Transaction Processing Dr Fawaz Alarfaj Al Imam Mohammed Ibn Saud Islamic University

Transcription

1 Processing Dr Fawaz Alarfaj Al Imam Mohammed Ibn Saud Islamic University ACKNOWLEDGEMENT Slides are adopted from: Elmasri & Navathe, Fundamentals of Database Systems Single-User vs. Multi-User System A DBMS is single-user if at most one user at a time can use the system A DBMS is multi-user if many user can use the system concurrently 2 A is an executing program that forms a logical unit of database processing. Includes : Deletion Modification Retrieval operations Insertion 3

2 Example: Khaled needs to transfer SR 700 from his bank account (412181) to his landlord's bank account (343213) EXEC transfer (412181, , 700) 4 CREATE PROCEDURE transfer(accounta CHAR(10), accountb CHAR(10), amount DECIMAL(12,2)) BEGIN DECLARE currentbalance DECIMAL(12,2); SELECT balance INTO currentbalance FROM account WHERE account.accountnumber = accounta; read(account) IF (currentbalance > amount) THEN UPDATE account SET account.balance = balance - amount WHERE account.accountnumber = accounta; write(account) UPDATE account SET account.balance = balance + amount WHERE account.accountnumber = accountb; ENDIF END write(account) 5 A transaction is a sequence of operations that form a single unit of work A transaction is often initiated by an application program begin a transaction: START TRANSACTION end a transaction: COMMIT or ROLLBACK A transaction T i transforms one consistent database state into another consistent database state 6

3 Concurrent users } Airline reservations Banking & credit card processing Online retail purchasing Supermarket checkouts fast response time high availability transaction processing systems 7 Concurrency Control Process of managing simultaneous operations on the database without having them interfere with one another. 8 Why concurrency Control Several problems can occur when concurrent transactions execute in an uncontrolled manner. Example: X : DB item (flight) Y : DB item (flight) N: number of records (reservations) M: number of records (reservations) 9

4 Why concurrency Control T1 Salem Hamad Ali {Sara X } N Mohammed Abeer Malik Saad S352 S353 S354 S355 } Y X Salem {Hamad N{ Mohammed Abeer Malik Saad Ali Sara S352 S353 S354 S355 }Y 10 Why concurrency Control T2 Salem Hamad Ali {Sara X Khaled Alaa Basel S360 S361 S362 } M M{ Salem Hamad Ali Sara Khaled Alaa Basel S360 S361 S362 }X The Lost Update Problem 2. The Temporary Update (or Dirty Read) Problem 3. The Incorrect Summary Problem 4. The Unrepeatable Read Problem 12

5 1. The Lost Update Problem This problem occurs when two transactions that access the same database items have their operations interleaved in a way that makes the value of some database items incorrect The Lost Update Problem The Lost Update Problem T1 T2 Salem {X Hamad Ali Sara X Salem {Hamad N N { Moham S352 Abeer S353 Malik S354 Saad S355 Moham Abeer Malik Saad Ali Sara S352 S353 S354 S355 Y Y Salem {X Hamad Ali Sara Salem Hamad Ali Sara { Khaled S360 M Alaa S361 Basel S362 Khaled Alaa Basel S360 S361 S362 X M 15

6 2. Temporary Update (or Dirty Read) Problem This problem occurs when one transaction updates a database item and then the transaction fails for some reason. Meanwhile, the updated item is accessed (read) by another transaction before it is changed back to its original value Temporary Update (or Dirty Read) Problem The Incorrect Summary Problem If one transaction is calculating an aggregate summary function on a number of database items while other transactions are updating some of these items, the aggregate function may calculate some values before they are updated and others after they are updated. 18

7 3. The Incorrect Summary Problem The Unrepeatable Read Problem If a transaction T reads the same item twice and the item is changed by another transaction T between the two reads. 20 Quiz 21

8 recovery A transaction is an atomic unit of work that should either be completed in its entirety or not done at all. 22 recovery The DBMS must not permit some operations of a transaction T to be applied to the database while other operations of T are not. If a transaction fails after executing some of its operations, the operations already executed must be undone and have no lasting effect. 23 recovery Types of Failures 1. A computer failure (system crash) 2. A transaction or system error 3. Local errors or exception conditions detected by the transaction 4. Concurrency control enforcement 5. Disk failure 6. Physical problems and catastrophes 24

9 Desirable Properties of s should possess several properties, often called the ACID properties. The following are the ACID properties: Atomicity Consistency preservation. Isolation Durability or permanency. 25 Desirable Properties of Atomicity A transaction is an atomic unit of processing; it should either be performed in its entirety or not performed at all. 26 Desirable Properties of Consistency preservation A transaction either creates a new and valid state of data, or, if any failure occurs, returns all data to its state before the transaction was started. 27

10 Desirable Properties of Isolation The execution of a transaction should not be interfered with by any other transactions executing concurrently. 28 Desirable Properties of Durability The changes applied to the database by a committed transaction must persist in the database. These changes must not be lost because of any failure. 29 Quiz 30

11 States Active initial state transaction is in this state while executing Partially committed after the last statement has been executed Committed after successful completion Failed after discovery that a normal execution is no longer possible Aborted after the rollback of a transaction 31 States 32