Week 6 - Discussion Forum HSIN

profileHeathersimf
HSCI6348-HealthcareDatabaseManagement_Chapter10-Tagged.pdf

Database Systems: Design, Implementation, and

Management Tenth Edition

Chapter 10 Transaction Management and Concurrency Control

Objectives

• In this chapter, you will learn: – About database transactions and their properties – What concurrency control is and what role it

plays in maintaining the database’s integrity

– What locking methods are and how they work

Database Systems, 10th Edition 2

Objectives (cont’d.)

– How stamping methods are used for concurrency control

– How optimistic methods are used for concurrency control

– How database recovery management is used to maintain database integrity

Database Systems, 10th Edition 3

What Is a Transaction?

• Logical unit of work that must be either entirely completed or aborted

• Successful transaction changes 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 – Equivalent of a single SQL statement in an

application program or transaction

Database Systems, 10th Edition 4

Database Systems, 10th Edition 5

Evaluating Transaction Results

• Not all transactions update database • SQL code represents a transaction because

database was accessed • Improper or incomplete transactions can have

devastating effect on database integrity – Some DBMSs provide means by which user can

define enforceable constraints

– Other integrity rules are enforced automatically by the DBMS

Database Systems, 10th Edition 6

Database Systems, 10th Edition 7

Figure 9.2

Transaction Properties

• Atomicity – All operations of a transaction must be

completed

• Consistency – Permanence of database’s consistent state

• Isolation – Data used during transaction cannot be used by

second transaction until the first is completed

Database Systems, 10th Edition 8

Transaction Properties (cont’d.)

• Durability – Once transactions are committed, they cannot

be undone

• Serializability – Concurrent execution of several transactions

yields consistent results

• Multiuser databases are subject to multiple concurrent transactions

Database Systems, 10th Edition 9

Transaction Management with SQL

• ANSI has defined standards that govern SQL database transactions

• Transaction support is provided by two SQL statements: COMMIT and ROLLBACK

• Transaction sequence must continue until: – COMMIT statement is reached

– ROLLBACK statement is reached

– End of program is reached

– Program is abnormally terminated

Database Systems, 10th Edition 10

The Transaction Log

• Transaction log stores: – A record for the beginning of transaction – For each transaction component:

• Type of operation being performed (update, delete, insert)

• Names of objects affected by transaction • “Before” and “after” values for updated fields

• Pointers to previous and next transaction log entries for the same transaction

– Ending (COMMIT) of the transaction

Database Systems, 10th Edition 11

Database Systems, 10th Edition 12

Concurrency Control

• Coordination of simultaneous transaction execution in a multiprocessing database

• Objective is to ensure serializability of transactions in a multiuser environment

• Three main problems: – Lost updates

– Uncommitted data

– Inconsistent retrievals

Database Systems, 10th Edition 13

Lost Updates

• Lost update problem: – Two concurrent transactions update same data

element

– One of the updates is lost • Overwritten by the other transaction

Database Systems, 10th Edition 14

Database Systems, 10th Edition 15

Uncommitted Data

• Uncommitted data phenomenon: – Two transactions are executed concurrently – First transaction rolled back after second already

accessed uncommitted data

Database Systems, 10th Edition 16

Database Systems, 10th Edition 17

Inconsistent Retrievals

• Inconsistent retrievals: – First transaction accesses data – Second transaction alters the data

– First transaction accesses the data again

• Transaction might read some data before they are changed and other data after changed

• Yields inconsistent results

Database Systems, 10th Edition 18

Database Systems, 10th Edition 19

Database Systems, 10th Edition 20

The Scheduler

• Special DBMS program – Purpose is to establish order of operations within

which concurrent transactions are executed

• Interleaves execution of database operations: – Ensures serializability

– Ensures isolation

• Serializable schedule – Interleaved execution of transactions yields

same results as serial execution

Database Systems, 10th Edition 21

Concurrency Control with Locking Methods

• Lock – Guarantees exclusive use of a data item to a

current transaction

– Required to prevent another transaction from reading inconsistent data

– Pessimistic locking • Use of locks based on the assumption that conflict

between transactions is likely

– Lock manager • Responsible for assigning and policing the locks

used by transactions Database Systems, 10th Edition 22

Lock Granularity

• Indicates level of lock use • Locking can take place at following levels:

– Database

– Table

– Page

– Row – Field (attribute)

Database Systems, 10th Edition 23

Lock Granularity (cont’d.)

• Database-level lock – Entire database is locked

• Table-level lock – Entire table is locked

• Page-level lock – Entire diskpage is locked

Database Systems, 10th Edition 24

Lock Granularity (cont’d.)

• Row-level lock – Allows concurrent transactions to access

different rows of same table • Even if rows are located on same page

• Field-level lock – Allows concurrent transactions to access same

row • Requires use of different fields (attributes) within

the row

Database Systems, 10th Edition 25

Database Systems, 10th Edition 26

Database Systems, 10th Edition 27

Database Systems, 10th Edition 28

Database Systems, 10th Edition 29

Lock Types

• Binary lock – Two states: locked (1) or unlocked (0)

• Exclusive lock – Access is specifically reserved for transaction

that locked object

– Must be used when potential for conflict exists

• Shared lock – Concurrent transactions are granted read

access on basis of a common lock

Database Systems, 10th Edition 30

Database Systems, 10th Edition 31

Two-Phase Locking to Ensure Serializability

• Defines how transactions acquire and relinquish locks

• Guarantees serializability, but does not prevent deadlocks – Growing phase

• Transaction acquires all required locks without unlocking any data

– Shrinking phase • Transaction releases all locks and cannot obtain

any new lock

Database Systems, 10th Edition 32

Two-Phase Locking to Ensure Serializability (cont’d.)

• 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

Database Systems, 10th Edition 33

Database Systems, 10th Edition 34

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

Database Systems, 10th Edition 35

Deadlocks (cont’d.)

• Three techniques to control deadlock: – Prevention – Detection

– Avoidance

• Choice of deadlock control method depends on database environment – Low probability of deadlock; detection

recommended

– High probability; prevention recommended

Database Systems, 10th Edition 36

Database Systems, 10th Edition 37

Concurrency Control with Time Stamping Methods

• Assigns global unique time stamp to each transaction

• Produces explicit order in which transactions are submitted to DBMS

• Uniqueness – Ensures that no equal time stamp values can

exist

• Monotonicity – Ensures that time stamp values always increase

Database Systems, 10th Edition 38

Wait/Die and Wound/Wait Schemes

• Wait/die – Older transaction waits and younger is rolled

back and rescheduled

• Wound/wait – Older transaction rolls back younger transaction

and reschedules it

Database Systems, 10th Edition 39

Database Systems, 10th Edition 40

Concurrency Control with Optimistic Methods

• Optimistic approach – Based on assumption that 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: read, validation, and write

Database Systems, 10th Edition 41

Database Recovery Management

• Restores database to previous consistent state • Based on atomic transaction property

– All portions of transaction are treated as single logical unit of work

– All operations are applied and completed to produce consistent database

• If transaction operation cannot be completed: – Transaction aborted

– Changes to database are rolled back

Database Systems, 10th Edition 42

Transaction Recovery

• Write-ahead-log protocol: ensures transaction logs are written before data is updated

• Redundant transaction logs: ensure physical disk failure will not impair ability to recover

• Buffers: temporary storage areas in primary memory

• Checkpoints: operations in which DBMS writes all its updated buffers to disk

Database Systems, 10th Edition 43

Transaction Recovery (cont’d.)

• Deferred-write technique – Only transaction log is updated

• Recovery process: identify last checkpoint – If transaction committed before checkpoint:

• Do nothing

– If transaction committed after checkpoint: • Use transaction log to redo the transaction

– If transaction had ROLLBACK operation: • Do nothing

Database Systems, 10th Edition 44

Transaction Recovery (cont’d.)

• Write-through technique – Database is immediately updated by transaction

operations during transaction’s execution

• Recovery process: identify last checkpoint – If transaction committed before checkpoint:

• Do nothing

– If transaction committed after last checkpoint: • DBMS redoes the transaction using “after” values

– If transaction had ROLLBACK or was left active: • Do nothing because no updates were made

Database Systems, 10th Edition 45

Database Systems, 10th Edition 46

Summary

• Transaction: sequence of database operations that access database – Logical unit of work

• No portion of transaction can exist by itself

– Five main properties: atomicity, consistency, isolation, durability, and serializability

• COMMIT saves changes to disk • ROLLBACK restores previous database state • SQL transactions are formed by several SQL

statements or database requests Database Systems, 10th Edition 47

Summary (cont’d.)

• Transaction log keeps track of all transactions that modify database

• Concurrency control coordinates simultaneous execution of transactions

• Scheduler establishes order in which concurrent transaction operations are executed

• Lock guarantees unique access to a data item by transaction

• Two types of locks: binary locks and shared/exclusive locks

Database Systems, 10th Edition 48

Summary (cont’d.)

• Serializability of schedules is guaranteed through the use of two-phase locking

• Deadlock: when two or more transactions wait indefinitely for each other to release lock

• Three deadlock control techniques: prevention, detection, and avoidance

• Time stamping methods assign unique time stamp to each transaction – Schedules execution of conflicting transactions

in time stamp order Database Systems, 10th Edition 49

Summary (cont’d.)

• Optimistic methods assume the majority of database transactions do not conflict – Transactions are executed concurrently, using

private copies of the data

• Database recovery restores database from given state to previous consistent state

Database Systems, 10th Edition 50

  • Database Systems: Design, Implementation, and Management Tenth Edition
  • Objectives
  • Objectives (cont’d.)
  • What Is a Transaction?
  • PowerPoint Presentation
  • Evaluating Transaction Results
  • Slide 7
  • Transaction Properties
  • Transaction Properties (cont’d.)
  • Transaction Management with SQL
  • The Transaction Log
  • Slide 12
  • Concurrency Control
  • Lost Updates
  • Slide 15
  • Uncommitted Data
  • Slide 17
  • Inconsistent Retrievals
  • Slide 19
  • Slide 20
  • The Scheduler
  • Concurrency Control with Locking Methods
  • Lock Granularity
  • Lock Granularity (cont’d.)
  • Lock Granularity (cont’d.)
  • Slide 26
  • Slide 27
  • Slide 28
  • Slide 29
  • Lock Types
  • Slide 31
  • Two-Phase Locking to Ensure Serializability
  • Two-Phase Locking to Ensure Serializability (cont’d.)
  • Slide 34
  • Deadlocks
  • Deadlocks (cont’d.)
  • Slide 37
  • Concurrency Control with Time Stamping Methods
  • Wait/Die and Wound/Wait Schemes
  • Slide 40
  • Concurrency Control with Optimistic Methods
  • Database Recovery Management
  • Transaction Recovery
  • Transaction Recovery (cont’d.)
  • Slide 45
  • Slide 46
  • Summary
  • Summary (cont’d.)
  • Slide 49
  • Slide 50