Two marks Questions with Answers

Two Marks Questions with Answers

Q.1 What is a transaction?    AU: May-04, Dec.05

Ans: A transaction can be defined as a group of tasks that form a single logical unit.

Q.2 What does time to commit mean?  AU: May-04

Ans:

• The COMMIT command is used to save permanently any transaction to database.

• When we perform, Read or Write operations to the database then those changes can be undone by rollback operations. To make these changes permanent, we should make use of commit

Q.3 What are the various properties of transaction that the database system maintains to ensure integrity of data.  AU: Dec.04

OR

Q.4 What are ACID properties?   AU: May-05,06,08,13,15,  Dec-07,14,17

Ans: In a database, each transaction should maintain ACID property to meet the consistency and integrity of the database. These are

(1) Atomicity (2) Consistency (3) Isolation (4) Durability

Q.5 Give the meaning of the expression ACID transaction.   AU: Dec.08

Ans: The expression ACID transaction represents the transaction that follows the ACID Properties.

Q.6 State the atomicity property of a transaction.  AU: May-09,13

Ans:This property states that each transaction must be considered as a single unit and must be completed fully or not completed at all.

No transaction in the database is left half completed.

Q.7 What is meant by concurrency control ? AU: Dec.15

Ans:

A mechanism which ensures that simultaneous execution of more than one transactions does not lead to any database inconsistencies is called concurrency control mechanism.

Q.8 State the need for concurrency control.   AU: Dec.17

OR

Q.9 Why is it necessary to have control of concurrent execution of transactions? How is it made possible?    AU: Dec.-02

Ans: Following are the purposes of concurrency control-

• To ensure isolation

• To resolve read-write or write-write conflicts

• To preserve consistency of database

Q.10 List commonly used concurrency control techniques. AU: Dec.11

Ans: The commonly used concurrency control techniques are -

i) Lock

ii) Timestamp

iii) Snapshot Isolation

Q.11 What is meant by serializability? How it is tested?   AU: May-14,18, Dec.-14,16

Ans.: Serializability is a concept that helps to identify which non serial schedule and find the transaction equivalent to serial schedule.

It is tested using precedence graph technique.

Q.12 What is serializable schedule?   AU: May-17

Ans.: The schedule in which the transactions execute one after the other is called serial schedule. It is consistent in nature. For example : Consider following two transactions T₁ and T₂

All the operations of transaction T₁ on data items A and then B executes and then in transaction T₂ all the operations on data items A and B execute. The R stands for Read operation and W stands for write operation.

Q.13 When are two schedules conflict equivalent?  AU: Dec.08

Ans.: Two schedules are conflict equivalent if :

• They contain the same set of the transaction.

• every pair of conflicting actions is ordered the same way.

For example -

Schedule S₂ is a serial schedule because, in this, all operations of T₁ are performed before starting any operation of T₂. Schedule S₁ can be transformed into a serial schedule by swapping non-conflicting operations of S₁.

Hence both of the above the schedules are conflict equivalent.

Q.14 Define two phase locking.   AU: May-13

Ans.: The two phase locking is a protocol in which there are two phases:

i) Growing Phase (Locking Phase): It is a phase in which the transaction may obtain locks but does not release any lock.

ii) Shrinking Phase (Unlocking Phase): It is a phase in which the transaction may release the locks but does not obtain any new lock.

Q.15 What is the difference between shared lock and exclusive lock? AU: May-18

Ans.:

Q.16 What type of lock is needed for insert and delete operations. AU: May-17

Ans.: The exclusive lock is needed to insert and delete operations.

Q.17 What benefit does strict two-phase locking provide? What disadvantages result?   AU: May-06,07, Dec.07

Ans.:

Benefits:

1. This ensure that any data written by an uncommitted transaction are locked in exclusive mode until the transaction commits and preventing other transaction from reading that data.

2. This protocol solves dirty read problem.

Disadvantage:

1. Concurrency is reduced.

Q.18 What is rigorous two phase locking protocol ?   AU: Dec.-13

Ans.: This is stricter two phase locking protocol. Here all locks are to be held until the transaction commits.

Q.19 Differentiate strict two phase locking and rigourous two phase locking protocol.   AU: May-16

Ans. :

• In Strict two phase locking protocol all the exclusive mode locks be held until the transaction commits.

• The rigourous two phase locking protocol is stricter than strict two phase locking protocol. Here all locks are to be held until the transaction commits

Q.20 Define deadlock.    AU: May-08,09,14

Ans.: Deadlock is a situation in which when two or more transactions have got a lock and waiting for another locks currently held by one of the other transactions.

Q.21 List four conditions for deadlock.   AU: Dec-16

Ans.: 1. Mutual exclusion condition                

2. Hold and wait condition      

3. No preemption condition             

4. Circular wait condition

Q.22 Why is recovery needed?  AU: May-09

Ans. :

• A recovery scheme that can restore the database to the consistent state that existed before the failure.

• Due to recovery mechanism, there is high availability of database to its users.

Q.23 What are states of transaction?    AU: May-19

Ans.: Various states of transaction are - (1) Active, (2) Partially Committed (3) Failed (4) Aborted (5) Committed.

Q.24 What is meant by log based recovery?

Ans.: Log is a most commonly used data structure for recording the modifications that can be made to actual database.

Log based recovery is a technique in which a log of each transaction is maintained in some stable storage so that if failure occurs then it can be recovered from there.

Q.25 List the responsibilities of a DBMS has whenever a transaction is submitted to the system for execution.       AU: Dec.-19

Ans.: The system is responsible for making sure that - (1) Either all the operations in the transaction are completed successfully and effect is recorded permanently in the database. (2) The transaction, has no effect whatsoever on the database or on the database or on any other transaction.

Q.26 Brief any two violations that may occur if a transaction executes a lower isolation level than serializable.     AU: Dec.-19

Ans.: (1) For non-repeatable Read the phantom read is allowed.

For read committed non-repeatable reads and phantom reads are allowed.