What is the complementary operation of "ApplyDelete" in Project 4?

[jaedongtang]

The complementary operation of Insert is ApplyDelete, but what is complementary operation of ApplyDelete? Is it Insert?

Suppose the last operation of transaction is Insert (inserted into slot rid_a in the log record). Then it starts to rollback and transactionManager::Abort function would append a ApplyDelete record (also with rid_a). Assume the the system crashes and these Insert and ApplyDelete are the last logs written on disk.

For the redo phase in recovery: if we perform Insert and ApplyDelete for slot rid_a, then record rid_a would be removed at the end. However, in the undo phase, how should we deal with the operation ApplyDelete? This operation is "CLR" type and we should not undo it. But it seems the system does not support this type of log record. If we decide undo this operation of ApplyDelete by Insert the tuple, the problem is Insert does not guarantee insert it at exact slot position rid_a (let us say it is inserted into rid_b now) so we might not undo the Insert with rid_a for the next step. How should we handle the ApplyDelete operation?

I also have another question about where should the log "ABORT" being appended? Should it being appended before any rollback logs (in the slides) or after rollback logs (in textbook and the codes in TransactionManager::Abort)?