Transaction Management and Concurrency Control

2

• Logical unit of work • Must be either entirely completed or aborted• No intermediate states are acceptable

What is a Transaction?

Figure 9.1

3

• Examine current account balance

• Consistent state after transaction• No changes made to Database

Example Transaction

SELECT ACC_NUM, ACC_BALANCEFROM CHECKACCWHERE ACC_NUM = ‘0908110638’;

4

• Register credit sale of 100 units of product X to customer Y for $500

• Consistent state only if both transactions are fully completed

• DBMS doesn’t guarantee transaction represents real-world event

Example Transaction

UPDATE PRODUCTSET PROD_QOH = PROD_QOH - 100WHERE PROD_CODE = ‘X’;UPDATE ACCT_RECEIVABLESET ACCT_BALANCE = ACCT_BALANCE + 500WHERE ACCT_NUM = ‘Y’;

5

• Atomicity – All transaction operations must be completed– Incomplete transactions aborted

• Durability – Permanence of consistent database state

• Consistency / Serializability – Conducts transactions in serial order– Important in multi-user and distributed databases

• Isolation – Transaction data cannot be reused until its execution

complete

Transaction Properties (ACID)

6

Example of Fund Transfer

• Transaction to transfer $50 from account A to account B:1. read(A)

2. A := A – 50

3. write(A)

4. read(B)

5. B := B + 50

6. write(B)

7

Example of Fund Transfer

• Consistency requirement – the sum of A and B is unchanged by the execution of the transaction.

• Atomicity requirement — if the transaction fails after step 3 and before step 6, the system should ensure that its updates are not reflected in the database, else an inconsistency will result.

8

Example of Fund Transfer

• Durability requirement — once the user has been notified that the transaction has completed (i.e., the transfer of the $50 has taken place), the updates to the database by the transaction must persist despite failures.

9

Example of Fund Transfer

• Isolation requirement — if between steps 3 and 6, another transaction is allowed to access the partially updated database, it will see an inconsistent database

(the sum A + B will be less than it should be).Can be ensured by running transactions serially, that is one after the other.

10

Transaction state

• Active– the initial state; the transaction stays in this state

while it is executing• Partially committed

– after the final statement has been executed.• Failed

– after the discovery that normal execution can no longer proceed.

11

Transaction state

• Aborted – after the transaction has been rolled back and the

database restored to its state prior to the start of the transaction. Two options after it has been aborted:

• restart the transaction – only if no internal logical error

• kill the transaction

• Committed, after successful completion.

12

DBMS Transaction Subsystem

Transaction Manager

Scheduler/ Lock

Manager

Recovery Manager

Buffer Manager

File Manager

Access Manager

Systems Manager

Database and system

catalog

Database Manager

13

DBMS Transaction Subsystem

• Trans. Mgr. – coordinates transactions on behalf of application

program. It communicates with scheduler.

• Scheduler – implements a strategy for concurrency control.

• Recovery manager – If any failure occurs, recovery manager handles it.

• Buffer manager – in charge of transferring data between disk storage and

main memory.

14

DBMS Transaction Subsystem

• File manager – manipulates the underlying storage files and manages

the allocation of storage space on disk.

• Access manager – File manager does not directly manage the physical

input and output of data, rather it passes the requests on to the access manager.

• System manager – Appropriate access method is used to either read or

write data into the system manager.

15

• Transaction support– COMMIT

– ROLLBACK

• User initiated transaction sequence must continue until: – COMMIT statement is reached

– ROLLBACK statement is reached

– End of a program reached

– Program reaches abnormal termination

Transaction Management with SQL

16

• Tracks all transactions that update database• May be used by ROLLBACK command• May be used to recover from system failure• Log stores

– Record for beginning of transaction– Each SQL statement

• Operation• Names of objects• Before and after values for updated fields• Pointers to previous and next entries

– Commit Statement

Transaction Log

17

Transaction Log Example

Table 9.1

18

• Coordinates simultaneous transaction execution in multiprocessing database– Ensure serializability of transactions in multiuser

database environment

– Potential problems in multiuser environments• Lost updates• Uncommitted data• Inconsistent retrievals

Concurrency Control

19

Lost Updates

Table 9.2

Table 9.3

20

Uncommitted Data

Table 9.5

Table 9.4

21

Inconsistent Retrievals

Table 9.6

Table 9.7

22

Inconsistent Retrievals (con’t.)

Table 9.8

23

• Establishes order of concurrent transaction execution

• Interleaves execution of database operations to ensure serializability

• Bases actions on concurrency control algorithms– Locking

– Time stamping

• Ensures efficient use of computer’s CPU

The Scheduler

24

Read/Write Conflict Scenarios:Conflicting Database Operations Matrix

Table 9.9

25

Concurrency Control with Locking Methods

• Lock guarantees current transaction exclusive use of data item

• Acquires lock prior to access• Lock released when transaction is

completed • DBMS automatically initiates and enforces

locking procedures• Managed by lock manager• Lock granularity indicates level of lock use

26

Database-Level Locking Sequence

Figure 9.2

27

Table-Level Lock Example

Figure 9.3

28

Page-Level Lock Example

Figure 9.4

29

Row-Level Lock Example

Figure 9.5

30

Field Level Lock

• Access same row as long as they require different fields (attributes) within that row

31

Lock Types

• Binary Locks• Shared/Exclusive Locks

32

• Two states– Locked (1)

– Unlocked (0)

• Locked objects unavailable to other objects– Unlocked objects open to any transaction

– Transaction unlocks object when complete

Binary Locks

33

Example of Binary Lock Table

Table 9.10

34

Shared/Exclusive Locks

• Shared– Exists when concurrent transactions granted READ

access – Produces no conflict for read-only transactions– Issued when transaction wants to read and exclusive

lock not held on item

• Exclusive– Exists when access reserved for locking transaction– Used when potential for conflict exists– Issued when transaction wants to update unlocked

data

35

Problems with Locking

• Transaction schedule may not be serializable– Managed through two-phase locking

• Schedule may create deadlocks– Managed by using deadlock detection and

prevention techniques

36

Two-Phase Locking

• Growing phase• Shrinking phase• Governing 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

37

Two-Phase Locking Protocol

Figure 9.6

38

Deadlocks

• Occurs when two transactions wait for each other to unlock data

• Called deadly embrace• Control techniques

– Deadlock prevention

– Deadlock detection

– Deadlock avoidance

39

How Deadlock Conditions Created

Table 9.11

40

Concurrency Control with Time Stamping Methods

• Assigns global unique time stamp to each transaction• Produces order for transaction submission• Properties

– Uniqueness– Monotonicity

• DBMS executes conflicting operations in time stamp order

• Each value requires two additional time stamps fields– Last time field read– Last update

41

Concurrency Control with Optimistic Methods

• Assumes most database operations do not conflict

• Transaction executed without restrictions until committed

• Phases:– Read Phase

– Validation Phase

– Write Phase

42

• Restores a database to previously consistent state

• Based on the atomic transaction property• Level of backup

– Full backup

– Differential

– Transaction log

Database Recovery Management

43

• Software• Hardware• Programming Exemption• Transaction• External

Causes of Database Failure

44

• Deferred-write and Deferred-update– Changes are written to the transaction log

– Database updated after transaction reaches commit point

• Write-through– Immediately updated by during execution

– Before the transaction reaches its commit point

– Transaction log also updated

– Transaction fails, database uses log information

to ROLLBACK

Transaction Recovery