Session10 Transaction

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Managing Transaction and

Lock

Vu Tuyet [email protected]

Hanoi University of Technology

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Transaction

A sequence of tasks that together constitute a logicalunit of work

Ensuring the highest level of integrity and consistencywithin a database

Used when statements are processed


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ACID Properties

Atomicity

All or none

Consistency Leave data in a consistent state

Isolation

Dont show data thats done

Durability

Modifications are permanent


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Overview

Transaction

Review of Transaction Log

Introduction to Transactions and Locks Managing Transactions

SQL Server Locking & Managing Locks


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Transaction Log Architecture

Write-Ahead transaction log

Flushing the page and dirty Pages

Use of checkpoints

Minimize what must be processed to recover

Truncating the transaction log

Shrinking the transaction log

DBCC SHRINKDATABASE

DBCC SHRINKFILE


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Transaction Recovery and Checkpoints

Transaction Recovery Action Required

None

Checkpoint System Failure

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22

33

44

55

Roll forward

Roll back

Roll forward

Roll back


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Concurrency Control

Pessimistic concurrency control

High contention for data

Optimistic concurrency control

Low contention for data


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Lockable Resources

ItemItem DescriptionDescription

RID Row identifier

Key Row lock within an index

Page

Extent

Table

Data page or index page

Group of pages

Entire table

Database Entire database


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Types of Locks

Basic locks

Shared (LS)

Exclusive (LX)

Special situation locks Intent

Update

Schema

Bulk update


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Lock Compatibility

Locks may or may not be compatible with other Locks

Examples Shared locks are compatible with all locks except exclusive

Exclusive locks are not compatible with any other locks

Update locks are compatible only with shared locks

LS LX

LS true false

LX false false


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Managing Locks

Session-level locking options

READ UNCOMMITTED

READCOMMITTED (default) REPEATABLE READ

SERIALIZABLE

SNAPSHOT

Locking timeout

Limits time waiting for a locked resource

SET LOCK_TIMEOUT


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Viewing Locking Information

Activity Monitor

sys.dm_tran_locks Dynamic Management View

EnumLocks SQL Server Profiler

System Monitor


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Considerations for Using Transactions

Transaction guidelines

Keep transactions as short as possible

Use caution with certain Transact-SQL statements Avoid transactions that require user interaction

Issues in nesting transactions

Allowed, but not recommended

Use @@trancount to determine nesting level


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Transaction Types

Explicit Transaction

Explicitly define start and end

Autocommit Transactions

Every statement is committed or rolled back

Implicit Transactions

Statement after Commit Transaction orRollback Transaction

starts a new transaction

SET IMPLICIT_TRANSACTIONSON


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Explicit Transaction

DefiningBEGIN TRANSACTION

COMMIT TRANSACTION

ROLLBACK TRANSACTION

BEGIN TRANSACTION

INSERT INTO Extensions (PrimaryExt)

VALUES ('5551212')INSERT INTO Extensions (VoiceMailExt)

VALUES ('5551212')

ROLLBACK TRANSACTION


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Implicit Transaction

Using

An implicit transaction starts when one of the following

statements is executed

Transaction must be explicitly completed with COMMIT orROLLBACK TRANSACTION

SET IMPLICIT_TRANSACTIONS ON

ALTER DATABASE

CREATE

DELETE

DROP

FETCH

GRANT

INSERT

OPEN

REVOKE

SELECT

TRUNCATE TABLE

UPDATE


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SQL Server Locking

Concurrency Problems Prevented by Locks

Dealing with Deadlocks


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Concurrency Problems Prevented by Locks

Lost update

Two simultaneous updates

Uncommitted dependency (Dirty Read)

Selecting data thats being updated

Inconsistent analysis (Nonrepeatable Read)

Reading data that changed between multiple reads

Phantoms reads

Reading data that doesnt exist on the second read


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Concurrency Control

Optimistic concurrency control Low contention for data

Pessimistic concurrency control High contention for data

Isolation level issues

Lower isolation level: increases concurrency; increases chanceof reading bad data

Higher isolation level: ensure data is correct; negatively affectsconcurrency


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Transaction Isolation Level

READ COMMITTED (DEFAULT)

Prevents SELECT statements from reading dirty, or uncommitted data

READ UNCOMMITTED

Allows dirty reads

REPEATABLE READ

No other rows can update/delete rows in the result set until released;inserting news rows is okay

SERIALIZABLE

No other transaction can make any changes


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Dynamic Locking Architecture

DynamicLocking

TablePageRow

Cost

GranularityLocking Cost

ConcurrencyCost


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Blocks


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Deadlocks

Occurs when two transactions are blocking each other


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Deadlock

How SQL server ends a deadlock

How to minimize deadlocks Access objects in same order Keep transactions short

Use low isolation level

Use bound connections

How to customize the lock time-out setting SET LOCK_TIMEOUT function


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Remarks

Keep transactions short

Design transactions to minimize deadlocks

Use SQL Server defaults for locking

Be careful when use locking options


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Documents

Session10 Transaction