GATE CS 2003 Q29 — Transactions and Concurrency Control

A schedule is recoverable if, for each pair of transactions $T_i$ and $T_j$ such that $T_j$ reads a data item previously written by $T_i$, the commit operation of $T_i$ appears before the commit operation of $T_j$.

If $T_j$ reads a value written by an uncommitted transaction $T_i$ (known as a dirty read) and commits before $T_i$ commits, the schedule becomes irrecoverable. If $T_i$ subsequently aborts, the database cannot roll back the already-committed $T_j$, violating the atomicity and durability properties.

Analyzing the Options:

• Option A: A transaction writes a data item after it is read by an uncommitted transaction (Write-After-Read). This does not create a dependency that affects recoverability.
• Option B: A transaction reads a data item after it is read by an uncommitted transaction (Read-After-Read). Read operations do not conflict or modify data, so this is completely safe.
• Option C: A transaction reads a data item after it is written by a committed transaction. Since the writing transaction is already committed, there is no risk of it aborting, making this scenario perfectly safe.
• Option D: A transaction reads a data item after it is written by an uncommitted transaction. This is a dirty read. If the reading transaction commits before the writing transaction, it leads to an irrecoverable error.