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2454 Compensating Transactions
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To reduce the frequency of long-duration waiting, we arrange for uncommitted updates to be exposed to other concurrently executing transactions Indeed, multilevel transactions may allow this exposure However, the exposure of uncommitted data creates the potential for cascading rollbacks The concept of compensating transactions helps us to deal with this problem Let transaction T be divided into several subtransactions t1 , t2 , , tn After a subtransaction ti commits, it releases its locks Now, if the outer-level transaction T has to be aborted, the effect of its subtransactions must be undone Suppose that subtransactions t1 , , tk have committed, and that tk+1 was executing when the decision to abort is made We can undo the effects of tk+1 by aborting that subtransaction However, it is not possible to abort subtransactions t1 , , tk , since they have committed already Instead, we execute a new subtransaction cti , called a compensating transaction, to undo the effect of a subtransaction ti Each subtransaction ti is required to have a
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compensating transaction cti The compensating transactions must be executed in the inverse order ctk , , ct1 Here are several examples of compensation: Consider the schedule of Figure 245, which we have shown to be correct, although not con ict serializable Each subtransaction releases its locks once it completes Suppose that T2 fails just prior to termination, after T2,2 has released its locks We then run a compensating transaction for T2,2 that subtracts 10 from A and a compensating transaction for T2,1 that adds 10 to B Consider a database insert by transaction Ti that, as a side effect, causes a B+ -tree index to be updated The insert operation may have modi ed several nodes of the B+ -tree index Other transactions may have read these nodes in accessing data other than the record inserted by Ti As in Section 179, we can undo the insertion by deleting the record inserted by Ti The result is a correct, consistent B+ -tree, but is not necessarily one with exactly the same structure as the one we had before Ti started Thus, deletion is a compensating action for insertion Consider a long-duration transaction Ti representing a travel reservation Transaction T has three subtransactions: Ti,1 , which makes airline reservations; Ti,2 , which reserves rental cars; and Ti,3 , which reserves a hotel room Suppose that the hotel cancels the reservation Instead of undoing all of Ti , we compensate for the failure of Ti,3 by deleting the old hotel reservation and making a new one If the system crashes in the middle of executing an outer-level transaction, its subtransactions must be rolled back when it recovers The techniques described in Section 179 can be used for this purpose Compensation for the failure of a transaction requires that the semantics of the failed transaction be used For certain operations, such as incrementation or insertion into a B+ -tree, the corresponding compensation is easily de ned For more complex transactions, the application programmers may have to de ne the correct form of compensation at the time that the transaction is coded For complex interactive transactions, it may be necessary for the system to interact with the user to determine the proper form of compensation
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The transaction concepts discussed in this section create serious dif culties for implementation We present a few of them here, and discuss how we can address these problems Long-duration transactions must survive system crashes We can ensure that they will by performing a redo on committed subtransactions, and by performing either an undo or compensation for any short-duration subtransactions that were active at the time of the crash However, these actions solve only part of the problem In typical database systems, such internal system data as lock tables and transactions timestamps are kept in volatile storage For a long-duration transaction to be resumed
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