HW4: Chapters 11 & 12

(11.4). What is the common characteristic of  all architectural styles that are geared to supporting software fault tolerance?: 

The common characteristic of architectural styles that are geared to support software fault tolerance is redundancy coupled with diversity. These characteristics combined give the system a high level of dependability and availability.

(11.7). It has been suggested that the control software for a radiation therapy machine, used to treat patients with cancer, should be implemented using N-version programming. Comment on whether or not you think this is a good suggestion.:

Any system that has the potential to harm a life-form should take all precautions to ensure that the system can consistently obtain an acceptable failure rate. N-Version programming accomplishes this by implementing a multi-layered system, where the layers can be designed independently, and rigorously tested to ensure all specifications are meet. This type of design entails reasonable and reliable fault tolerance mechanisms that can produce acceptable failure rates in a consistent manner. Because of this, I think it N-Versin programing is reasonable/good suggestion.

(11.9).  Explain why you should explicitly handle all exceptions in a system that is indeed to have a high level of availability. 

Needing a high level of availability corresponds to the consistency of which an entity is needed. This need may serve different purposes, but if an exception is not handled in manner that allows the system to run, its failure can have adverse affects. These effects can include danger to health, a large sum loss in revenue,  a detrimental breach in security, etc. Due to the irrevocable affects that a non-handled exception can have in a high level of availability type system, it is essential to address all exceptions.

(12.5). A train protection system automatically apples the brakes... There are two critical-saftey requirements for this train protection system:

• The train shall not enter a segment of track that is signaled with a red light.
• The train shall not exceed the specified speed limit for  a section of track.

..., propose five possible functional system requirements for the onboard software that may be generated from the system safety requirements. 

a). Get specified speed limit, if within 'X' percentage of limit, reduce speed.
b). Send warning notifications to the conductor and other impertinent personnel.
c). Document location, time, and speed and send to management.
d). When train is within 'X' proximity of red light segment, apply emergency brakes.
e). Recalculate distance to red light segment, if within 'X' distance, perform 'b' and 'd' .