Managing Multiuser Databases Questions and Answers

Answer: B

Correct answer: False

The name is right in substance and wrong in wording: reading a changed record that has not been committed is a dirty read, not a filthy one. The term matters because it is one of the three anomalies the isolation levels are defined against, alongside nonrepeatable reads and phantom reads.

Answer: D

Correct answer: Serializable

Serializable is the strictest level and permits none of the three anomalies: no dirty reads, no nonrepeatable reads and no phantoms. It gives the same result as running the transactions one after another, which is where the name comes from. The cost is the concurrency given up to achieve it, which is why lower levels exist at all.

Answer: A

Correct answer: True

Serializable transactions produce a result identical to some serial execution of the same transactions, one after another. It is the standard against which concurrency control is judged: the transactions may interleave in any way at all, so long as the outcome could have been produced by running them in some order.

Answer: C

Correct answer: Keyset

A keyset cursor saves the primary key of every row in the recordset when it is opened, and fetches the current values through that key each time a row is accessed. So changes made to rows already in the set are seen, but rows inserted afterwards are not, since their keys were never captured - it sits between static and dynamic.

Answer: A

Correct answer: atomic.

Atomicity is the all-or-nothing property: the transaction either completes in full or leaves the database exactly as it found it. It is the first of the four ACID properties, the others being consistency, isolation and durability. It is what makes a multi-step operation such as a transfer between accounts safe.

Answer: A

Correct answer: Nonrepeatable read

A non-repeatable read occurs when a transaction reads a row, another transaction modifies or deletes it and commits, and the first transaction reads it again and sees something different. The remedy is the repeatable-read isolation level, which holds read locks to the end of the transaction.

Answer: C

Correct answer: Exclusive lock

An exclusive lock excludes everyone else entirely: no other transaction may read the item or change it while the lock is held. It is what a writing transaction needs. A shared lock is the weaker form, permitting concurrent readers but no writer, which is why several may be held on the same item at once.

Answer: B

Correct answer: False

A record of a page after it has changed is an after image; a before image is the copy taken beforehand. Both are kept, and each has its own use in recovery: after images are applied to roll a transaction forward, before images to roll it back.

Answer: B

Correct answer: False

What is described is a dirty read - seeing data from a transaction that is only part way through. The lost update problem is different: two transactions each read the same value, each compute a new one from it, and each write back, so the second silently destroys the first. Resource locking exists to prevent it.

Answer: A

Correct answer: the lock is obtained only after the transaction has processed.

Optimistic locking assumes conflict will be rare: the transaction reads, does its work without holding locks, and only at the end takes a lock long enough to check that nothing it relied on has changed. If something has, the transaction is rolled back and retried. Locks are therefore held for a very short time, which is where the concurrency gain comes from.