ITPEC FE Morning October 2018 Question 30

Exclusive control — locking stops two transactions from updating the same data at once, so no update is lost or read half-finished.

Without it, two programs can read the same value, each change it, and write it back, so the second write silently overwrites the first (a lost update). With exclusive control, a transaction locks the data it updates; others that want the same data wait until the lock is released at commit or rollback. Shared locks let several transactions read together, while an exclusive lock blocks both reading and writing by others.

Answer (a)

Why not others:

• •(b) Integrity constraint — rules on the values stored (primary key, foreign key, NOT NULL, CHECK); they do not coordinate concurrent transactions
• •(c) Normalization — a design technique that removes redundancy from tables, done before the database is in use
• •(d) Reorganization — rewriting the physical storage to remove fragmentation and restore performance

Key rule: Concurrent updates are kept consistent by concurrency control (locking); integrity constraints guard values, and normalization guards the design.

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