ITPEC IP April 2019 Question 26
Among the processes where multiple users simultaneously access the same database, which of the following is a process that does not need a countermeasure for securing transaction consistency?
Explanation
A search process for library information — a search only reads data, so users searching at the same time cannot corrupt it.
Consistency breaks when several transactions read and then update the same data at once. Two users both see seat 5A as free and both book it; two orders both take the last item in stock. Such processes need concurrency control: transactions and exclusive locks that make the updates happen one after another.
A search of a library catalogue changes no record. Any number of users can read the same rows at once, since reads do not affect each other (shared locks are compatible).
Answer (c)
Why not others:
- •(a) — each bid updates the current highest price; two bids at the same moment must be serialised, or one overwrites the other
- •(b) — each order reduces the stock; without control, two simultaneous orders can both buy the last unit
- •(d) — each reservation claims a seat; without control, the same seat can be sold twice
Key rule: Concurrency control is needed wherever data is read and then updated by several users; read-only access can share data freely.
Databases · AI-written, may contain errors
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