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Dependability TSW 10 Anders P. Ravn Aalborg University November 2009
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Characteristics of a RTS Timing Constraints Dependability Requirements Concurrent control of separate components Facilities to interact with special purpose hardware
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Dependability - impediments Faults Errors Failures BW Ch 2,... FaultErrorFailure... Fault
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System and Component
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Dependability - attributes Availability Reliability Safety Confidentiality Integrity Maintainability BW Ch 2
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Dependability - means Fault prevention Fault tolerance Error Removal Failure Forecasting BW Ch 2
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Fault classification Origin Kind Property physical (internal/external) logical (design/interaction) omission value timing byzantine duration (permanent, transient) consistency (determinate, nondeterminate) autonomy (spontaneous, event-dependent)
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Error Classification (Fault Error) Effect Extent latent effective local distributed
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Failure Classification (Fault Failure) Consequence benign malign (a mishap) BW (Failure modes) Ch 2
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Dependability - means Fault prevention Fault tolerance Error Removal Failure Forecasting
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Fault Prevention Careful Design Conservative Design process (procedures) notations tools robust functionality testability tracability
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Dependability - means Fault prevention Fault tolerance Error Removal Failure Forecasting
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Error Removal Verification (analysis of design) Test (analysis of implementation)
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Dependability - means Fault prevention Fault tolerance Error Removal Failure Forecasting
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Calculation – analysis of design Simulation – measurement on design Test -- measurement on implementation
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Dependability - means Fault prevention Fault tolerance Error Removal Failure Forecasting BW Ch 2
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Fault Tolerance Means to isolate component faults Prevents system failures May increase system dependability... And mask them
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Fault Tolerance
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FT - levels Full tolerance Graceful Degradation Fail safe BW Ch 2
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FT basis: Redundancy Time Space TryRetry... Try... BW Ch 2
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N-version programming V1 V2 V3 Driver (comporator) Comparison vectors (votes) Comparison status indicators BW Ch 2 Comparison points
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Fault classification (scope of N-VP) Origin Kind Property physical (internal/external) logical (design/interaction) omission value timing byzantine duration (permanent, transient) consistency (determinate, nondeterminate) autonomy (spontaneous, event-dependent) + (+) ++ (+) + / (+) + / +
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Dynamic Redundancy 1.Error detection 2.Damage confinement and assessment 3.Error recovery 4.Fault treatment and continued service BW Ch 2
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Error Detection f: State x Input State x Output Environment (exception) Application BW Ch 2 Assertion: precondition (input) postcondition (input, output) invariant(state, state’) Timing: WCET(f, input) Deadline (f,input) D
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Damage Confinement Static structure Dynamic structure BW Ch 2 object I I
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Error Recovery Forward Backward BW Ch 2 Repair the state – if you can ! define recovery points checkpoint state at r. p. roll back retry Domino effect
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Recovery blocks ENSURE acceptance_test BY { module_1 } ELSE BY { module_2 }... ELSE BY { module_m } ELSE ERROR BW Ch 2
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The ideal FT-component Exception HandlerNormal mode Request/response Interface exception Interface exception Failure exception Failure exception BW Ch 2
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