Week 3 - Transactions and Concurrency

Question 1

transactions - What is a schedule?

Answer 1

A schedule is
an organization of the transactions that
interleave the actions of these transactions from a database perspective.

Question 2

Serial Schedule

Answer 2

schedule that does NOT interleave the actions of different transactions

Question 3

Equivalent Schedule

Answer 3

For any database state, the effect of executing the first schedule is identical to the effect of executing the second schedule

Question 4

Serializable Schedule

Answer 4

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

Question 5

Two Schedule are in Conflict equivalent if (2 items)

Answer 5

1) Involve the same actions of the same transactions

2) Every pair of conflicting actions is ordered the same way

Question 6

Schedule S is conflict serializable if ?

Answer 6

S is conflict equivalent to some serial schedule

Question 7

What is a conflicting action

Answer 7

A conflicting action is when a transaction is
writing into something and the other transaction wants to read that data item.

Because that might lead to confusion,
might lead to inconsistency

Question 8

When are two schedules conflict equivalent?

1) If they have the same actions of the same transactions.
2) If they have the same actions of two different transactions but are ordered the same in both schedules.
3) If they have the same actions of the same transactions, and every pair of conflicting actions has the same order in both schedules.

Answer 8

3) If they have the same actions of the same transactions, and every pair of conflicting actions has the same order in both schedules.

Question 9

Dependency Graph

Answer 9

Has transaction T2 written or read any item that has been last written by transaction T1

(bad banking example)

Question 10

What is the result of the following database transaction if the initial values of both A and B is 100?

BEGIN

A=A+100A=A+100A=A+100

B=A+100B=A+100B=A+100

END

1) A=200, B=200
2) A=100, B=200
3) A=200, B=100
4) A=200, B=300

Answer 10

4) A=200, B=300

Question 11

Consider the following two database transactions with initial values of A=500 and B=500:

T1:A=A+100,B=A−100T1: A=A+100, B=A-100T1:A=A+100,B=A−100

T2:A=A∗1.06,B=B∗1.06T2: A=A1.06, B=B1.06T2:A=A∗1.06,B=B∗1.06

Assuming T1 arrives first, what will be the final values of A and B if the two transactions are processed using a serial schedule?

1) A=636, B=568.16
2) A=630, B=561.80
3) A=636, B=530
4) A=630, B=530

Answer 11

3) A=636, B=530

Question 12

Consider the following two database transactions with initial values of A=500 and B=500:

T1:A=A+100,B=A−100T1: A=A+100, B=A-100T1:A=A+100,B=A−100

T2:A=A∗1.06,B=B∗1.06T2: A=A1.06, B=B1.06T2:A=A∗1.06,B=B∗1.06

Assuming T1 arrives first, what will be the final values of A and B if the database management system interleaves transactions?

1) A=630, B=530
2) A=636, B=568.16
3) A=630, B=561.80
4) A=636, B=530

Answer 12

2) A=636, B=568.16