Reliability

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Discussion

Reliability is a Quality Characteristic concerning the degree to which a subject performs specified functions under specified conditions for a specified period.

Reliability can apply to:

A Candidate SolutionA Distributed SystemA NodeA Qualified NodeA Node SetA serviceA software componentA hardware componentAn Execution FacilityAn Operational ResourceAnother subject required to perform specified functionsReliability does not mean that a subject never fails. Reliability concerns whether the subject satisfies defined expectations for operation under stated conditions for a stated period.

Reliability can concern:

AvailabilityContinuity of operationFailure frequencyFailure rateFault toleranceDegraded operationRecoveryState preservationInformation preservationEvidence continuitySuccessful completionService-level behaviorAnother reliability-related propertyReliability can identify:

The evaluated subjectThe specified functionsThe specified operating conditionsThe evaluation periodThe applicable workloadThe applicable Test EnvironmentThe applicable ConfigurationThe applicable failure conditionsThe applicable fault conditionsThe applicable failure classificationsThe applicable Reliability measuresThe applicable measurement unitsThe required measurement precisionThe permitted toleranceThe applicable DIDO BaselineThe applicable Validation CriteriaThe applicable Acceptance CriteriaIts EvidenceIts ProvenanceIts TraceabilityReliability can be evaluated using measures such as:

Successful operations per attempted operationSuccessful Test Runs per attempted Test RunFailures per unit of timeFailures per number of operationsMean time between failuresMean time to failureAvailability during the evaluation periodSuccessful recovery attempts per recovery attemptInformation or state preserved after a failureEvidence continuity after a failure or restartAnother measure established by the Acceptance CriteriaWithin the Governed Node Service Market, Reliability supports:

Trust in repeated governed workService continuityEvidence continuityProvider comparisonCompetitive SubstitutionA Qualified Node demonstrates Reliability by performing its governed functions according to the applicable conditions, service expectations, Governance Policies, and Acceptance Criteria.

Definition

Quality Characteristic concerning the degree to which a subject performs specified functions under specified conditions for a specified period

Source

Adapted from:

Financial Systems Archetype, Part 7: Governed Node Service MarketQuality CharacteristicDIDO Reference Implementation Conceptual ModelDIDO-TE draft Requirements RegisterThe original shared definition described Reliability as the ability of a Qualified Node to perform a governed function correctly and consistently under stated operating conditions.

This revision generalizes Reliability for use throughout the shared DIDO corpus while preserving the original Qualified Node and Governed Node Service Market application.

Note

Reliability differs from Availability:

Availability concerns whether a subject is operational and accessible when requiredReliability concerns whether a subject performs specified functions under specified conditions for a specified periodAvailability can contribute to a Reliability evaluation, but Availability and Reliability are not interchangeable.

Reliability differs from Functional Correctness:

Reliability concerns sustained performance of specified functionsFunctional Correctness concerns whether produced results are correct with the required precisionA subject can perform an incorrect function consistently.

A subject can produce correct results during successful operations while failing too frequently to satisfy its Reliability criteria.

Reliability differs from Operational Stability:

Reliability concerns performance of specified functions over a specified periodOperational Stability concerns whether selected operational characteristics remain within permitted variationReliability differs from Recoverability:

Reliability concerns performance under specified conditions for a specified periodRecoverability concerns restoration of an affected state and required information after an interruption or failureRecoverability can contribute to Reliability but does not replace Reliability.

A completed Test Run assigned a failing Verdict does not necessarily indicate a Reliability failure. The Verdict can concern Functional Correctness, Performance Efficiency, Security, Interoperability, or another Evaluation Characteristic.

A Test Execution Exception does not necessarily indicate failure of the evaluated subject. The exception can originate in:

The Test EnvironmentAn Execution FacilityAn Operational ResourceAn external systemAnother supporting subjectThe applicable Acceptance Criteria should distinguish:

Failure of the evaluated subjectFailure of DIDO-TEFailure of the Test EnvironmentFailure of a supporting Operational ResourceA Test Execution Exception unrelated to the evaluated subject

Example

A persistence Node records Evidence references and retention metadata during normal operating conditions and after a controlled restart.

The applicable Acceptance Criteria establish:

The specified persistence functionsThe operating conditionsThe evaluation periodThe permitted failure frequencyThe required availabilityThe required recovery behaviorThe information and Evidence that must remain intactThe permitted recovery durationThe Node exhibits Reliability when it performs the specified functions according to those conditions and limits throughout the evaluation period.

In a separate DIDO-TE evaluation, three Candidate Solutions perform the same function for 72 hours under a common workload. DIDO-TE records each attempted operation, successful operation, failure, period of unavailability, recovery attempt, Test Execution Exception, and supporting Evidence.

The resulting Test Results support a comparative evaluation of Candidate Solution Reliability.

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