In multiversion timestamp ordering, read operations can access tentative versions of objects. Give an example to show how cascading aborts can happen if all read operations are allowed to proceed immediately.

Question

What will be an ideal response?

Answer 1

The answer gives the read rule for multiversion timestamp ordering. read operations are delayed to ensure recoverability. In fact this delay also prevents dirty reads and cascading aborts.
Suppose now that read operations are not delayed, but that commits are delayed as follows to ensure recoverability. If a transaction, T has observed one of U’s tentative objects, then T’s commit is delayed until U commits or aborts. Cascading aborts can occur when U aborts because T has done a dirty read and will have to abort as well.

To find an example, look for an answer to Exercise 13.19 where a read operation was delayed (i.e. (c) or (d)). Consider the diagram for (c). With delays, T’s x := read(i) is delayed until U commits or aborts. Now consider allowing T’s x := read(i) to use U’s tentative version immediately. Then consider the situation in which T asks to commit and U subsequently aborts. Note that T’s commit request is delayed until the outcome of U is known, so the situation is recoverable, but T has performed a ‘dirty read’ and must be aborted. Cascading aborts can now arise because some other transactions may have observed T’s tentative objects.