Software Availability and Reliability
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Questions and Answers

What does availability refer to in the context of software?

  • The downtime due to periodic maintenance of the software
  • The concept of reliability and robustness of the software
  • The ability of the software to carry out its task when needed (correct)
  • The ability of the software to repair itself when it breaks

    • How does availability differ from reliability?

  • Availability encompasses robustness, while reliability does not
  • Reliability ensures that the cumulative service outage period does not exceed a required value, while availability does not
  • Availability includes the concept of recovery when the system breaks, while reliability does not (correct)
  • Reliability focuses on periodic maintenance, while availability does not

    • What does it mean for a system to mask or repair faults?

  • Ensuring that faults do not become externally visible failures (correct)
  • Preventing faults from occurring in the system
  • Making the faults invisible to external observers
  • Repairing faults as soon as they occur

    • What is considered a failure in the context of a system?

    <p>The deviation of the system from its specification, visible externally</p> Signup and view all the answers

    What is the cause of a failure called in the context of a system?

    <p>Fault</p> Signup and view all the answers

    How does availability relate to robustness and reliability?

    <p>Availability encompasses concepts of reliability, robustness, and any other quality attribute involving unacceptable failure</p> Signup and view all the answers

    What is the relationship between availability and security?

    <p>Availability is distinct from security, but closely related.</p> Signup and view all the answers

    What does the term 'observability' refer to in the context of system failures?

    <p>Whether or not a failure could have been observed, regardless of whether it was actually observed.</p> Signup and view all the answers

    How can the availability of a system be measured?

    <p>By the probability of providing specified services within required bounds over a specified time interval.</p> Signup and view all the answers

    What is the main concern of safety in relation to system availability?

    <p>Keeping the system from entering a hazardous state and limiting the damage when it does.</p> Signup and view all the answers

    What is the expression used to derive steady-state availability?

    <p>MTBF/(MTBF + MTTR)</p> Signup and view all the answers

    Study Notes

    Availability in Software

    • Refers to the proportion of time a system is operational and accessible for use.
    • Typically expressed as a percentage, indicating the uptime versus downtime.

    Difference Between Availability and Reliability

    • Availability focuses on the system being operational when required.
    • Reliability emphasizes the likelihood of a system performing correctly over a period.
    • A system can be available yet unreliable if it fails transiently while in use.

    Fault Masking and Repair

    • Fault masking involves concealing faults from users or systems, allowing uninterrupted operation.
    • Repair includes the identification and resolution of faults to restore normal system function.

    System Failure

    • Defined as the condition where a system can no longer perform its required functions.
    • Can manifest as a complete outage or a partial inability to fulfill tasks.

    Cause of Failure

    • The origin of a failure is referred to as a fault or defect, which can arise from various factors like hardware malfunctions or software bugs.

    Availability in Relation to Robustness and Reliability

    • Robustness indicates a system's ability to maintain performance under adverse conditions.
    • High availability often requires both robust design and reliable components for minimizing downtime and failures.

    Relationship Between Availability and Security

    • High availability systems must also ensure security, particularly in safeguarding against attacks that can compromise access or functionality.
    • Security incidents can drastically affect availability by causing system outages.

    Observability in System Failures

    • Refers to the ability to monitor and understand a system's internal states and behaviors, facilitating the diagnosis of failures.
    • Enhances the capacity to track issues and optimize system performance.

    Measuring System Availability

    • Commonly assessed using metrics like uptime, downtime, and mean time between failures (MTBF).
    • Can be expressed as a percentage, e.g., "99.9% availability" means the system is unavailable roughly 8.76 hours annually.
    • Safety in system availability focuses on ensuring that operations do not lead to hazards, especially in critical systems like healthcare and transportation.
    • Ensures user and system protection while maintaining functional performance.

    Steady-State Availability Expression

    • Steady-state availability can be calculated using the formula: [ A = \frac{MTBF}{MTBF + MTTR} ]
    • Where MTBF is mean time between failures, and MTTR is mean time to repair.

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    Description

    Explore the concept of software availability and reliability, including the relationship between the two and additional considerations such as downtime due to periodic maintenance. Test your understanding of this important property of software.

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