問77
頻出度★☆☆
DBMSにおいて, データへの同時アクセスによる矛盾の発生を防止し, データの一貫性を保つための機能はどれか。
平成27年度 春期 ITパスポート試験 問77
テクノロジ系データベース › トランザクション処理
This question's explanation and your answers so far.
解説
問 77
Incorrectア
Normalization is a database design technique: tables are split up to reduce duplication and update anomalies and to tidy up the data structure. It is about removing structural redundancy at design time, not a function that resolves the conflicts of concurrent access at run time.
Incorrectイ
A deadlock is a state in which several transactions each keep waiting for the locks held by the others to be released, so none of them can move forward. It is rather a troublesome phenomenon (a failure) that can occur as a result of performing exclusive control, not the function that prevents inconsistencies itself.
- ウ
正解 The correct answer is ウ, “exclusive control (mutual exclusion).” In a DBMS, several users or transactions may access the same data at the same time, so with no control at all, inconsistencies arise: an update can be overwritten and lost, or someone can read a half-finished value that has not been committed yet. Exclusive control puts a lock on the data a transaction is working on and makes other transactions wait before they read or write it, coordinating the order of concurrent access and protecting data consistency. There are two kinds of lock: an exclusive lock (for writing), which forbids others both from reading and from updating, and a shared lock (for reading), which lets others read only. This matches exactly what the question asks for — the function that prevents inconsistencies caused by concurrent access and maintains consistency.
Incorrectエ
Restore is the operation of writing backed-up data back so that the database recovers to the state it was in before the failure. Its purpose is recovery from a failure (repair after the fact), a different role from controlling concurrent access while the system is running.
ここがポイント
- Exclusive control (locking) = the DBMS function that coordinates how concurrently running transactions access data, preventing lost updates and reads of half-finished values, and thereby keeping data consistent. Learn the distinction between an exclusive lock (others can neither read nor update) and a shared lock (others may read).
- How the confusable terms divide up: normalization = removing redundancy at design time; deadlock = a failure phenomenon where locks wait on each other and work stops; restore (plus rollback / roll-forward) = means of recovery after a failure. When the question asks for the function that prevents inconsistencies, the answer is exclusive control.
用語