問41
頻出度★★★
システムの能力や品質をあらわすものとして, 可用性, 性能, データの一貫性, 保守性などがある。可用性に関する記述として, 適切なものはどれか。
平成26年度 春期 ITパスポート試験 問41
テクノロジ系システム構成要素 › システムの評価指標
This question's explanation and your answers so far.
解説
問 41
Incorrectア
“Keeping related data in a consistent state” describes data consistency (integrity), which is also listed in the question. It is the property that no contradictory data remains even when there are multiple update operations, and it is maintained by transaction commit/rollback and by exclusive (concurrency) control. This is about whether the data is correct, a quality characteristic different from availability, which expresses whether the system can be used; therefore it is wrong.
Incorrectイ
“The time from giving an instruction to the system until a result is obtained” is the response time, which describes performance. It is an indicator of processing speed, and its meaning differs from availability, which expresses the proportion of time the system can be used. A fast system has low availability if it stops due to a failure, while a slow system that never stops has high availability, so the two are independent characteristics.
- ウ
正解 The correct answer is ウ “The degree to which functions can be used even when a failure occurs in the system.” Availability means the degree to which a system can be used continuously whenever it is needed; it expresses how little time the system is unusable because of failures or outages. Quantitatively it is measured by the availability rate: availability rate = MTBF ÷ (MTBF + MTTR), that is, the proportion of the total time during which the system was operating normally (MTBF is mean time between failures, MTTR is mean time to repair). Therefore ウ, which states the degree to which functions can still be used even after a failure, is the appropriate description of availability. A typical way to raise availability is redundancy: duplicating equipment or servers so that operation continues even if one of them breaks, or preparing a cluster configuration or hot standby so that switchover time is short — such measures raise the availability rate. The other three options describe performance, data consistency, and maintainability, which are listed alongside availability in the question; the item tests whether you can distinguish quality characteristics that are easily confused with availability.
Incorrectエ
“The degree of ease with which a program can be modified” describes maintainability. It expresses how easily specification changes and defect fixes can be handled, and it mainly concerns the work efficiency of development and maintenance, which differs from availability, the degree to which the system can be used. High maintainability shortens the repair time MTTR and can therefore help improve the availability rate, but the wording of this option only states ease of modification, so as a description of availability it is wrong.
ここがポイント
- Availability is the degree to which a system can be used whenever it is needed, quantified as availability rate = MTBF ÷ (MTBF + MTTR). Making the system keep running even during a failure through redundancy and duplication is the typical countermeasure for raising availability.
- Quality characteristics of a system are easy to mix up, so organize the correspondences: availability = the degree to which the system runs without stopping and can be used; performance = processing speed such as response time and throughput; data consistency = the state in which related data does not contradict itself; maintainability = ease of change and correction. The options in this question line up exactly these four.
用語