Hướng dẫn học Microsoft SQL Server 2008 part 133 ppt

The CASTfunction converts it to readable text: SELECT CT.SYS_CHANGE_VERSION, CT.DepartmentID, CT.SYS_CHANGE_OPERATION, d.Name, d.GroupName, D.ModifiedDate, CASTSYS_CHANGE_CONTEXT as VARC

Trang 1

Nielsen c59.tex V4 - 07/21/2009 3:58pm Page 1282

Part VIII Monitoring and Auditing

Here’s the catch: The context isn’t automatic — it must be added to each and every DML command In

addition, it uses aWITHclause, just like a common table expression, so the syntax is confusing While

I’m glad it’s possible to capture the context, I’m not a huge fan of the implementation

The following code creates thevarbinaryvariable and passes it to Change Tracking as part of an

UPDATEcommand

DECLARE @AppContext VARBINARY(128)

= CAST(‘Maui/Pn’ as VARBINARY(128));

WITH Change_Tracking_Context (@AppContext)

UPDATE HumanResources.Department SET GroupName = ‘Certified Master w/Context’

WHERE Name = ‘Row Three’;

When queryingChangeTable, thesys_Change_Contextcolumn returns the context data The

CAST()function converts it to readable text:

SELECT CT.SYS_CHANGE_VERSION, CT.DepartmentID, CT.SYS_CHANGE_OPERATION, d.Name, d.GroupName, D.ModifiedDate,

CAST(SYS_CHANGE_CONTEXT as VARCHAR) as ApplicationContext

FROM ChangeTable (Changes HumanResources.Department, 5) as CT LEFT OUTER JOIN HumanResources.Department d

ON d.DepartmentID = CT.DepartmentID ORDER BY CT.SYS_CHANGE_VERSION;

Removing Change Tracking

It’s as easy to remove Change Tracking as it is to enable it: Disable it from every table, and then remove

it from the database

If the goal is to reduce Change Tracking by a single table, then the sameALTERcommand that enabled

Change Tracking can disable it:

ALTER TABLE HumanResources.Department Disable Change_tracking;

When Change Tracking is disabled from a table, all storedChangeTabledata — the PKs and columns

updated — are lost

If the goal is to remove Change Tracking from the database, then Change Tracking must first

be removed from every table in the database One way to accomplish this is to leverage the

sp_MSforeachtablestored procedure:

EXEC sp_MSforeachtable

‘ALTER TABLE ? Disable Change_tracking;’;

Trang 2

Change Tracking 59

However, after much testing, I can only warn that in many casessp_msforeachtableoften fails to

remove Change Tracking from every table

A less elegant, but more reliable, method of ensuring that Change Tracking is completely removed from

every table in the database is to actually cursor through the sys.change_tracking_tablestable:

DECLARE @SQL NVARCHAR(MAX)=‘’;

SELECT @SQL = @SQL + ‘ALTER TABLE ’ + s.name + ‘.’ + t.name +

‘ Disable Change_tracking;’

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;

PRINT @SQL;

EXEC sp_executesql @SQL;

Only after Change Tracking is disabled from every table can Change Tracking be removed from the

database:

ALTER DATABASE AdventureWorks2008

SET Change_tracking = off;

Even though Change Tracking is removed from the database, it doesn’t reset the Change Tracking

version number, so if Change Tracking is restarted it won’t cause a synchronization nightmare

Summary

Designing a DIY synchronization system involves triggers that either update row timestamps or write

keys to a table Change Tracking does all the hard work, adds auto cleanup, is relatively easy to set up

and use, and reliably returns the net changes Without question, using Change Tracking sets you up for

success with ETL processes and mobile device synchronization

Microsoft introduces several new auditing and monitoring technologies with SQL Server 2008 The

next chapter continues exploring these new technologies with Change Tracking’s big brother, Change

Data Capture

Trang 3

Trang 4

Change Data Capture

IN THIS CHAPTER

High-end BI ETL Leveraging the T-Log

Iknow almost nothing about the CDC in Atlanta The little I do know about

the Centers for Disease Control comes from watching Dustin Hoffman in the

movie Outbreak Fortunately for me and you, this chapter is about the other

CDC — Change Data Capture

There’s power hidden in the transaction log (T-Log), and Change Data Capture

(CDC) harnesses the transaction log to capture data changes with the least

possible impact on performance

Any data written to the transaction log can be asynchronously captured using

CDC from the transaction log after the transaction is complete, so it doesn’t affect

the original transaction’s performance CDC can track any data from the T-Log,

including any DMLINSERT,UPDATE,DELETE, andMERGEcommand, and DDL

CREATE,ALTER, andDROP

Changes are stored in change tables — tables created by CDC with the same

columns as the tracked tables plus a few extra CDC-specific columns All the

changes are captured, so CDC can return all the intermediate values or just

the net changes

Because CDC gathers its data by reading the log, the data in the change tables is

organized the same way the transaction log is organized — by T-log log sequence

numbers, known as LSNs (Kalen Delaney told a joke about Oracle’s founder

Larry Ellison being inside SQL Server — just look at the transaction log and

there’s LSN! Ha!)

There are only a few drawbacks to CDC:

■ Cost: It requires Enterprise Edition

■ Code: Personally, this really irks me CDC uses system stored

proce-dures instead of standardizedALTERstatements

Trang 5

■ Code: There’s no UI for configuring change data capture in Management Studio

■ T-Log I/O: The transaction log will experience about twice as much I/O because CDC reads from the log

■ Performance hit: Although it can vary greatly, expect an approximate 10% performance hit

on the OLTP server running CDC

■ Disk space: Because CDC essentially stores copies of every transaction data, there’s the potential that it can grow like a transaction log gone wild

Where change data capture shines is in gathering data for ETL from a high-traffic OLTP database to a

data warehouse Of the possible options, change data capture has the least performance hit, and it does

a great job of providing the right set of data for the Business Intelligence ETL (extract-transform-load)

When you think big-dollar BI, think change data capture

Enabling CDC

Change Data Capture is enabled at the database level first, and then for every table that needs to be

tracked Because change data capture reads from the transaction log, one might think that CDC requires

the database to be set to full recovery model so that the transaction log is kept However, SQL Server

doesn’t flush the log until after the transactions have been read by CDC, so CDC will work with any

recovery model, even simple

Also, and this is very important, change data capture uses SQL Agent jobs to capture and clean up the

data, so SQL Agent must be running or data will not be captured

Enabling the database

To enable the database, execute thesys.sp_cdc_enable_dbsystem stored procedure in the current

database It has no parameters:

EXEC sys.sp_cdc_enable_db

Theis_cdc_enabledcolumn insys.databasescan be used to determine which databases have

CDC enabled on them:

SELECT * FROM sys.databases WHERE is_cdc_enabled = 1

This procedure creates six system tables in the current database:

■ cdc.captured_columns: Stores metadata for tracked table’s columns

■ cdc.change_tables: Stores metadata for tracked tables

■ cdc.ddl_history: Tracks DDL activity

■ cdc.index_columns: Tracks table indexes

■ cdc.lsn_time_mapping: Used for calculating clean-up time

Trang 6

Change Data Capture 60

■ dbo.systranschemas: Tracks schema changes

These are listed in Object Explorer under the Database➪ Tables ➪ System tables node

Enabling tables

Once the database has been prepared for CDC, tables may be set up for CDC using thesys.sp_

cdc_enable_tablestored procedure, which has several options:

■ @source_schema: The name of the table to be tracked

■ @source_name: The tracked table’s schema

■ @role_name: The role with permission to view CDC data

The last six parameters are optional:

■ @capture_instance: May be used to create multiple capture instances for the table This is

useful if the schema is changed

■ @supports_net_changes: Allows seeing just the net changes, and requires the primary

key The default is true

■ @index_name: The name of the unique index, if there’s no primary key for the table (but

you’d never do that, right?)

■ @captured_column_list: Determines which columns are tracked The default is to track

all columns

■ @filegroup_name: The filegroup the CDC will be stored on If not specified, then the

change table is created on the default filegroup

■ @allow_partition_switch: AllowsALTER TABLE SWITCH PARTITIONon

CDC table

Note that the last parameter,@allow_partition_switch, was changed late in development of SQL

Server 2008, and some sources incorrectly list it as@partition_switch

The following batch configures CDC to track changes made to theHumanResources.Department

table:

EXEC sys.sp_cdc_enable_table

@source_schema = ‘HumanResources’,

@source_name = ‘Department’,

@role_name = null;

With the first table that’s enabled, SQL Server generates two SQL Agent jobs:

■ cdc.dbname_capture

■ cdc.dbname_cleanup

With every table that’s enabled for CDC, SQL Server creates a change table:

■ cdc.change_tables

Trang 7

■ cdc.index_columns

■ cdc.captured_columns

For an excellent article on tuning the performance of change data capture under various loads, see http://msdn.microsoft.com/en-us/library/dd266396.aspx

Working with Change Data Capture

It isn’t difficult to work with change data capture The trick is to understand the transaction log’s log

sequence numbers

AssumingAdventureWorks2008has been freshly installed, the following scripts make some data

changes so there will be some activity in the log for change data capture to gather:

INSERT HumanResources.Department (Name, GroupName) VALUES (’CDC New Row’, ‘SQL Rocks’),

(’Test Two’ , ‘CDC Rocks ’);

UPDATE HumanResources.Department SET Name = ‘Changed Name’

WHERE Name = ‘CDC New Row’;

INSERT HumanResources.Department (Name, GroupName) VALUES (’Row Three’, ‘PBM Rocks’),

(’Row Four’ , ‘TVP Rocks’);

UPDATE HumanResources.Department SET GroupName = ‘T-SQL Rocks’

WHERE Name = ‘Test Two’;

DELETE FROM HumanResources.Department WHERE Name = ‘Row Four’;

With five transactions complete, there should be some activity in the log The following DMVs can

reveal information about the log:

SELECT * FROM sys.dm_cdc_log_scan_sessions SELECT *

FROM sys.dm_repl_traninfo SELECT *

FROM sys.dm_cdc_errors

Trang 8

Examining the log sequence numbers

The data changes are organized in the change tables by log sequence number (LSN) Converting a

given date time to LSN is essential to working with change data capture Thesys.fn_cdc_map_

time_to_lsnfunction is designed to do just that The first parameter defines the LSN search (called

LSN boundary options), and the second parameter is the point in time Possible searches are as follows:

■ smallest greater than

■ smallest greater than or equal

■ largest less than

■ largest less than or equal

Each of the search options defines how the function will locate the nearest LSN in the change tables

The following sample query defines a range beginning with Jan 20 and ending with Jan 24, and returns

the LSNs that bound that range:

select

sys.fn_cdc_map_time_to_lsn

(’smallest greater than or equal’, ‘20090101’)

as BeginLSN,

(’largest less than or equal’, ‘20091231’)

as EndLSN;

Result:

-0x0000002F000001330040 0x0000003B000002290001

Thesys.fn_cdc_get_min_lsn()andsys.fn_cdc_get_max_lsn()functions serve as anchor

points to begin the walk through the log Theminfunction requires a table and returns the oldest log

entry Themaxfunction has no parameters and returns the most recent LSN in the change tables:

DECLARE

@BeginLSN VARBINARY(10) =

sys.fn_cdc_get_min_lsn(’HumanResources_Department’);

SELECT @BeginLSN;

DECLARE

@EndLSN VARBINARY(10) =

sys.fn_cdc_get_max_lsn();

SELECT @EndLSN;

There’s not much benefit to knowing the hexadecimal LSN values by themselves, but the LSNs can be

passed to other functions to select data from the change tables

Trang 9

Querying the change tables

Change tracking creates a function for each table being tracked using the namecdc.fn_cdc_get_

all_changesconcatenated with the schema and name of the table The following script uses the

sys.fn_cdc_map_time_to_lsnfunction to determine the LSN range values, store them in variables,

and then pass the variables to the department tables’ custom change data capture function:

with variables DECLARE

@BeginLSN VARBINARY(10) =

(’smallest greater than or equal’, ‘20090101’),

@EndLSN VARBINARY(10) =

(’largest less than or equal’, ‘20091231’);

SELECT $start_lsn, $seqval, $operation,

$update_mask, DepartmentID Name, GroupName, ModifiedDate FROM cdc.fn_cdc_get_all_changes_HumanResources_Department

(@BeginLSN, @EndLSN, ‘all’) ORDER BY $start_lsn

Result:

- - -0x0000005400001D6E0008 0x0000005400001D6E0003 2

0x0000005400001D6E0008 0x0000005400001D6E0006 2 0x0000005400001D700007 0x0000005400001D700002 4 0x0000005400001D7D0008 0x0000005400001D7D0003 2 0x0000005400001D7D0008 0x0000005400001D7D0006 2 0x0000005400001D7F0004 0x0000005400001D7F0002 4 0x0000005400001D810005 0x0000005400001D810003 1

$update_mask Name GroupName ModifiedDate - - -0x0F 17 SQL Rocks 2009-03-07 11:21:48.720 0x0F 18 CDC Rocks 2009-03-07 11:21:48.720 0x02 17 SQL Rocks 2009-03-07 11:21:48.720 0x0F 19 PBM Rocks 2009-03-07 11:21:55.387 0x0F 20 TVP Rocks 2009-03-07 11:21:55.387 0x04 18 T-SQL Rocks 2009-03-07 11:21:48.720 0x0F 20 TVP Rocks 2009-03-07 11:21:55.387

It’s also possible to pass the functions directly to the table’s change data capture function This is

essen-tially the same code as the previous query, but slightly simpler, which is usually a good thing:

SELECT * FROM cdc.fn_cdc_get_all_changes_HumanResources_Department (sys.fn_cdc_map_time_to_lsn

Trang 10

(’smallest greater than or equal’, ‘20090101’), sys.fn_cdc_map_time_to_lsn

(’largest less than or equal’, ‘20091231’),

‘all’) as CDC

ORDER BY $start_lsn

You can also convert an LSN directly to a time using thefn_cdc_map_lsn_to_time()function The

next query extends the previous query by returning the time of the transaction:

with lsn converted to time

SELECT

sys.fn_cdc_map_lsn_to_time( $start_lsn) as StartLSN, *

FROM cdc.fn_cdc_get_all_changes_HumanResources_Department

(sys.fn_cdc_map_time_to_lsn

(’smallest greater than or equal’, ‘20090101’),

sys.fn_cdc_map_time_to_lsn

(’largest less than or equal’, ‘20091231’),

‘all’) as CDC

ORDER BY $start_lsn

The $Operationcolumn returned by the change data capture custom table functions identifies the

type of DML that caused the data change Similar to a DML trigger, the data can be the before (deleted

table) or after (inserted table) image of an update

The default‘all’parameter directs CDC to only return the after, or new, image from an update

oper-ation The‘all update old’option, shown in the following example, tells CDC to return a row for

both the before update image and the after update image

This query uses a row constructor subquery to spell out the meaning of the operation:

SELECT

sys.fn_cdc_map_lsn_to_time( $start_lsn) as StartLSN,

Operation.Description as ‘Operation’,

DepartmentID, Name, GroupName

FROM cdc.fn_cdc_get_all_changes_HumanResources_Department

(sys.fn_cdc_map_time_to_lsn(’smallest greater than or equal’,

‘20090101’),

sys.fn_cdc_map_time_to_lsn(’largest less than or equal’,

‘20091231’),

‘all update old’) as CDC

JOIN

(VALUES

(1, ‘delete’),

(2, ‘insert’),

(3, ‘update/deleted’), ‘all update old’ option to view

(4, ‘update/inserted’)

) as Operation(OperationID, Description)

ON CDC $operation = Operation.OperationID

ORDER BY $start_lsn

Định dạng
Số trang	10
Dung lượng	0,97 MB