Fault Tolerance: Basic Mechanisms mMIC-SFT September 2003 Anders P. Ravn Aalborg University

Fault Tolerance Means to isolate component faults Prevents system failures May increase system dependability... And mask them

Dependability - means Fault prevention Fault tolerance Error Removal Failure Forecasting BW p. 106,...

Fault Tolerance

FT - levels Full tolerance Graceful Degradation Fail safe BW p. 107

FT basis: Redundancy Time Space TryRetry... Try... BW p. 109

N-version programming V1 V2 V3 Driver (comporator) Comparison vectors (votes) Comparison status indicators BW p. 109 Comparison points

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) + (+) ++ (+) + / (+) + / +

Dynamic Redundancy 1.Error detection 2.Damage confinement and assessment 3.Error recovery 4.Fault treatment and continued service BW p. 114

Error Detection f: State x Input  State x Output Environment (exception) Application BW p. 115 Assertion: precondition (input) postcondition (input, output) invariant(state, state’) Timing: WCET(f, input) Deadline (f,input) D

Damage Confinement Static structure Dynamic structure BW p. 117 object I I

Error Recovery Forward Backward BW p. 118 Repair the state – if you can ! define recovery points checkpoint state at r. p. roll back retry Domino effect

Recovery blocks ENSURE acceptance_test BY { module_1 } ELSE BY { module_2 }... ELSE BY { module_m } ELSE ERROR BW p. 120

The ideal FT-component Exception HandlerNormal mode Request/response Interface exception Interface exception Failure exception Failure exception BW p. 126