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CMPT 454 Quiz 9 - ACID Transactions, Schedules and P2L Exam T or F: A single transaction may result in multiple actions - Answer True Transactions should have the ACID properties. What are they? - Answer 1. Atomic -either all or none of a transaction's actions are carried out
- Consistent - transaction must preserve the DB consistency
- Isolated - transaction should make sense in isolation, should stand on their own andshould be protected from the effects of other concurrently executing transactions
- Durable - once a transaction is completed, its effects should persist even in the eventof a system crash
How can a DB remain consistent? - Answer If transactions are:
- consistent- processed as if they occur in some serial order
- atomic, no partial transactions occur What does it mean for a DB to be in a consistent state? - Answer - Satisfies theconstraints of the DB schema
- Transactions leave the DB in a consistent state
- Transactions are executed serially Where are transactions processed? - Answer Main memory How can we achieve durability of transactions? - Answer Maintain backups of thedatabase
What is the scheduler? Where does it take place? - Answer The process responsible for
executing reads & writes. It takes place in the main memory, not the disk. What are the two possible last actions of a transaction? - Answer Commit or abort What must happen if a transaction is aborted? - Answer All of its actions must be undone What is the difference between a complete schedule and a serial schedule? - Answer Acomplete schedule is one that contains either an abort or commit action for each of its transactions A serial schedule is one where actions from different transactions are not interleaved What are the 3 types of conflicts when two actions act on the same data? - AnswerWrite-read conflicts (WR) Read-write conflicts (RW) Write-write conflicts (WW) What is a dirty read? - Answer When one transaction writes data, and a secondtransaction reads that data before it commits
What is a write-read conflict? - Answer When one transaction writes data, and a secondtransaction reads same data before it commits
What is a read-write conflict? (AKA unrepeatable read) - Answer When one transactionreads data which is then written by a second transaction
What is a write-write conflict? (AKA lost update) - Answer When one transaction writesdata that has already been read or written by a second, incomplete transaction
Two schedules are view-equivalent if - Answer They contain the same transactions
What is the lock manager? - Answer Keeps track of which locks have been issued totransactions
What is a lock table? - Answer A hash table with the data object ID as the key What does each lock table entry contain? - Answer The number of transactions holdinga lock on the object Type of lock (shared or exclusive)A pointer to a queue of lock requests
When are the only times a shared lock request is granted? - Answer The request queueis empty The object is not locked in exclusive mode When are the only times a exclusive lock is granted? - Answer If there is no lock on theobject and request queue is empty
What are update locks? - Answer Allows a transaction to read a record, but can be laterupgraded to an exclusive lock
When can update locks be granted? - Answer When another transaction has a sharedlock
What are increment locks? - Answer They allow objects to be incremented ordecremented
T or F: Only one increment lock is allowed on the same object - Answer F. Multiple. Can other locks be granted on objects with increment locks? - Answer No.
