Transaction Transaction ManagementManagement

Chapter 9Chapter 9

What is a Transaction?What is a Transaction?

A logical unit of work on a databaseA logical unit of work on a database An entire programAn entire program A portion of a programA portion of a program A single commandA single command

The entire series of steps necessary to The entire series of steps necessary to accomplish a logical unit of workaccomplish a logical unit of work

Successful transactions change the database Successful transactions change the database from one CONSISTENT STATE to another from one CONSISTENT STATE to another

(One where all data integrity constraints are (One where all data integrity constraints are satisfied)satisfied)

Example of a TransactionExample of a Transaction

Updating a RecordUpdating a Record Locate the Record on DiskLocate the Record on Disk Bring record into BufferBring record into Buffer Update Data in the BufferUpdate Data in the Buffer Writing Data Back to DiskWriting Data Back to Disk

4 Properties of a 4 Properties of a TransactionTransaction

Atomic – All or NothingAtomic – All or Nothing

All parts of the transaction must be All parts of the transaction must be completed and committed or it must completed and committed or it must be aborted and rolled backbe aborted and rolled back

Consistent Consistent

Each user is responsible to ensure Each user is responsible to ensure that their transaction (if executed by that their transaction (if executed by itself) would leave the database in a itself) would leave the database in a consistent stateconsistent state

4 Properties of a 4 Properties of a TransactionTransaction

IsolationIsolationThe final effects of multiple The final effects of multiple simultaneous transactions must be the simultaneous transactions must be the same as if they were executed one same as if they were executed one right after the otherright after the other

DurabilityDurabilityIf a transaction has been committed, If a transaction has been committed, the DBMS must ensure that its effects the DBMS must ensure that its effects are permanently recorded in the are permanently recorded in the database (even if the system crashes)database (even if the system crashes)

Transaction Management Transaction Management with SQLwith SQL

SQL Statements SQL Statements Commit / Rollback Commit / Rollback When a transaction sequence is initiated When a transaction sequence is initiated

it must continue through all succeeding it must continue through all succeeding SQL statements until:SQL statements until:

1.1. A Commit Statement is ReachedA Commit Statement is Reached

2.2. A Rollback Statement is ReachedA Rollback Statement is Reached

3.3. The End of the Program is Reached The End of the Program is Reached (Commit)(Commit)

4.4. The Program is Abnormally Terminated The Program is Abnormally Terminated (Rollback)(Rollback)

ExampleExampleBEGIN TRANBEGIN TRAN DECLARE @ErrorCode INT, @TranSuccessful INTDECLARE @ErrorCode INT, @TranSuccessful INT SET @TranSuccessful = 1SET @TranSuccessful = 1

INSERT INTO tblCatalog (CatalogYear) INSERT INTO tblCatalog (CatalogYear) VALUES('2002')VALUES('2002') SET @ErrorCode = @@ERROR; IF (@ErrorCode <> 0) SET @TranSuccessful = 0 –FalseSET @ErrorCode = @@ERROR; IF (@ErrorCode <> 0) SET @TranSuccessful = 0 –False

INSERT INTO tblCatalog (CatalogYear) INSERT INTO tblCatalog (CatalogYear) VALUES('2003')VALUES('2003')

SET @ErrorCode = @@ERROR; IF (@ErrorCode <> 0) SET @TranSuccessful = 0 –FalseSET @ErrorCode = @@ERROR; IF (@ErrorCode <> 0) SET @TranSuccessful = 0 –False

IF @TranSuccessful = 0IF @TranSuccessful = 0 BEGINBEGIN

ROLLBACK TRANROLLBACK TRAN RAISERROR ('Rolledback transaction: Insert Catalog Year.', 16,1)RAISERROR ('Rolledback transaction: Insert Catalog Year.', 16,1)

ENDEND ELSEELSE BEGINBEGIN COMMIT TRANCOMMIT TRAN PRINT 'Successfully inserted catalog years...'PRINT 'Successfully inserted catalog years...' ENDENDGOGO

Transaction LogTransaction Log

Keeps track of all transactions that update Keeps track of all transactions that update the databasethe database Record for the beginning of the transactionRecord for the beginning of the transaction Type of operation (insert / update / delete)Type of operation (insert / update / delete) Names of objects/tables affected by the Names of objects/tables affected by the

transactiontransaction Before and After Values for Updated FieldsBefore and After Values for Updated Fields Pointers to Previous and Next Transaction Log Pointers to Previous and Next Transaction Log

Entries for the same transactionEntries for the same transaction The Ending of the Transaction (Commit)The Ending of the Transaction (Commit)

Used for recovery in case of a RollbackUsed for recovery in case of a Rollback

Concurrency ControlConcurrency Control Coordination of simultaneous transaction Coordination of simultaneous transaction

execution in a multiprocessing database execution in a multiprocessing database systemsystem

Ensure transaction serializability in a Ensure transaction serializability in a multi-user databasemulti-user database

Lack of Concurrency Control can create Lack of Concurrency Control can create data integrity and consistency problems:data integrity and consistency problems: Lost UpdatesLost Updates Uncommitted DataUncommitted Data Inconsistent RetrievalsInconsistent Retrievals

Lost UpdatesLost UpdatesTimTim

eeJack’s TransJack’s Trans Jill’s TransJill’s Trans BalanBalan

cece

T1T1 BeginBegin

T2T2 Read BalanceRead Balance BeginBegin 10001000

T3T3 Read BalanceRead Balance 10001000

T4T4 Bal = Bal – 50 Bal = Bal – 50 (950)(950)

10001000

T5T5 Write Bal (950)Write Bal (950) Bal = Bal + Bal = Bal + 100 (1100)100 (1100)

950950

T6T6 CommitCommit 950950

T7T7 Write Bal Write Bal (1100)(1100)

11001100

T8T8 CommitCommit 11001100

Uncommitted DataUncommitted DataTimTimee

DepositDeposit InterestInterest BalBal

T1T1 Begin TransactionBegin Transaction 10010000

T2T2 Read Bal (1000)Read Bal (1000) 10010000

T3T3 Bal = Bal + 1000 Bal = Bal + 1000 (2000)(2000)

10010000

T4T4 Write Bal (2000)Write Bal (2000) Begin Begin TransactionTransaction

20020000

T5T5 Read Bal (2000)Read Bal (2000) 20020000

T6T6 Bal = Bal*1.05 Bal = Bal*1.05 (2100)(2100)

20020000

T7T7 RollbackRollback 10010000

T8T8 Write Bal (2100)Write Bal (2100) 21021000

T9T9 CommitCommit 21021000

Inconsistent RetrievalsInconsistent RetrievalsTimeTime SumBalSumBal TransferTransfer Bal Bal

AABal Bal BB

Bal Bal CC

SumSum

T1T1 Begin TransBegin Trans 50050000

50050000

50050000

T2T2 Sum = 0Sum = 0 Begin TransBegin Trans 50050000

50050000

50050000

T3T3 Read BalA (5000)Read BalA (5000) 50050000

50050000

50050000

T4T4 Sum = Sum + Sum = Sum + BalA (5000)BalA (5000)

Read BalA (5000)Read BalA (5000) 50050000

50050000

50050000

T5T5 Read BalB (5000)Read BalB (5000) BalA = BalA -1000 BalA = BalA -1000 (4000)(4000)

50050000

50050000

50050000

T6T6 Sum = Sum+BalB Sum = Sum+BalB (10000)(10000)

Write BalA (4000)Write BalA (4000) 40040000

50050000

50050000

T7T7 Read BalCRead BalC 40040000

50050000

50050000

T8T8 BalC =BalC + 1000 BalC =BalC + 1000 (6000)(6000)

40040000

50050000

50050000

T9T9 Write BalC (6000)Write BalC (6000) 40040000

50050000

60060000

T10T10 Read BalCRead BalC CommitCommit 40040000

50050000

60060000

T11T11 Sum=Sum + Sum=Sum + BalC (16000)BalC (16000)

40040000

50050000

60060000

T12T12 Write Sum Write Sum (16000)(16000)

40040000

50050000

60060000

1600160000

T13T13 CommitCommit 40040000

50050000

60060000

1600160000

Serial Execution of Serial Execution of TransactionsTransactions

Serial Execution of transaction means Serial Execution of transaction means that the transactions are performed one that the transactions are performed one after another.after another.

No interaction between transactions - No interaction between transactions - No Concurrency Control ProblemsNo Concurrency Control Problems

Serial Execution will never leave the Serial Execution will never leave the database in an inconsistent state database in an inconsistent state Every Serial Execution is considered Every Serial Execution is considered correct (Even if a different order would correct (Even if a different order would cause different results)cause different results)

SerializabilitySerializability

If 2 Transactions are only reading data items If 2 Transactions are only reading data items – They do not conflict – They do not conflict Order is Order is unimportantunimportant

If 2 Transactions operate (Read/Write) on If 2 Transactions operate (Read/Write) on Separate Data Items Separate Data Items

– – They do not conflict They do not conflict Order is Order is unimportantunimportant

If 1 Transaction Writes to a Data Item and If 1 Transaction Writes to a Data Item and Another Reads or Writes to the Same Data Another Reads or Writes to the Same Data Item Item The Order of Execution IS Important The Order of Execution IS Important

The SchedulerThe Scheduler

Special DBMS Program to establish Special DBMS Program to establish the order of operations in which the order of operations in which concurrent transactions are concurrent transactions are executesexecutes

Interleaves the execution of Interleaves the execution of database operations to ensure:database operations to ensure:

SerializabilitySerializability

Isolation of TransactionsIsolation of Transactions

The SchedulerThe Scheduler

Bases its actions on Concurrency Bases its actions on Concurrency Control Algorithms (Locking / Time Control Algorithms (Locking / Time Stamping)Stamping)

Ensures the CPU is used efficiently Ensures the CPU is used efficiently (Scheduling Methods)(Scheduling Methods)

Facilitates Data Isolation Facilitates Data Isolation Ensure Ensure that 2 transactions do not update the that 2 transactions do not update the same data at the same timesame data at the same time

Concurrency Control Concurrency Control AlgorithmsAlgorithms

LockingLockingA Transaction “locks” a database object to A Transaction “locks” a database object to

prevent another object from modifying prevent another object from modifying the objectthe object

Time-StampingTime-StampingAssign a global unique time stamp to Assign a global unique time stamp to each transactioneach transaction

OptimisticOptimisticAssumption that most database Assumption that most database operations do not conflictoperations do not conflict

LockingLocking

Lock guarantees exclusive use of Lock guarantees exclusive use of data item to current transactiondata item to current transaction

Prevents reading Inconsistent DataPrevents reading Inconsistent Data Lock Manager is responsible for Lock Manager is responsible for

assigning and policing the locks assigning and policing the locks used by the transactionused by the transaction

Locking GranularityLocking Granularity

Indicates the level of lock useIndicates the level of lock use Database Level – Entire Database is Database Level – Entire Database is

LockedLocked Table Level – Entire Table is LockedTable Level – Entire Table is Locked Page Level – Locks an Entire Diskpage Page Level – Locks an Entire Diskpage

(Most Frequently Used)(Most Frequently Used) Row Level – Locks Single Row of TableRow Level – Locks Single Row of Table Field Level – Locks a Single Attribute Field Level – Locks a Single Attribute

of a Single Row (Rarely Done)of a Single Row (Rarely Done)

Types of Locks: Types of Locks: BinaryBinary

Binary Locks – Lock with 2 StatesBinary Locks – Lock with 2 States Locked – No other transaction can use that objectLocked – No other transaction can use that object Unlocked – Any transaction can lock and use objectUnlocked – Any transaction can lock and use object

All Transactions require a Lock and Unlock All Transactions require a Lock and Unlock Operation for Each Operation for Each

Object Accessed (Handled by DBMS)Object Accessed (Handled by DBMS)

Eliminates Lost UpdatesEliminates Lost Updates Too Restrictive to Yield Optimal Concurrency Too Restrictive to Yield Optimal Concurrency

ConditionsConditions

Types of Locks:Types of Locks:Shared / Exclusive LocksShared / Exclusive Locks

Indicates the Nature of the LockIndicates the Nature of the Lock Shared Lock – Concurrent Transactions are granted Shared Lock – Concurrent Transactions are granted

READ access on the basis of a common lockREAD access on the basis of a common lock Exclusive Lock – Access is reserved for the Exclusive Lock – Access is reserved for the

transaction that locked the objecttransaction that locked the object 3 States: Unlocked, Shared (Read), Exclusive 3 States: Unlocked, Shared (Read), Exclusive

(Write)(Write) More Efficient Data Access SolutionMore Efficient Data Access Solution More Overhead for Lock ManagerMore Overhead for Lock Manager

Type of lock needed must be known Type of lock needed must be known 3 Operations: 3 Operations:

Read_Lock – Check to see the type of lockRead_Lock – Check to see the type of lock Write_Lock – Issue a LockWrite_Lock – Issue a Lock Unlock – Release a LockUnlock – Release a Lock

Allow Upgrading / Downgrading of LocksAllow Upgrading / Downgrading of Locks

Problems with LockingProblems with Locking

Transaction Schedule May Not be Transaction Schedule May Not be SerializableSerializable Can be solved with 2-Phase LockingCan be solved with 2-Phase Locking

May Cause DeadlocksMay Cause Deadlocks A deadlock is caused when 2 A deadlock is caused when 2

transactions wait for each other to transactions wait for each other to unlock dataunlock data

Two Phase LockingTwo Phase Locking

Defines how transactions Acquire and Defines how transactions Acquire and Relinquish LocksRelinquish Locks

1.1. Growing Phase – The transaction acquires Growing Phase – The transaction acquires all locks (doesn’t unlock any data)all locks (doesn’t unlock any data)

2.2. Shrinking Phase – The transaction releases Shrinking Phase – The transaction releases locks (doesn’t lock any additional data)locks (doesn’t lock any additional data)

Transactions acquire all locks it needs Transactions acquire all locks it needs until it reaches locked pointuntil it reaches locked point

When locked, data is modified and locks When locked, data is modified and locks are releasedare released

DeadlocksDeadlocks Occur when 2 transactions exist in the Occur when 2 transactions exist in the

following mode:following mode:T1 = access data item X and YT1 = access data item X and YT2 = Access data items Y and XT2 = Access data items Y and X

If T1 does not unlock Y, T2 cannot beginIf T1 does not unlock Y, T2 cannot beginIf T2 does not unlock X, T1 cannot continueIf T2 does not unlock X, T1 cannot continue

T1 & T2 wait indefinitely for each other to unlock T1 & T2 wait indefinitely for each other to unlock datadata

Deadlocks are only possible if a transactions Deadlocks are only possible if a transactions wants an Exclusive Lock (No Deadlocks on wants an Exclusive Lock (No Deadlocks on Shared Locks)Shared Locks)

Controlling DeadlocksControlling Deadlocks Prevention – A transaction requesting a new Prevention – A transaction requesting a new

lock is aborted if there is the possibility of a lock is aborted if there is the possibility of a deadlock – Transaction is rolled back, Locks deadlock – Transaction is rolled back, Locks are released, Transaction is rescheduledare released, Transaction is rescheduled

Detection – Periodically test the database Detection – Periodically test the database for deadlocks. If a deadlock is found, for deadlocks. If a deadlock is found, abort / rollback one of the transactionsabort / rollback one of the transactions

Avoidance – Requires a transaction to Avoidance – Requires a transaction to obtain all locks needed before it can obtain all locks needed before it can execute – requires locks to be obtained in execute – requires locks to be obtained in successionsuccession

Time StampingTime Stamping Creates a specific order in which the transactions Creates a specific order in which the transactions

are processed by the DBMSare processed by the DBMS 2 Main Properties2 Main Properties

1.1. Uniqueness – Assumes that no equal time stamp value can Uniqueness – Assumes that no equal time stamp value can exist (ensures serializability of the transactions)exist (ensures serializability of the transactions)

2.2. Monotonicity – Ensures that time stamp values always Monotonicity – Ensures that time stamp values always increasesincreases

All operations within the same transaction have the All operations within the same transaction have the same time stampsame time stamp

If Transactions conflict, one is rolled back and If Transactions conflict, one is rolled back and rescheduledrescheduled

Each value in Database requires 2 Additional Each value in Database requires 2 Additional Fields: Last Time Read / Last Time UpdatedFields: Last Time Read / Last Time Updated

Increases Memory Need and Processing OverheadIncreases Memory Need and Processing Overhead

Time Stamping SchemesTime Stamping Schemes Wait / Die SchemeWait / Die Scheme

The older transaction will waitThe older transaction will waitThe younger transaction will be rolled backThe younger transaction will be rolled back

Wound / Wait SchemeWound / Wait SchemeThe older transaction will preempt (wound) The older transaction will preempt (wound)

the younger transaction and roll it the younger transaction and roll it backback

The younger transaction waits for the older The younger transaction waits for the older transaction to release the lockstransaction to release the locks

Without time-out values, Deadlocks may be Without time-out values, Deadlocks may be createdcreated

Optimistic MethodOptimistic Method Most database operations do not conflictMost database operations do not conflict No locking or time stampingNo locking or time stamping Transactions execute until commitTransactions execute until commit

Read Phase – Read database, execute computations, Read Phase – Read database, execute computations, make local updates (temporary update file)make local updates (temporary update file)

Validate Phase – Transaction is validated to ensure Validate Phase – Transaction is validated to ensure changes will not effect integrity of databasechanges will not effect integrity of database

If Validated If Validated Go to Write Phase Go to Write Phase If Not Validated If Not Validated Restart Transaction and discard initial Restart Transaction and discard initial

changeschanges Write Phase – Commit Changes to databaseWrite Phase – Commit Changes to database

Good for Read / Query Databases (Few Good for Read / Query Databases (Few Updates)Updates)

Database RecoveryDatabase Recovery Restore a database from a given state to a Restore a database from a given state to a

previous consistent stateprevious consistent state Atomic Transaction Property (All or None)Atomic Transaction Property (All or None) Backup Levels:Backup Levels:

Full BackupFull Backup Differential BackupDifferential Backup Transaction Log BackupTransaction Log Backup

Database / System Failures:Database / System Failures: Software (O.S., DBMS, Application Programs, Viruses)Software (O.S., DBMS, Application Programs, Viruses) Hardware (Memory Chips, Disk Crashes, Bad Sectors)Hardware (Memory Chips, Disk Crashes, Bad Sectors) Programming Exemption (Application Program Programming Exemption (Application Program

rollbacks)rollbacks) Transaction (Aborting transactions due to deadlock Transaction (Aborting transactions due to deadlock

detection)detection) External (Fire, Flood, etc)External (Fire, Flood, etc)

Transaction RecoveryTransaction Recovery Recover Database by using data in the Transaction Recover Database by using data in the Transaction

LogLog Write-Ahead-Log – Transaction logs need to be Write-Ahead-Log – Transaction logs need to be

written before any database data is updatedwritten before any database data is updated Redundant Transaction Logs – Several copies of log Redundant Transaction Logs – Several copies of log

on different deviceson different devices Database Buffers – Buffers are used to increase Database Buffers – Buffers are used to increase

processing time on updates instead of accessing data processing time on updates instead of accessing data on diskon disk

Database Checkpoints – Process of writing all Database Checkpoints – Process of writing all updated buffers to disk updated buffers to disk While this is taking place, While this is taking place, all other requests are not executesall other requests are not executes Scheduled several times per hourScheduled several times per hour Checkpoints are registered in the transaction logCheckpoints are registered in the transaction log