Session – 11

CONCURRENCY CONTROL

USER ACCESS CONTROL

Matakuliah : M0184 / Pengolahan Data DistribusiTahun : 2005Versi :

OBJECTIVE

• Basic Transaction Concept• Transaction as the basic unit of work in

DBMS• Transaction Types

TRANSACTION

• The concept of a transaction is used within the database domain as a basic unit of consistent and reliable computing

• A database in consistent state if it obey all of consistency (integrity) constraint defined over it

• Transaction consistency refers to the action of concurrent transaction.

• The module responsible for concurrency control in DBMS known as a scheduler, since its job is to schedule transactions correctly to avoid interference

Transaction Model

Begin transaction T

End transaction TExecution of

transaction T

DB in consistent state

DB in consistent state

DB may temporarily in an inconsistent way during execution

Four basic properties of transaction

• Atomicity, the “all or nothing” property; a transaction is an indivisible unit

• Consistency, transaction transform the DB from one consistent state to another consistent state

• Independence, transaction execute independently of one another

• Durability, the effects of a successfully completed transaction are permanently recorded in the DB and can not be undone

Transaction in Application Program

• The execution of an application program in a database environment can be viewed as a series of atomic transaction with non-database process taking place in between

Transaction in Application Program

ProgramStart

Program End

Time

TransactionT1

TransactionT2

TransactionT3

Non-Database

Distributed Transaction

• Transaction may access data stored at more than one site

• Each transaction is divided into a number of sub-transactions

Distributed Transaction Example

Begin transaction T1Begin Transaction T1(A)

read balance(x)balance(x) = balance(x) – 100if balance(x) < - then

print “No Fund”abort T1(A)

end ifwrite balance(x)

commit T1(A)

Distributed Transaction Example

Begin Transaction T1(B)read balance(y)balance(y) = balance(y) + 100write balance(y)

commit T1(B)Commit T1