Transaction management

Question 1

Interleaving of operations

Answer 1

mix (digital signals) by alternating between them.

Question 2

pessimistic locking

Answer 2

The use of locks based on the assumption that conflict between transactions is likely.

Question 3

three main transaction problems

Answer 3

lost updates, uncommitted data, and inconsistent retrievals

Question 4

lost updates

Answer 4

Question 5

uncommitted data

Answer 5

Question 6

inconsistent retrievals

Answer 6

Question 7

1. Explain the following statement: A transaction is a logical unit of work.

Answer 7

A transaction is a sequence of database operations that access the database. A trans- action is a logical unit of work; that is, all parts are executed or the transaction is aborted. A transaction takes a database from one consistent state to another.

Question 8

2. What is a consistent database state, and how is it achieved?

Answer 8

A consis- tent database state is one in which all data integrity constraints are satisfied.

Question 9

6. What is a transaction log, and what is its function?

Answer 9

The transaction log keeps track of all transactions that modify the database. The information stored in the transaction log is used for recovery (ROLLBACK) purposes

Question 10

4. List the four individual transaction properties.

Answer 10

Transactions have four main properties: atomicity, consistency, isolation, and dura- bility.

Question 11

Atomicity

Answer 11

Atomicity means that all parts of the transaction must be executed; otherwise, the transaction is aborted.

Question 12

Consistency

Answer 12

Consistency means that the database’s consistent state is maintained.

Question 13

Isolation

Answer 13

Isolation means that data used by one transaction cannot be accessed by another transaction until the first one is completed.

Question 14

Durability

Answer 14

Durability means that changes made by a transaction cannot be rolled back once the transaction is committed

Question 15

B. (2 points) What is two-phase locking (2PL)? Give an example to illustrate how deadlock may happen with two phase locking.

Answer 15

The two-phase locking schema has a growing phase, in which the transaction acquires all of the locks that it needs without unlocking any data, and a shrinking phase, in which the transaction releases all of the locks without acquiring new locks.

Question 16

What is a lock, and how does it work in general?

Answer 16

The scheduler uses locking to ensure the serializability of transactions.

A lock guarantees unique access to a data item by a transaction. The lock prevents one transaction from using the data item while another transaction is using it

Question 17

9. What are the different levels of lock granularity?

Answer 17

database, table, page, row, and field

Question 18

11. What is concurrency control, and what is its objective?

Answer 18

Concurrency control coordinates the simultaneous execution of transactions. The objective of concurrency control is to ensure the serializability of transactions in a multiuser database environment

Question 19

serializability

Answer 19

the result of the concurrent execution of transactions is the same as that of the transactions being executed in serial order.

Question 20

12. What is an exclusive lock, and under what circumstances is it granted?

Answer 20

An exclusive lock is issued when a transaction wants to update (write to) the database and no other locks (shared or exclusive) are held on the data.

Question 21

binary lock

Answer 21

binary lock can have only two states: locked (1) or unlocked (0).

Question 22

shared lock

Answer 22

A shared lock is used when a transac- tion wants to read data from a database and no other transaction is updating the same data. Several shared or “read” locks can exist for a particular item

Question 23

13. What is a deadlock, and how can it be avoided? Discuss several strategies for dealing with deadlocks.

Answer 23

When two or more transactions wait indefinitely for each other to release a lock, they are in a deadlock, also called a deadly embrace. There are three deadlock control techniques: prevention, detection, and avoidance.

Question 24

Two-Phase Locking

Answer 24

Question 25

Difference between lost update and uncommitted data

Answer 25

• In uncomitted data one transaction must have accessed uncommited data from another transaction. In lost update this doesn’t happens