Question

Because the log contains a record of every write operation that changes the value of some database item, it is possible to ___ the effect of these write operations of a transaction T by tracing backward through the log and resetting all items changed by a write operation of T to their old_values.

Answer 1

The missing word is 'undo'. By tracing the log backward, the database management system can undo the effects of a transaction by resetting changed items to their old values, maintaining database consistency after errors.

Step-by-step explanation:

The student's question pertains to a concept in database management, specifically the process of undoing changes made by transactions within a database system. The term that completes the sentence is undo, meaning to reverse the effects of certain operations. A database log, which records every write operation, can be used to rollback a transaction by reversing actions in reverse order of execution. To undo a transaction (often in the case of an aborted transaction), the system administrator or the database management system itself will start at the most recent log record for transaction T and work backward, resetting any affected items to their previous state as recorded in the log. This is a vital part of transaction management, ensuring that in the event of a failure, the database can be restored to a consistent state.

For example, if a transaction T changed the balance of a bank account and then encountered an error, the log would contain an entry with the old balance (before the transaction) and the new balance (after the transaction). By tracing the log backward, one can find this entry and restore the balance to the original amount, effectively undoing the transaction's effect on the database.