T2 reads a value of X written by T1. For the schedule to be recoverable, what must hold?
Correct answer B. T1 must commit before T2 commits
Explanation
If T2 commits after reading T1's uncommitted write and T1 later aborts, T2 has committed based on a value that never officially existed, and that cannot be undone. A recoverable schedule therefore requires that whenever Tj reads data written by Ti, Ti commits before Tj does. A stronger property, cascadeless, avoids even reading uncommitted data, preventing cascading rollbacks.
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