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Dependability. TSW 10 Anders P. Ravn Aalborg University November 2009. Characteristics of a RTS. Timing Constraints Dependability Requirements Concurrent control of separate components Facilities to interact with special purpose hardware. . Fault. Error. Failure. Fault.

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dependability

Dependability

TSW 10

Anders P. Ravn

Aalborg University

November 2009

characteristics of a rts
Characteristics of a RTS
  • Timing Constraints
  • Dependability Requirements
  • Concurrent control of separate components
  • Facilities to interact with special purpose hardware
dependability impediments

...

Fault

Error

Failure

Fault

Dependability - impediments
  • Faults
  • Errors
  • Failures

BW Ch 2, ...

dependability attributes
Dependability - attributes
  • Availability
  • Reliability
  • Safety
  • Confidentiality
  • Integrity
  • Maintainability

BW Ch 2

dependability means
Dependability - means
  • Fault prevention
  • Fault tolerance
  • Error Removal
  • Failure Forecasting

BW Ch 2

fault classification

byzantine

Fault classification
  • physical (internal/external)
  • logical (design/interaction)
  • Origin
  • Kind
  • Property
  • omission
  • value
  • timing
  • duration (permanent, transient)
  • consistency (determinate, nondeterminate)
  • autonomy (spontaneous, event-dependent)
error classification
Error Classification
  • (Fault  Error)
  • Effect
  • Extent
  • latent
  • effective
  • local
  • distributed
failure classification
Failure Classification
  • (Fault  Failure)
  • Consequence
  • benign
  • malign (a mishap)

BW (Failure modes) Ch 2

dependability means1
Dependability - means
  • Fault prevention
  • Fault tolerance
  • Error Removal
  • Failure Forecasting
fault prevention
Fault Prevention
  • process (procedures)
  • notations
  • tools
  • Careful Design
  • Conservative Design
  • robust functionality
  • testability
  • tracability
dependability means2
Dependability - means
  • Fault prevention
  • Fault tolerance
  • Error Removal
  • Failure Forecasting
error removal
Error Removal
  • Verification (analysis of design)
  • Test (analysis of implementation)
dependability means3
Dependability - means
  • Fault prevention
  • Fault tolerance
  • Error Removal
  • Failure Forecasting
failure forecasting
Failure Forecasting
  • Calculation – analysis of design
  • Simulation – measurement on design
  • Test -- measurement on implementation
dependability means4
Dependability - means
  • Fault prevention
  • Fault tolerance
  • Error Removal
  • Failure Forecasting

BW Ch 2

fault tolerance
Fault Tolerance

Means to isolate componentfaults

... And mask them

Prevents systemfailures

May increase systemdependability

ft levels

Full tolerance

  • Graceful Degradation
  • Fail safe
FT - levels

BW Ch 2

ft basis redundancy

Retry

...

...

Try

Try

Try

FT basis: Redundancy
  • Time
  • Space

Try

Retry

BW Ch 2

n version programming
N-version programming

V1

V3

V2

Comparison vectors (votes)

Driver (comporator)

Comparison status indicators

Comparison points

BW Ch 2

fault classification scope of n vp

byzantine

Fault classification (scope of N-VP)

+

+

(+)

++

(+)

+ / (+)

+ / +

+ / +

  • physical (internal/external)
  • logical (design/interaction)
  • Origin
  • Kind
  • Property
  • omission
  • value
  • timing
  • duration (permanent, transient)
  • consistency (determinate, nondeterminate)
  • autonomy (spontaneous, event-dependent)
dynamic redundancy
Dynamic Redundancy
  • Error detection
  • Damage confinement and assessment
  • Error recovery
  • Fault treatment and continued service

BW Ch 2

error detection

D

Error Detection

f: State x Input  State x Output

  • Environment (exception)
  • Application
  • Assertion:
  • precondition (input)
  • postcondition (input, output)
  • invariant(state, state’)
  • Timing:
  • WCET(f, input)
  • Deadline (f,input)

BW Ch 2

damage confinement

object

I

object

I

Damage Confinement
  • Static structure
  • Dynamic structure

BW Ch 2

error recovery
Error Recovery
  • Forward
  • Backward

Repair the state – if you can !

  • define recovery points
  • checkpoint state at r. p.
  • roll back
  • retry

Domino effect

BW Ch 2

recovery blocks
Recovery blocks

ENSURE acceptance_test

BY { module_1 }

ELSE BY { module_2 }

...

ELSE BY { module_m }

ELSE ERROR

BW Ch 2

the ideal ft component

Failure

exception

Interface

exception

Request/response

Interface

exception

Failure

exception

Request/response

The ideal FT-component

Normal mode

Exception Handler

BW Ch 2