CONCURRENCY AND SCHEDULING

Question 1

What is concurrency?

Answer 1

Multiple transactions are allowed to run simultaneously in the system

Question 2

What are the advantages of using concurrency?

Answer 2

Increased processor and disk utilization: leading to better transaction throughput. One transaction can be using the CPU while another transaction reads from the disk.

Reduced average response time for transactions: Short transactions need not to wait for long ones

Question 3

What are the expectations of concurrency control?

Answer 3

Isolation is maintained: Users must be able to execute each transactions as if they were the only user.
DBMS handles the details of interleaving various TXNs

Consistency is maintained: Transactions must leave the DB in a consistent state
DBMS handles the details of enforcing integrity constraints

Question 4

What problems can be caused by concurrency?

Answer 4

Dirty Read/ Reading Uncommitted data
Lost update problem
Inconsistent analysis problem

Question 5

Describe dirty read

Answer 5

Occurs because of a WriteRead conflict
It occurs when one transaction is allowed to see the intermediate results of another transaction before it is committed.

*See doc for pic

Question 6

What is dirty data?

Answer 6

Data written by an uncommitted transaction due to system failure or rollback

Question 7

Describe the lost update problem

Answer 7

Occurs because of a Write Write conflict
An apparently successfully completed update operation by one user can be overridden by another user.

*See notes for pic

Question 8

Describe the inconsistent analysis problem

Answer 8

Occurs when a transaction reads several values form the database but a second transaction updates some of them during the execution of the first.

*See notes for pic

Question 9

What is a schedule?

Answer 9

Sequences that indicate the order in which instructions of concurrent transactions are executed

Question 10

What is a serial schedule?

Answer 10

A schedule where one transaction executes fully before the next transaction starts.
A serial schedule is one that does not interleave the actions of different transactions

Question 11

What is a non-serial schedule?

Answer 11

A schedule where two transactions are interleaved

Question 12

What situations may lead to the interleaving of transactions?

Answer 12

Individual Transactions might be slow- don’t want to block other users during execution

Disk access may be slow- let some transactions use CPUs while others accessing disk!

Question 13

What is the prerequisite for interleaving transactions?

Answer 13

DBMS must pick an interleaving or schedule such that isolation and consistency are maintained

Must be as if the transactions had executed serially!

Question 14

When is a schedule said to be not serializable?

Answer 14

An interleaved schedule is said to be not serializable if it gives different results from any serial schedule

Question 15

What are equivalent schedules?

Answer 15

A and B are equivalent schedules if, for any database state, the effect on DB of executing A is identical to the effect of executing B

Question 16

What is a serializable schedule?

Answer 16

A serializable schedule is a schedule that is equivalent to some serial execution of the transactions

Question 17

What is concurrency control?

Answer 17

The general process of ensuring that transactions preserve consistency, when executing simultaneously.
Interactions among concurrently executing transactions, can cause the database to become inconsistent, even when there is no system failure.
Thus, the timings of different transactions need to be regulated in some manner.

Question 18

What are the two main approaches to concurrency control?

Answer 18

Pessimistic approach: Locks

Optimistic Approach: Timestamps, multi-version, validation

Question 19

What is concurrency control

Answer 19

Concurrency control: The process of managing simultaneous operations on the database without having them interfere with one another.