Hướng dẫn học Microsoft SQL Server 2008 part 131 pptx

Removing the session configuration requires aDROP command: ALTER EVENT SESSION [CheckPoint] ON SERVER State = STOP; DROP EVENT SESSION [CheckPoint] ON SERVER Summary Extended Events may

SQL Server MVP and all-around smart guy, Jonathan Kehayias, has developed ‘‘Extended Events Manager,’’ a UI for Extended Events that you can download from www.codeplex com/ExtendedEventManager

Jonathan is also writing a white paper on Extended Events for Microsoft Having read a draft,

if you’re interested in implementing an Extended Events solution, I highly recommend it See

www.sqlserverbible.com for a link.

Packages

At the top level, Extended Events organizes its components into three Microsoft-developed packages (it’s

really four, but the fourth package,SecAudit, is a private package dedicated to SQL Audit):

■ Package0: Provides base functionality used by the other packages

■ sqlos: Works with the SQL/OS events

■ sqlserver: Works with the rest of SQL Server Each of these packages contains several objects To see the full list of objects within packages, query the

following DMVs:

SELECT p.NAME AS Package, o.object_type,

o.NAME AS Object, o.description FROM sys.dm_xe_objects o

JOIN sys.dm_xe_packages p

ON o.type_package_guid = p.guid ORDER BY p.NAME, o.object_type

Result (abbreviated):

Package object_type Object description

- - -

-package0 action tsql_stack Collect Transact-SQL stack

package0 action sql_text Collect SQL text

package0 action plan_handle Collect plan handle

package0 action create_dump_single_thread Create mini dump for the

current thread package0 action create_dump_all_threads Create mini dump including

all threads

.

Theobject_typecolumn reveals seven types of objects: actions, events, maps, pred_compares,

pred_sources, targets, and types

Building a crosstab query from the previous query analyzes the distribution of object types within

the packages:

SELECT Package, action, event, map, pred_compare, pred_source, target, type FROM (SELECT o.Name AS Object, p.Name AS Package, o.object_type

FROM sys.dm_xe_objects o JOIN sys.dm_xe_packages p

Extended Events 58

ON o.type_package_guid = p.guid) sq PIVOT (COUNT(Object)

FOR object_type IN

(action, event, map, pred_compare, pred_source, target, type)

) AS pt;

Result:

Package action event map pred_compare pred_source target type

- -

Objects

Within each package is a host of objects Creating an Extended Event is basically a process of evoking

the right objects To dig into these object types:

■ Events: Similar to SQL Trace events, XE has more events and their columns are listed in

sys.dm_xe_object_columns

Extended Events borrows the concept of event channels from Event Tracing for Windows

(ETW) In theChannelread-only column, every event is in one of four predetermined

channels: admin, analytic, debug, or operational

■ Actions: XE actions fire as the result of an event and can gather additional data about the

context of the event

■ Types and Maps: A high-performance optimization, types and maps both serve as pointers to

data Instead of having to gather every data point synchronously, a type or map can point

to the data in memory

■ Pred_Compares and Pred_Sources: Predicates are essentially filters, similar to SQL Trace

filters

■ Targets: The target object defines the destination for the event data and the synchronous or

asynchronous nature of the target

XE Sessions

The Extended Events equivalent of starting a SQL Trace is a session Sessions can be launched, queried,

and stopped using T-SQL code

The following example launches a simple event that tracks when SQL Server performs checkpoints

Using thering_bufferas the target captures the data in memory:

CREATE EVENT SESSION [CheckPoint]

ON SERVER

ADD EVENT sqlserver.checkpoint_end

ADD TARGET package0.ring_buffer;

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The session is now created, but it’s not running until anALTERcommand actually starts the session:

ALTER EVENT SESSION [CheckPoint]

ON SERVER State = START;

The session can now be seen in the session DMV:

SELECT address, name FROM sys.dm_xe_sessions

Result:

- -0x0000000080576081 system_health

0x0000000080360D61 CheckPoint

Sure enough, there’s theCheckPointsession SQL Server automatically started a default

system_healthsession as well, which is worth exploring

Joining the session DMV withsys.dm_xe_session_targetswill retrieve the event data from the

ring_buffertarget and expose the data for every captured event within a single XML value:

SELECT CONVERT(XML, st.target_data) AS ring_buffer

FROM sys.dm_xe_sessions s JOIN sys.dm_xe_session_targets st

ON s.address = st.event_session_address WHERE NAME = ‘CheckPoint’

Result (abbreviated and opened in the browser window):

<RingBufferTarget eventsPerSec="0" processingTime="0"

totalEventsProcessed="13"

eventCount="13" droppedCount="0" memoryUsed="546">

<event name="checkpoint_end" package="sqlserver" id="86" version="1"

timestamp="2009-05-11T07:12:00.220Z">

<data name="database_id">

<type name="uint16" package="package0" />

<value>3</value>

<text />

</data>

</event>

<event name="checkpoint_end" package="sqlserver" id="86" version="1"

timestamp="2009-05-11T07:12:00.719Z">

<data name="database_id">

<type name="uint16" package="package0" />

<value>12</value>

<text />

Extended Events 58

</data>

</event>

.

Other types of events and actions will return other elements within the XML data You can find

addi-tional examples on the chapter’s downloadable SQL script

Extracting the data from the XML into relational data requires a little XPath:

SELECT

node.event_data.value(’(data/value)[1]’, ‘BIGINT’) AS

source_database_id,

node.event_data.value(’(timestamp)[1]’, ‘Datetime’) AS test

FROM (SELECT CONVERT(XML, st.target_data) AS ring_buffer

FROM sys.dm_xe_sessions s JOIN sys.dm_xe_session_targets st

ON s.address = st.event_session_address WHERE NAME = ‘CheckPoint’ ) AS sq

CROSS APPLY sq.ring_buffer.nodes(’//RingBufferTarget/event’) node

(event_data)

Result:

source_database_id test

-

To stop the session, use anotherALTERcommand Removing the session configuration requires aDROP

command:

ALTER EVENT SESSION [CheckPoint]

ON SERVER

State = STOP;

DROP EVENT SESSION [CheckPoint]

ON SERVER

Summary

Extended Events may seem cryptic at first, but with a few attempts, I think you’ll find they’re not that

complicated They offer better performance and granularity than SQL Trace, but at the cost of raw

T-SQL and XPath

The key to working with Extended Events is exploring the objects so you know which events and

actions to monitor

The next chapter also deals with a new monitoring technology in SQL Server 2008, Change Tracking

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Change Tracking

IN THIS CHAPTER

Lightweight synchronization for data warehouse ETL and mobile applications Net changes Auto cleanup

Change Tracking is one of the best-named software features I’ve come

across All Change Tracking does is track changes; it says to the world:

‘‘This row was changed, here’s the PK.’’ Clean and simple, no fuss,

no muss It’s a piece of cake to configure and easy to query

While Change Data Capture (another of the new auditing technologies in SQL

Server 2008, covered in the next chapter) is limited to only the Enterprise

Edi-tion, Change Tracking is available in all the SQL Server editions, even SQL Server

Express

Change Tracking occurs synchronously within the transaction It simply records

in an internal table the primary key values of the rows that are modified

Although there’s a cost to recording the changes within the transaction, it means

that SQL Agent is not required

Optionally, Change Tracking can store which columns were changed, using a

bit-mapped method similar to how triggers know which column was included in

the DML code

The real purpose of Change Tracking is to support synchronization By easily and

reliably recording the primary keys of which rows were inserted, updated, or

deleted since the last synchronization, it becomes much simpler to perform the

synchronization

Change Tracking returns the net changes If a row is inserted and updated since

the last synchronization, then Change Tracking will list it as an insert If the

row is inserted and deleted, then it won’t even show in the Change Tracking

results — which is perfect for applications that need synchronization

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Several applications can benefit from using Change Tracking:

■ The Microsoft Synch Framework

■ ETL processes that keep a data warehouse in synch with the OLTP database

■ Caching data in middle tiers for performance

■ Synchronizing occasionally connected mobile applications There is a performance hit for using Change Tracking, but it’s a lightweight feature with about the same

performance hit as adding an index The performance cost depends on the size of the primary key and

the number of rows affected by the transaction A single column integer primary will have less

perfor-mance cost than a wide composite primary key Adding column tracking also adds to the perforperfor-mance

overhead

As cool as Change Tracking is for applications that need synchronization, it isn’t adequate for OLTP

auditing (who changed what rows to what new values when and from where?) for two reasons First,

Change Tracking does have a context option, but it’s not very pretty Second, Change Tracking returns

the net changes, so intermediate changes wouldn’t be audited

Configuring Change Tracking

Compared to other optional SQL Server technologies, Change Tracking is relatively easy to turn on and

configure It’s simply turned on for the database, and then for each table

Enabling the database

Change Tracking may be enabled for the database in Object Explorer’s Database Properties dialog,

available from each database’s context menu (as shown in Figure 59-1) Changing the values in the

drop-down boxes immediately changes the database settings when you close the properties dialog

Change Tracking may also be configured with T-SQL I like that it uses normal SQLALTER SET

commands and not system stored procedures:

ALTER DATABASE AdventureWorks2008 SET Change_tracking = on

(change_retention = 24 hours, auto_cleanup = on);

The current Change Tracking database configuration can be viewed in the Object Explorer Database

Properties dialog, or by querying thesys.change_tracking_databasesDMV:

SELECT d.name, ct.is_auto_cleanup_on, ct.retention_period, ct.retention_period_units, ct.retention_period_units_desc

FROM sys.change_tracking_databases ct

JOIN sys.databases d

ON ct.database_id = d.database_id;

The database must be in 9.0 compatibility mode, and at least db_owner role permission is required, to

enable the database for Change Tracking

Change Tracking 59

FIGURE 59-1

The Database Properties’ Change Tracking page displays the current settings and may be used to

enable or disable Change Tracking

Auto cleanup

Change Tracking can create a lengthy audit trail, but it can also optionally automatically clean up

old Change Tracking data The retention period can be set to any number ofDays,Hours, or

Minutes The default is to retain the data for two days (which is probably too short for most

applications)

Auto_cleanupand the retention period can be set when Change Tracking is initially enabled, or it

can be modified later by reissuing the setChange_Trackingoption with the new retention settings

In this situation, because Change Tracking is already enabled, re-enabling Change Tracking would

generate an error It’s only necessary to change the option setting:

ALTER DATABASE [AdventureWorks2008]

SET change_tracking (change_retention = 7 days)

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Be careful with the auto cleanup retention period Change Tracking doesn’t know when

synchroniza-tions occur If the synchronization application doesn’t run before the retention period expires, then the

changes will not be seen by the synchronization

If there’s a problem and the synchronization won’t occur, then use theALTER DATABASEcommand to

toggle auto cleanup:

ALTER DATABASE [AdventureWorks2008]

SET change_tracking (auto_cleanup = off)

Best Practice

Estimate the longest possible period between synchronizations and then triple that time Other than the

disk space usage, there’s no risk in a longer retention period, but there’s considerable risk in a retention

period that’s too short

Enabling tables

The ease of configuring Change Tracking continues with enabling Change Tracking of each table Using

Management Studio, table Change Tracking is viewed or enabled/disabled in the Table Properties dialog,

on the Change Tracking page, as shown in Figure 59-2

Like the database, Change Tracking is enabled using T-SQL for tables with anALTERcommand:

ALTER TABLE HumanResources.Department Enable Change_tracking

With (track_columns_updated = on);

The only option is to enable or disable whether Change Tracking tracks which columns were changed

By default, column tracking is disabled There’s a section later in this chapter that explains how to use

column tracking

TheALTER TABLEpermission is required to enable the table for Change Tracking

Enabling Change Tracking for a table can affect other tasks:

■ The primary key constraint/index cannot be dropped while Change Tracking is enabled

■ If the table is dropped, then Change Tracking is first removed from the table

■ The table can’t use a partitioned table’sALTER TABLE .SWITCH PARTITIONcommand

■ Change Tracking does not track changes made by theTRUNCATE TABLEcommand In this case, the synch target table should also be truncated

Change Tracking 59

FIGURE 59-2

The Table Properties dialog may be used to view the table’s Change Tracking settings

To view the current tables with Change Tracking enabled, query thesys.change_tracking_tables

DMV:

SELECT s.name + ‘.’ + t.name as [table],

ct.is_track_columns_updated_on,ct.min_valid_version,

ct.begin_version, ct.cleanup_version

FROM sys.change_tracking_tables ct

JOIN sys.tables t

ON ct.object_id = t.object_id

JOIN sys.schemas s

ON t.schema_id = s.schema_id

ORDER BY [table];

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