Quiz 4 (Transactions)

Question 1

Transaction

Answer 1

An execution of a user program, seen by the DBMS as a series or list of operations; operations may include read, write, commit, rollback (abort).

Question 2

ACID

Answer 2

Four properties of transactions to maintain data in the face of concurrency and possible failures; Atomicity, Consistency, Isolation, Durability.

Question 3

Atomicity

Answer 3

Either all actions are carried out or none are.

Question 4

Consistency

Answer 4

When the user submits a transaction against a database that is in a consistent state, and that transation runs to completion, the data will remain in a consistent state; consistency is the responsibility of the user.

Question 5

Isolation

Answer 5

Transactions are isolated from the effects of other concurrently running (interleaved) transaction.

Question 6

Durability

Answer 6

Once the DBMS informs the user that a transaction has been successfully completed, its effects should persist regardless of system crashes or failures.

Question 7

Schedule

Answer 7

A list of actions (reading, writing, committing, rolling back (aborting) from a set of transactions; the order in which two actions of a transaction T appear in a schedule must be the same order as in the transaction.

Question 8

Concurrency

Answer 8

Running the individual steps of multiple transactions interleaved together to take advantage to improve performance; isolation of transactions must be maintained.

Question 9

Serializable Schedule

Answer 9

A schedule of a set of transactions that is equivalent to running them serially.

Question 10

Conflicting Actions

Answer 10

Actions in two or more transactions that may conflict with each other; always involves a write operation as one of the actions.

Question 11

Reading Uncommitted Data Conflicting Action

Answer 11

A transaction T2 could read a database object A that has been previously modified by another transaction T1 (dirty read).

Question 12

Unrepeatable Read Conflicting Action

Answer 12

Transaction T2 could change the value of an object A that has been read by a transaction T1 while T1 is still in progress.

Question 13

Overwriting Uncommitted Data Conflicting Action

Answer 13

A transaction T2 could overwrite the value of an object AA which has already been modified by transaction T1 while T1 is still in progress.

Question 14

Strict Two-Phase Locking (Strict 2PL)

Answer 14

A protocol used by DBMS’s to ensure that only serializable recoverable schedules are allowed and that no action of committed transactions are lost while undoing aborted transactions.
Strict 2PL has two rules:

1. If a transaction T wants to read (or modify) an object, it first requests a shared (or exclusive) lock on the object.
2. All locks held by a transaction are released when the transaction is completed.

Question 15

Table Locking

Answer 15

Place a shared or exclusive lock on an entire table.

Question 16

Row Locking

Answer 16

Place a share or exclusive lock on individual row(s) in a table.

Question 17

Phantom Row

Answer 17

A transaction retrieves a collection of tuples twice and sees different results because another transaction added a new row that wasn’t previously part of a row level lock.