Transaction Concurrency Control

Question 1

• any action that reads from or writes to a database.
• sequence of database requests that accesses the database.

Answer 1

Transaction

Question 2

a logical unit of work; that is, it must be entirely completed or aborted—no intermediate ending states are accepted

Answer 2

Transaction

Question 3

Transaction support is provided by two SQL statements

Answer 3

COMMIT and ROLLBACK.

Question 4

____ require that when a transaction sequence is initiated by a user or an
application program, the sequence must continue through all succeeding SQL statements

Answer 4

ANSI standards

Question 5

This keeps track of all transactions that update the database

Answer 5

transaction log

Question 6

A DBMS feature that coordinates the simultaneous execution of transactions in a
multiprocessing database system while preserving data integrity

Answer 6

Concurrency Control?

Question 7

Three main problems of Concurrency Control

Problem occurs when two concurrent transactions, T1 and T2, are updating the same
data element and one of the updates is lost (overwritten by the other transaction).

Answer 7

Lost Updates

Question 8

It occurs when two transactions, T1 and T2, are executed concurrently and the first transaction (T1) is rolled back after the second transaction (T2) has already accessed the uncommitted data—thus violating the isolation property of transactions.

Answer 8

Uncommitted Data

Question 9

It occur when a transaction accesses data before and after one or more other transactions finish working with such data.

Answer 9

Inconsistent Retrieval

Question 10

a special DBMS process that establishes the order in which the operations are
executed within concurrent transactions.

Answer 10

Scheduler

Question 11

The scheduler ________ the execution of database operations to ensure
serializability and isolation of transactions

Answer 11

interleaves

Question 12

scheduler’s main job

Answer 12

to create a serializable schedule of a transaction’s operations

Question 13

guarantees exclusive use of a data item to a current transaction.

Answer 13

lock

Question 14

The use of locks based on the assumption that conflict between transactions is likely usually referred to as

Answer 14

pessimistic locking

Question 15

issued when a transaction requests permission to update a data item and no locks are held on that data item by any other transaction. An exclusive lock does not allow other transactions to access the database

Answer 15

exclusive lock

Question 16

allows other read only transactions to access the database.

Answer 16

shared lock

Question 17

a condition in which only one transaction at a time can own an exclusive lock on the same object.

Answer 17

mutual exclusive rule

Question 18

assigns a global,
unique time stamp to each transaction.

Answer 18

time stamping

Question 19

ensures that no equal time stamp values can exist

Answer 19

Uniqueness

Question 20

ensures that time stamp values always increase.

Answer 20

monotonicity

Question 21

________ is based on the assumption that the majority of database operations do not conflict.

Answer 21

optimistic approach

Question 22

In this phase the transaction reads the database, executes the needed computations, and makes the updates to a private copy of the database values.

Answer 22

read phase

Question 23

During this phase, the transaction is validated to ensure that the changes made will not affect the integrity and consistency of the database.

Answer 23

validation phase

Question 24

During this phase, the changes are permanently applied to the database.

Answer 24

write phase

Question 24

The ____ defines transaction management based on transaction isolation levels.

Answer 24

ANSI SQL standard (1992)

Question 25

a transaction can read data that is not yet committed.

Answer 25

Dirty read

Question 26

a transaction reads a given row at time t1, and then it reads the same row at time t2, yielding different results. The original row may have been updated or deleted.

Answer 26

Nonrepeatable read

Question 27

a transaction executes a query at time t1, and then it runs the same query at time t2, yielding additional rows that satisfy the query.

Answer 27

Phantom read

Question 28

At this isolation level, the database does not place any locks on the data, which increases transaction
performance but at the cost of data consistency

Answer 28

Read Uncommitted

Question 29

forces transactions to read only committed data.

Answer 29

Read Committed

Question 30

isolation level ensures that queries return consistent results. This type of isolation level uses shared locks to ensure other transactions do not update a row after the original query reads.

Answer 30

The Repeatable Read

Question 31

restores a database from a given state (usually inconsistent) to a previously consistent state.

Answer 31

Database recovery

Question 32

all portions of the transaction must be treated as a single, logical unit of work in which all operations are applied and completed to produce a consistent database

Answer 32

atomic transaction property

Question 33

ensures that transaction logs are always written before any database data is actually updated.

Answer 33

write-ahead-log protocol

Question 34

(several copies of the transaction log) ensure that a physical disk failure will not impair the DBMS’s ability to recover data

Answer 34

Redundant transaction logs

Question 35

are temporary storage areas in primary memory used to speed up disk operations

Answer 35

Database buffers

Question 36

are operations in which the DBMS writes all of its updated buffers in memory (also known as dirty buffers) to disk.

Answer 36

Database checkpoints