12 - CriticalSystems

12 - CriticalSystems - CO502 Advanced Computer Architecture...

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CO502 – Advanced Computer Architecture Computer Architecture & Dependability Roshan Ragel – Department of Computer Engineering Background – Critical Systems Based on: John Rushby, Critical System Properties: Survey and Taxonomy , Reliability Engineering and System Safety , 43 (1994) pp 189-219 Critical System triangle45right There is a lot of disagreement concerning the definition of a “critical system”. There are FOUR different views on the subject: triangle45right 1. Dependability Approach 2. Safety Approach 3. Security Approach 4. Real Time Systems Approach triangle45right There are differing views on the relationship between properties and the compatibility of techniques from these approaches. Dependability Approach Safety Approach Real Time Security Approach Systems Approach
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Dependability Approach triangle45right Introduced by Jean Claude Laprie triangle45right A dependable system is one for which the reliance may justifiably be placed on both its “ correctness ” and continuityofitsdelivery triangle45right “correctness” pertains to the conformity with requirements, specifications, etc. triangle45right “Dependability” encapsulates the technical meaning of terms such as reliability, safety, survivability, security and fault tolerance. Failure vs. Fault? triangle45right Failure is defined as the inability to provide a required service from the system because of faults . triangle45right Failure is a property of the external behavior of a system. triangle45right Failures can be either: triangle45right Benign triangle45right The consequences of failure = Benefits provided by normal operation triangle45right Catastrophic triangle45right The consequences of failure >> Benefits provided by normal operation Failure and Internal States of a System triangle45right Suppose a system progresses through a set of states S 1 , S 2 , S 3 ,………S N S 1 S 3 S 4 S N S 2 Failure Occurs Fault Activated Latent Error Fault Tolerant System triangle45right The error or fault is latent from the state of activation until it manifests itself in the effective fault state. triangle45right A faulttolerantsystem attempts to detect and correct such latenterrors before they become effective.
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Steps in Fault Tolerance triangle45right Error Detection – Detect the latent error before the effective fault state. head2right This can be done by internal consistency checks or by comparison with redundant computations.
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