The transactions T and U are run with timestamp ordering concurrency control. Describe the information written to the log file on behalf of T and U, allowing for the fact that U has a later timestamp than T and must wait to commit after T. Why is it essential that the commit entries in the log file should be ordered by timestamps? Describe the effect of recovery if the server crashes (i) between the two Commits and (ii) after both of them.

Question

What are the advantages and disadvantages of early writing with timestamp ordering?

Answer 1

The timestamps of T and U are put in the recovery file. Call them t(T) and t(U), where t(T)


The entry at P5 shows that U has committed, but must be ordered after T. If the transaction T aborts or the server fails before T commits, this entry indicates that U has committed.



It essential that the commit entries in the log file should be ordered by timestamps because recovery works through the log file backwards. The effects of later transactions must be over write the effects of earlier ones.



The effect of recovery:



(i) if the server crashes between the two commits, we lose the entry at P8, but we have P6: Trans t(U) waiting to commit. Transaction U can be committed as the transaction it waits for has either committed, or if it has not yet committed, it will be aborted.



(ii)if the server crashes after both the commits, both will be recovered.



The advantage of early writing with timestamp ordering is that commitment is quicker. Transactions can always commit i