Transaction Isolation and Atomicity
Recoverable Schedule :
Consider the following schedule-9 :
Here, T6 contains only one transaction.
This schedule is said to be ‘Partial Schedule’, because in T6, there is no either Commit or Abort operations.
Suppose, T6 fails before it commits, T7 has read the value of data item A written by T6.
Therefore, T7 is said to be dependent on T6. We must abort T7 to ensure atomicity.
Here, T7 is already committed. So, it can not be aborted.
So, it is impossible to recover this schedule.
The above schedule-9 is said to be ‘non-recoverable schedule’.
A ‘recoverable schedule’ is one where, for each pair of transactions Ti and Tj, such that Tj reads a data item previously written by Ti, the commit operation of Ti appears before the commit operation of Tj.
In the above schedule-9 to be recoverable, T6 would have to delay committing until T5 commits.