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Dependability. How much can you trust your program? How important is it that you trust your program? Note : Usefulness and trustworthiness are not the same thing. . Dependability. Availability How likely that system will be up and running?. Reliability

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Presentation Transcript
Dependability l.jpg
Dependability

  • How much can you trust your program?

  • How important is it that you trust your program?

  • Note: Usefulness and trustworthiness are not the same thing.

Critical Systems and Dependability


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Dependability

  • Availability

    • How likely that system will be up and running?

  • Reliability

    • How likely that system behaves correctly?

  • Safety

    • How likely is system to cause damage?

  • Security

    • How likely can system defend against accidental or deliberate intrusion?

Critical Systems and Dependability


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Cost/dependability curve

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Dependability

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M

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H

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V

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U

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Critical Systems and Dependability


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Dependability vs. Performance

  • High levels of dependability often achieved at expense of performance

  • Justification:

    • Undependable critical systems won’t be used

    • Cost of system failure very high

    • Data integrity/reliability very important

    • Dependability hard to retrofit

    • Possible to compensate for performance deficit

Critical Systems and Dependability


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Critical Systems

  • Systems where failures can result in physical damage, threats to life, significant economic damage

    • Safety-critical systems

    • Mission-critical systems

    • Business-critical systems

  • Software only one “link in the chain”

    • hardware

    • human operator

Critical Systems and Dependability


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Availability and Reliability

  • Both can be quantified as probabilities

  • Reliability assumes availability, but might have different requirements, owing to:

    • reboot time

    • perceptions

Critical Systems and Dependability


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Availability and Reliability

  • ReliabilityThe probability of failure-free operation over a specified time in a given environment for a specific purpose

  • AvailabilityThe probability that a system, at a point in time, will be operational and able to deliver the requested services

Critical Systems and Dependability


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Reliability and context

  • Perception of reliability influenced by:

    • patterns of use

    • human perceptions

    • situations where failures occur

Critical Systems and Dependability


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Reliability Terminology

  • System failure

    • when system doesn’t deliver user-expected service

  • System error

    • when system doesn’t behave to spec

  • System fault

    • incorrect system state

  • Human error or mistake

    • source of error is human behavior

Critical Systems and Dependability


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Improving Reliability

  • Fault Avoidance

    • Conservative programming idioms

  • Fault Detection and Removal

    • Rigorous testing

  • Fault Tolerance

    • Designing system to recover gracefully from runtime faults

Critical Systems and Dependability


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Number of Bugs != Unreliability

  • Removing 60% of product defects improved reliability only 3% [Mills (1987)]

  • It’s how often the bugs are tickled that matters

    • and unfortunately, this could relate to user behavior

Critical Systems and Dependability


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Safety and Security

  • Harder to quantify than reliability or availability

  • Process strategies

    • avoidance

    • detection and removal

    • limitation of influence

Critical Systems and Dependability


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Safety

How likely is system to cause damage?

A system can be reliable but still not 100% safe:

  • incomplete specification

  • hardware malfunction

  • (human) operator error

Critical Systems and Dependability


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SafetyProcess Strategies

  • Hazard Avoidance

    • Defending against occurrence of hazards(e.g., traffic restrictions)

  • Hazard Detection/Removal

    • Noticing/reacting to dangerous conditions(e.g., restricting/denying access)

  • Damage Limitation

    • Additional system features to minimize extent of damage (fail-safes)

Critical Systems and Dependability


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Security

  • Types of damage attributed to security failures:

    • Denial of service

    • Corruption of programs or data

    • Disclosure of confidential info

Critical Systems and Dependability


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SecurityProcess Strategies

  • Vulnerability Avoidance

    • Direct defense against attack(e.g., denying external access)

  • Attack Detection/Neutralization

    • Noticing/reacting to potential attacks before security is compromised(e.g., virus checker)

  • Exposure Limitation

    • Additional system features to minimize extent of damage (e.g., system backups)

Critical Systems and Dependability


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