Explain how the two-phase commit protocol for nested transactions ensures that if the top-level transaction commits, all the right descendents are committed or aborted.

What will be an ideal response?

Whenever a nested transaction commits, it reports its status and the status of its descendants to its parent. Therefore when a transaction enters the committed state, it has a correct list of its committed descendants. Therefore when the top-level transaction starts the two-phase commit protocol, its list of committed descendants is correct. It checks the descendants and makes sure they can still commit or must abort. There may be nodes that ran unsuccessful descendants which are not included in the two-phase commit protocol. These will discover the outcome by querying the top-level transaction.