Chapter 9. Transaction Management and Concurrency Control. Database Systems: Design, Implementation, and Management, Sixth Edition, Rob and Coronel

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1 Chapter 9 Transaction Management and Concurrency Control Database Systems: Design, Implementation, and Management, Sixth Edition, Rob and Coronel 1

2 In this chapter, you will learn: What a database transaction is and what its properties are How database transactions are managed What concurrency control is and what role it plays in maintaining the database s integrity What locking methods are and how they work How database recovery management is used to maintain database integrity 2

3 What is a Transaction? Any action that reads from and/or writes to a database may consist of Simple SELECT statement to generate a list of table contents A series of related UPDATE statements to change the values of attributes in various tables A series of INSERT statements to add rows to one or more tables A combination of SELECT, UPDATE, and INSERT statements 9 3

4 What is a Transaction? (continued) A logical unit of work that must be either entirely completed or aborted Successful transaction changes the database from one consistent state to another One in which all data integrity constraints are satisfied Most real-world database transactions are formed by two or more database requests The equivalent of a single SQL statement in an application program or transaction 4

5 The Relational Schema for the Ch09_SaleCo Database 9 5

6 Evaluating Transaction Results Not all transactions update the database SQL code represents a transaction because database was accessed Improper or incomplete transactions can have a devastating effect on database integrity Some DBMSs provide means by which user can define enforceable constraints based on business rules Other integrity rules are enforced automatically by the DBMS when table structures are properly defined, thereby letting the DBMS validate some transactions 9 6

7 Tracing the Transaction in the Ch09_SaleCo Database 9 Figure 9.2 7

8 Atomicity Transaction Properties Requires that all operations (SQL requests) of a transaction be completed Durability Indicates permanence of database s consistent state 8

9 Transaction Properties (continued) 9 Serializability Ensures that the concurrent execution of several transactions yields consistent results Isolation Data used during execution of a transaction cannot be used by second transaction until first one is completed 9

10 Transaction Management with SQL ANSI has defined standards that govern SQL database transactions Transaction support is provided by two SQL statements: COMMIT and ROLLBACK ANSI standards require that, when a transaction sequence is initiated by a user or an application program, it must continue through all succeeding SQL statements until one of four events occurs 10

11 The Transaction Log 9 Stores A record for the beginning of transaction For each transaction component (SQL statement) Type of operation being performed (update, delete, insert) Names of objects affected by the transaction (the name of the table) Before and after values for updated fields Pointers to previous and next transaction log entries for the same transaction The ending (COMMIT) of the transaction 11

12 A Transaction Log 9 12

13 Concurrency Control Coordination of simultaneous transaction execution in a multiprocessing database system Objective is to ensure transaction serializability in a multiuser database environment 13

14 Concurrency Control Important simultaneous execution of transactions over a shared database can create several data integrity and consistency problems lost updates uncommitted data inconsistent retrievals 14

15 Normal Execution of Two Transactions 15

16 Lost Updates 16

17 Correct Execution of Two Transactions 17

18 An Uncommitted Data Problem 18

19 Retrieval During Update 19

20 Transaction Results: Data Entry Correction 9 20

21 Inconsistent Retrievals 21

22 The Scheduler Special DBMS program: establishes order of operations within which concurrent transactions are executed Interleaves the execution of database operations to ensure serializability and isolation of transactions 22

23 The Scheduler (continued) Bases its actions on concurrency control algorithms Ensures computer s central processing unit (CPU) is used efficiently Facilitates data isolation to ensure that two transactions do not update the same data element at the same time 23

24 Read/Write Conflict Scenarios: Conflicting Database Operations Matrix 9 24

25 Lock Concurrency Control with Locking Methods Guarantees exclusive use of a data item to a current transaction Required to prevent another transaction from reading inconsistent data 9 Lock manager Responsible for assigning and policing the locks used by the transactions 25

26 Lock Granularity Indicates the level of lock use Locking can take place at the following levels: Database Table Page Row Field (attribute) 26

27 Lock Granularity (continued) Database-level lock Entire database is locked Table-level lock Entire table is locked Page-level lock Entire diskpage is locked 27

28 Row-level lock Lock Granularity (continued) Allows concurrent transactions to access different rows of the same table, even if the rows are located on the same page Field-level lock Allows concurrent transactions to access the same row, as long as they require the use of different fields (attributes) within that row 28

29 A Database-Level Locking Sequence 29

30 An Example of a Table-Level Lock 30

31 Example of a Page-Level Lock 31

32 An Example of a Row-Level Lock 32

33 Binary lock Lock Types Has only two states: locked (1) or unlocked (0) Exclusive lock Access is specifically reserved for the transaction that locked the object Must be used when the potential for conflict exists Shared lock Concurrent transactions are granted Read access on the basis of a common lock 9 33

34 An Example of a Binary Lock 34

35 Two-Phase Locking to Ensure Serializability Defines how transactions acquire and relinquish locks Guarantees serializability, but it does not prevent deadlocks Growing phase, in which a transaction acquires all the required locks without unlocking any data Shrinking phase, in which a transaction releases all locks and cannot obtain any new lock 35

36 Two-Phase Locking to Ensure Serializability (continued) 9 Governed by the following rules: Two transactions cannot have conflicting locks No unlock operation can precede a lock operation in the same transaction No data are affected until all locks are obtained that is, until the transaction is in its locked point 36

37 Two-Phase Locking Protocol 37

38 Deadlocks Condition that occurs when two transactions wait for each other to unlock data Possible only if one of the transactions wants to obtain an exclusive lock on a data item No deadlock condition can exist among shared locks Control through Prevention Detection Avoidance 38

39 How a Deadlock Condition Is Created 39

40 Concurrency Control with Time Stamping Methods 9 Assigns a global unique time stamp to each transaction Produces an explicit order in which transactions are submitted to the DBMS Uniqueness Ensures that no equal time stamp values can exist Monotonicity Ensures that time stamp values always increase 40

41 Wait/Die and Wound/Wait Schemes Wait/die Older transaction waits and the younger is rolled back and rescheduled Wound/wait Older transaction rolls back the younger transaction and reschedules it 41

42 Wait/Die and Wound/Wait Concurrency Control Schemes 9 42

43 Concurrency Control with Optimistic Methods 9 Optimistic approach Based on the assumption that the majority of database operations do not conflict Does not require locking or time stamping techniques Transaction is executed without restrictions until it is committed Phases are read, validation, and write 43

44 Database Recovery Management Database recovery Restores database from a given state, usually inconsistent, to a previously consistent state Based on the atomic transaction property All portions of the transaction must be treated as a single logical unit of work, in which all operations must be applied and completed to produce a consistent database If transaction operation cannot be completed, transaction must be aborted, and any changes to the database must be rolled back (undone) 44

45 Transaction Recovery Makes use of deferred-write and writethrough Deferred write Transaction operations do not immediately update the physical database Only the transaction log is updated Database is physically updated only after the transaction reaches its commit point using the transaction log information 45

46 Transaction Recovery (continued) Write-through Database is immediately updated by transaction operations during the transaction s execution, even before the transaction reaches its commit point 46

47 A Transaction Log for Transaction Recovery Examples 9 47

48 Transaction Summary Sequence of database operations that access the database Represents real-world events Must be a logical unit of work No portion of the transaction can exist by itself Takes a database from one consistent state to another One in which all data integrity constraints are satisfied 48

49 Summary (continued) SQL provides support for transactions through the use of two statements: COMMIT and ROLLBACK Concurrency control coordinates the simultaneous execution of transactions Scheduler is responsible for establishing order in which concurrent transaction operations are executed 49

50 Summary (continued) Lock guarantees unique access to a data item by a transaction Database recovery restores the database from a given state to a previous consistent state 50

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