Transaction Processing

Question 1

what is transaction Processing

Answer 1

A mechanism for describing logical units of database processing.

Systems with large databases and hundreds of concurrent users

Question 2

give examples of transaction processing systems (3)

Answer 2

airline reservations

banking

credit card processing

Question 3

describe single vs multi user

Answer 3

single- only one access at one time

multi- many users accessing the database

Question 4

what is interleaving

Answer 4

processes are interleaved to achieve the illusion of concurrency

Question 5

what are transactions

Answer 5

executing programs

Question 6

what is a DBMS buffer

Answer 6

In ram, contents swapped out to disk when full

using LRU

Question 7

what is concurrency

Answer 7

several accesses at the same time are likely to allow concurrent access

Question 8

what are the problems caused by concurrency (4)

Answer 8

lost update

unrepeatable read

dirty read/temporary update

incorrect summary

Question 9

what is the lost update problem

Answer 9

2 transactions access the same items such that a value becomes incorrect

Question 10

what is the dirty read/temporary update problem

Answer 10

a process will read a value when it is in a state. Then the transaction that set that will fail and it will be reverted back.
but the other process has already operated on the incorrect data

Question 11

what is the unrepeatable read problem

Answer 11

a process will read a value and read it again and it will be different each time

Question 12

what is the incorrect summary problem

Answer 12

one of the transactions is executing an aggregate operation on several data items, and other transactions are updating one or more of those data items. This causes a inconsistent database state

Question 13

what risks are there to transactions

Answer 13

computer failure

physical problems

transaction error

logical error or exception

concurrency control enforcement

Question 14

what happens if a transaction aborts

Answer 14

roll back

Question 15

what are the transaction states

Answer 15

active
partially committed
failed
committed
aborted
terminated

Question 16

what does it mean to say a transaction is atomic

Answer 16

it will either complete or it will not

Question 17

what are the desirable properties of a transaction (ACID)

Answer 17

atomic- either completes or doesn’t

consistency preservation- state to state

isolated- appear to work as though isolated

durability or permanency- changes made by a transaction must not be lost

Question 18

what is a schedule

Answer 18

consists of a number of transactions with an ordering of operations

Question 19

what are the 3 conditions of conflicting operations

Answer 19

one is write
different transactions
access same variable

Question 20

what is a complete schedule

Answer 20

operations are exactly T1 to N
for 2 operations in T, order in S is preserved
for 2 conflicting operations, one occurs before the other

Question 21

what is the easy approach to create a serialised schedule

Answer 21

each transaction runs after another in full

Question 22

when is a schedule serial

Answer 22

if it is equivaled to some serial schedule of the same n transactions

Question 23

how many possible serial schedules are there

Answer 23

n!

Question 24

what is result equivalence

Answer 24

if two schedules produce the same result

Question 25

what is conflict equivalence

Answer 25

the relative order of any two conflicting operations is the same

Question 26

what is non conflicting equivalence

Answer 26

when conflicting operations are applied in different orders in two schedules

Question 27

how do we graph transactions

Answer 27

have a graph for each transaction. if they conflict, draw a line from one to the other.
r(x) -> w(x)
w(x) -> r(x)
w(x) -> w(x)
if there is a cycle, the schedule is not serializable