource held by a lower priority process

Question 16

[Priority inheritance protocol] push-through blocking

Answer 16

occurs when medium priority process is blocked by a lower priority process which inherited a higher priority from a process it directly blocks

Question 17

In this context, briefly explain why a fault does not always lead to an error. Give an example.
(ii) Briefly describe the difference between ’fault prevention’ and ’fault tolerance’.

Answer 17

A fault wrong state in system / subsystem (wrong data element in memory, register. Error is activated, when computation accesses the faulty data (this can be quite a bit later). If the faulty memory location is not accessed, this may not lead to an error.
(ii)
Fault prevention attempts to eliminate any possibility of faults creeping into a system before it goes operational.
Fault tolerance enables a system to continue functioning even in the presence of faults