Be aborted the rollback can be done in the usual

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be aborted, the rollback can be done in the usual manner by the recov- ery algorithm, during the undo pass. If the nested-transaction must be restarted, any incomplete subtransactions that need to be rolled back can be rolled back as above. To restart the nested-transaction, state information about the transaction, such as locks held and execu- tion state, must have been noted on the log, and must restored during recovery. Mini-batch transactions (discussed in Section 24.1.10) are an example of nested transactions that must be restarted. Recovery with multi-level transactions : In addition to what is done in the previous case, we have to handle the problems caused by exposure of updates performed by committed subtransactions of incomplete upper-level transactions. A committed
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4 Chapter 26 Advanced Transaction Processing subtransaction may have released locks that it held, so the compen- sating transaction has to reacquire the locks. This is straightforward in the case of transaction failure, but is more complicated in the case of system failure. The problem is, a lower level subtransaction a of a higher level transaction A may have released locks, which have to be reacquired to compensate A during recovery. Unfortunately, there may be some other lower level subtransaction b of a higher level transaction B that started and acquired the locks released by a , before the end of A . Thus undo records for b may precede the operation commit record for A . But if b had not finished at the time of the system failure, it must first be rolled back and its locks released, to allow the compensating transaction of A to reacquire the locks. This complicates the undo pass; it can no longer be done in one backward scan of the log. Multilevel recovery is described in detail in David Lomet, MLR : A Recovery Method for Multi-Level Systems , ACM SIGMOD Conf. on the Management of Data 1992, San Diego.
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