Computer Science 202-CH9-Transactions

(1)

Barry Irwin August 2003

Barry Irwin August 2003 Rhodes University CS202 : DatabasesRhodes University CS202 : Databases 11

Computer Science 202 Computer Science 202

Transaction Management and Transaction Management and

Concurrency Control Concurrency Control

2 2 Barry Irwin August 2003

Barry Irwin August 2003 Rhodes University CS202 : DatabasesRhodes University CS202 : Databases

Objectives Objectives

To learn what a transaction is.

To learn how an incomplete transaction can yield an To learn how an incomplete transaction can yield an inconsistent database.

inconsistent database.

To learn how an inconsistent database can be avoided To learn how an inconsistent database can be avoided through proper transaction management.

through proper transaction management.

To learn what concurrency control is.

To learn how concurrency control failure affects a To learn how concurrency control failure affects a database.

database.

To learn how concurrency control is accomplished.

To learn what database recovery management is.

To understand the use of transaction logs in recovery To understand the use of transaction logs in recovery management.

management.

What is a Transaction?

A logical unit of work on a database A logical unit of work on a database

n

Read (select) and write (Update/insert/delete) Read (select) and write (Update/insert/delete) Database request is equivalent to a single SQL Database request is equivalent to a single SQL statement

statement

A transaction can consist of many DB Requests A transaction can consist of many DB Requests A transaction must be completed, or must be A transaction must be completed, or must be aborted

aborted

Consistency of a database is paramount Consistency of a database is paramount DB should be maintained in a consistent state DB should be maintained in a consistent state

Example Transaction Example Transaction

So what's the problem?

To ensure consistency To ensure consistency

n

A Transaction must start with the DB in a A Transaction must start with the DB in a known state

known state

n

The transaction must complete and leave the The transaction must complete and leave the DB in a known state

DB in a known state

A transaction is likely to modify DB A transaction is likely to modify DB contents

contents

n

Protection must be given for the areas of the Protection must be given for the areas of the DB that the transaction works with.

DB that the transaction works with.

Transaction Evaluation (I) Transaction Evaluation (I)

Examine current account balance Examine current account balance

Consistent state after transaction Consistent state after transaction No changes made to Database No changes made to Database SELECT ACC_NUM, ACC_BALANCE FROM CHECKACC

WHERE ACC_NUM = ‘0908110638’;

(2)

Transaction Evaluation (II) Transaction Evaluation (II)

Register credit sale of 100 units of product X to customer Register credit sale of 100 units of product X to customer Y for R500

Y for R500

Consistent state only if both transactions are fully Consistent state only if both transactions are fully completed

completed

DBMS doesn’t guarantee that the semantic meaning of a DBMS doesn’t guarantee that the semantic meaning of a transaction represents real

transaction represents real- -world event world event UPDATE PRODUCT

SET PROD_QOH = PROD_QOH - 100 WHERE PROD_CODE = ‘X’;

UPDATE ACCT_RECEIVABLE

SET ACCT_BALANCE = ACCT_BALANCE + 500 WHERE ACCT_NUM = ‘Y’;

Transaction Properties Transaction Properties

Atomicity Atomicity

n

All parts of must be completed OR none All parts of must be completed OR none

n

Indivisible Work unit Indivisible Work unit Durability

Durability

n

On completion DB is in a consistent state On completion DB is in a consistent state Serializability

Serializability

n

Concurrent execution of multiple transactions Concurrent execution of multiple transactions

n

Concurrency can mean that transactions are handled Concurrency can mean that transactions are handled in a serial manner

in a serial manner Isolation

Isolation

n

Data used in a transaction cannot be used by another Data used in a transaction cannot be used by another un till the first completes

un till the first completes

Database Properties Database Properties

Single User Single User

n

Automatically ensures serializability and isolation, by Automatically ensures serializability and isolation, by design ( only single user)

design ( only single user)

n

Durability & Atomicity need to be provided by the Durability & Atomicity need to be provided by the DBMS

DBMS Multi User Multi User

n

Multiple concurrent transactions Multiple concurrent transactions

n

DBMS must enforce serializability and isolation DBMS must enforce serializability and isolation

n

Provide concurrency control management Provide concurrency control management

SQL Transaction Management SQL Transaction Management

Transaction support Transaction support

n

COMMIT COMMIT

n

ROLLBACK ROLLBACK

User initiated transaction sequence must User initiated transaction sequence must continue until:

continue until:

n

COMMIT statement is reached COMMIT statement is reached

n

ROLLBACK statement is reached ROLLBACK statement is reached

n

End of a program reached (COMMIT) End of a program reached (COMMIT)

n

Abnormal program termination ( Abnormal program termination (ROLLBACK) ROLLBACK)

SQL Example SQL Example

BEGIN TRANSACTION; --Not ALL support THIS UPDATE PRODUCT

SET PROD_QOH = PROD_QOH - 100 WHERE PROD_CODE = ‘X’;

UPDATE ACCT_RECEIVABLE

SET ACCT_BALANCE = ACCT_BALANCE + 500 WHERE ACCT_NUM = ‘Y’;

COMMIT;

Transaction Log Transaction Log

Transaction log keeps track of all updates Transaction log keeps track of all updates affecting the database

affecting the database

Log is maintained by the DBMS Log is maintained by the DBMS Used for DB recovery in the event of a Used for DB recovery in the event of a ROLLBACK

ROLLBACK

Can be used by some DBMS for recovery Can be used by some DBMS for recovery following a crash

following a crash

Used to help ensure Durability of Used to help ensure Durability of Transactions

Transactions

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Transaction Log Transaction Log

Transaction log holds information to allow Transaction log holds information to allow a recovery

a recovery

Concurrency Control Concurrency Control

Ensures serializability of transactions Ensures serializability of transactions Protects against problems that could arise Protects against problems that could arise from simultaneous execution

from simultaneous execution

n

Data Integrity Data Integrity

n

Consistency Consistency Handles Handles

n

Lost Updates Lost Updates

n

Uncommitted Data Uncommitted Data

n

Inconsistent Data Retrievals Inconsistent Data Retrievals

Concurrency Issues Concurrency Issues

n

An update is lost and the result is Erroneous data An update is lost and the result is Erroneous data being retrieved

being retrieved Uncommitted Data Uncommitted Data

n

Two concurrent transactions Two concurrent transactions

n

T1 Rolls back after T2 Has already accessed updated T1 Rolls back after T2 Has already accessed updated information

information

Inconsistent Retrievals Inconsistent Retrievals

n

Usually with Aggregation functions Usually with Aggregation functions

n

T2 performs an aggregation while T1 is Updating T2 performs an aggregation while T1 is Updating values

values

Lost Updates Lost Updates

Normal Transaction Normal Transaction

Lost Update results in Error Lost Update results in Error

Correct Processing

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Problems resulting from Uncommitted Data Problems resulting from Uncommitted Data

Inconsistent Data Retrieval Inconsistent Data Retrieval

Retrieval during an Update Retrieval during an Update

Inconsistent Data Retrieval Inconsistent Data Retrieval

Resultant Output Resultant Output

The Scheduler The Scheduler

Scheduler provides overall management of the Scheduler provides overall management of the concurrency process

concurrency process Ordering of transactions

Ordering of transactions à à serializability serializability Ordering based on Control Algorithms Ordering based on Control Algorithms

n n

Locking Locking

n

Time stamping Time stamping

Why Not use FIFO ordering ? Why Not use FIFO ordering ?

n

NOT effective use of CPU NOT effective use of CPU

n

Poor Response times Poor Response times

Conflicting Operations Conflicting Operations

Locking Methods Locking Methods

A Lock guarantees exclusive access to a A Lock guarantees exclusive access to a DB object

DB object

Prevents access to inconsistent Data Prevents access to inconsistent Data Automatically managed by DBMS Automatically managed by DBMS A

A Lock Manager Lock Manager process is responsible for process is responsible for all Locking within the DB

all Locking within the DB

Lock Granularity can have a big effect on Lock Granularity can have a big effect on performance

performance

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Locking granularity Locking granularity

The Level at which the locks are applied The Level at which the locks are applied

n

Database Level Database Level

n

Table Level Table Level

n

Page Level Page Level

n

Row Level Row Level

n

Field Level Field Level

Locks can be of two types Locks can be of two types

n n

Binary Binary

n

Shared/Exclusive Shared/Exclusive

Entire Database is locked Entire Database is locked Prevents use of

Prevents use of any any other tables by other tables by transactions T2, T3…. until T1 has transactions T2, T3…. until T1 has completed

completed Very Slow

Very Slow performance performance

Locks are applied to specific tables Locks are applied to specific tables T2 can access tables within the DB T2 can access tables within the DB provided that they are not locked by T1 provided that they are not locked by T1 Better performance than DB Level locks Better performance than DB Level locks Can still be problematic with multiple Can still be problematic with multiple accesses to a Table

accesses to a Table

Slowdown even if T2 and T1 are Slowdown even if T2 and T1 are accessing different portions of a table.

accessing different portions of a table.

Page Level Page Level

Locks are specific to a physical disk page Locks are specific to a physical disk page Disk page is equivalent to a storage block Disk page is equivalent to a storage block on disk

on disk

Size of page is dependant on underlying Size of page is dependant on underlying HW, OS and File system

HW, OS and File system

n

4K , 8K , 16K , 32K 4K , 8K , 16K , 32K

T2 cannot access a page locked by T1

T2 CAN access the same table if the

portion needed is in a different disk page

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Page Level Page Level

Row Level Row Level

Non restrictive locking mechanism Non restrictive locking mechanism

Locks are restricted to the particular ROW Locks are restricted to the particular ROW that T1, may be using. T2 May access the that T1, may be using. T2 May access the rest of the DB.

rest of the DB.

Higher granularity results in increased Higher granularity results in increased overhead

overhead

Row Level Row Level

Field Level Field Level

Allows Transactions to access the same Allows Transactions to access the same row

row

Provides locking on fields within the row Provides locking on fields within the row Extremely high overhead

Extremely high overhead

Binary Locks Binary Locks

Only has two states Only has two states

n n

Locked Locked

n

Unlocked Unlocked

Applies to all levels of granularity Applies to all levels of granularity DBMS manages the locking process DBMS manages the locking process Binary locks are generally exclusive Binary locks are generally exclusive

Binary Locks

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Shared/Exclusive Locks Shared/Exclusive Locks

Exclusive Locks Exclusive Locks

n

Only the locking transaction has access Only the locking transaction has access

n

Used for write operations Used for write operations Shared Locks

Shared Locks

n

Concurrent transactions may have READ access to a Concurrent transactions may have READ access to a locked object

locked object

n

No conflict provided all transactions are READ No conflict provided all transactions are READ

n

Cannot be issued if an exclusive lock for the object Cannot be issued if an exclusive lock for the object exists

exists

Shared/Exclusive Locks Shared/Exclusive Locks

Locking states Locking states

n

READ_LOCK READ_LOCK

n

WRITE_LOCK WRITE_LOCK

n

UNLOCK UNLOCK Locking problems Locking problems

n

Transactions may not be serializable Transactions may not be serializable Can be solved with TWO

Can be solved with TWO- -PHASE locking PHASE locking

n

Scheduler may create deadlocks Scheduler may create deadlocks

Two

Two- - Phase Locking Phase Locking

Growing phase Growing phase Shrinking phase Shrinking phase Governing rules Governing rules

n

Two transactions cannot have conflicting Two transactions cannot have conflicting locks

locks

n

No unlock operation can precede a lock No unlock operation can precede a lock operation in the same transaction operation in the same transaction

n

No data objects are updated until all locks are No data objects are updated until all locks are obtained

obtained

Two

Two- -Phase Locking Phase Locking

Deadlocks Deadlocks

Occurs when two transactions are waiting Occurs when two transactions are waiting for the other to unlock data

for the other to unlock data T1 = Accesses DATA X & DATA Y T1 = Accesses DATA X & DATA Y T2 = Accesses DATA Y & DATA X T2 = Accesses DATA Y & DATA X Also known as a ‘

Also known as a ‘deadly embrace deadly embrace’ ’

Deadlock Example

(8)

Deadlock Control Deadlock Control

Deadlock Prevention Deadlock Prevention

n

New locks are aborted if there is a likelihood they New locks are aborted if there is a likelihood they would cause a deadlock

would cause a deadlock

n

Transaction re Transaction re- -scheduled scheduled Deadlock Detection Deadlock Detection

n

DBMS checks DB for deadlocks DBMS checks DB for deadlocks

n

In the case of a deadlock one transaction is aborted In the case of a deadlock one transaction is aborted and then restarted

and then restarted Deadlock Avoidance Deadlock Avoidance

n

Transaction must obtain ALL locks it needs prior to Transaction must obtain ALL locks it needs prior to execution (two

execution (two - -phase locking) phase locking)

n

Much slower due to serialization of transactions Much slower due to serialization of transactions

Time stamping Time stamping

Timestamps can be used to assist with Timestamps can be used to assist with scheduling algorithms

scheduling algorithms

Each transaction is assigned a unique stamp Each transaction is assigned a unique stamp based on the order of submission to DBMS based on the order of submission to DBMS R/W operations can take place provided they R/W operations can take place provided they have the same timestamp

have the same timestamp

Transactions are serialized based on timestamp Transactions are serialized based on timestamp order

order

Disadvantages Disadvantages

n

Increased admin Increased admin

n

Increased memory (Last read & Last Update) Increased memory (Last read & Last Update)

Optimistic Methods Optimistic Methods

Assumption that most transactions to do Assumption that most transactions to do not conflict

not conflict

Transaction executed without restrictions Transaction executed without restrictions until committed

until committed Phases:

Phases:

n

Read Phase Read Phase

n

Validation Phase Validation Phase

n

Write Phase Write Phase

Database Recovery Management Database Recovery Management

Restores a database from an inconsistent Restores a database from an inconsistent to a previously consistent state

to a previously consistent state

Based on the atomic transaction property Based on the atomic transaction property Level of backup

Level of backup

n

Full backup Full backup

n

Differential Differential

n

Transaction log Transaction log

Database Failures Database Failures

Software Software

n

Viruses, Operating System, DBMS Viruses, Operating System, DBMS Hardware

Hardware

n

Hard disks, Memory, Networks Hard disks, Memory, Networks Programming Exemption Programming Exemption

n

Abnormal termination, premature termination Abnormal termination, premature termination Transaction

Transaction

n

Deadlock results in termination of a transaction Deadlock results in termination of a transaction External

External

n

Fire, Flood, Theft Fire, Flood, Theft

Transaction Recovery Transaction Recovery

Write

Write- -Ahead Logging Ahead Logging

n

Logs written BEFORE data is modified Logs written BEFORE data is modified Redundant transaction logs Redundant transaction logs

n

Multiple copies of transaction log maintained Multiple copies of transaction log maintained Buffers

Buffers

n

Memory buffers lead to faster processing Memory buffers lead to faster processing Checkpoints

Checkpoints

n

Updated buffers committed to disk Updated buffers committed to disk

n

Registered in Transaction log Registered in Transaction log

n

Buffers and Physical copies synchronised Buffers and Physical copies synchronised

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Transaction Recovery Transaction Recovery

Deferred

Deferred- -write and Deferred write and Deferred- -update update

n

Changes are written to the transaction log Changes are written to the transaction log

n

Database updated after transaction reaches commit Database updated after transaction reaches commit point

point Write Write- -through through

n

Immediately updated by during execution Immediately updated by during execution

n

Before the transaction reaches its commit point Before the transaction reaches its commit point

n

Transaction log also updated Transaction log also updated

n

Transaction fails, database uses log information Transaction fails, database uses log information to ROLLBACK

to ROLLBACK