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Enabling Change Data Capture

This topic covers how to enable change data capture for a database and a table.

Before a capture instance can be created for individual tables, a member of the sysadmin fixed server role must first enable the database for change data capture. This is done by running the stored procedure sys.sp_cdc_enable_db (Transact-SQL) in the database context. To determine if a database is already enabled, query the is_cdc_enabled column in the sys.databases catalog view.

When a database is enabled for change data capture, the cdc schema, cdc user, metadata tables, and other system objects are created for the database. The cdc schema contains the change data capture metadata tables and, after source tables are enabled for change data capture, the individual change tables serve as a repository for change data. The cdc schema also contains associated system functions used to query for change data.

Change data capture requires exclusive use of the cdc schema and cdc user. If either a schema or a database user named cdc currently exists in a database, the database cannot be enabled for change data capture until the schema and or user are dropped or renamed.

See the Enable Database for Change Data Capture template for an example of enabling a database.

Important noteImportant

To locate the templates in SQL Server Management Studio, go to View, click Template Explorer, and then select SQL Server Templates. Change Data Capture is a sub-folder. Under this folder, you will find all the templates referenced in this topic. There is also a Template Explorer icon on the SQL Server Management Studio toolbar.

-- ================================

-- Enable Database for CDC template

-- ================================

USE MyDB
GO

EXEC sys.sp_cdc_enable_db
GO

After a database has been enabled for change data capture, members of the db_owner fixed database role can create a capture instance for individual source tables by using the stored procedure sys.sp_cdc_enable_table. To determine whether a source table has already been enabled for change data capture, examine the is_tracked_by_cdc column in the sys.tables catalog view.

The following options can be specified when creating a capture instance:

Columns in the source table to be captured.

By default, all of the columns in the source table are identified as captured columns. If only a subset of columns need to be tracked, such as for privacy or performance reasons, use the @captured_column_list parameter to specify the subset of columns.

A filegroup to contain the change table.

By default, the change table is located in the default filegroup of the database. Database owners who want to control the placement of individual change tables can use the @filegroup_name parameter to specify a particular filegroup for the change table associated with the capture instance. The named filegroup must already exist. Generally, it is recommended that change tables be placed in a filegroup separate from source tables. See the Enable a Table Specifying Filegroup Option template for an example showing use of the @filegroup_name parameter.

===================================================

-- Enable a Table Specifying Filegroup Option Template

-- ===================================================

USE MyDB
GO

EXEC sys.sp_cdc_enable_table
@source_schema = N'dbo',
@source_name   = N'MyTable',
@role_name     = N'MyRole',
@filegroup_name = N'MyDB_CT',
@supports_net_changes = 1
GO

A role for controlling access to a change table.

The purpose of the named role is to control access to the change data. The specified role can be an existing fixed server role or a database role. If the specified role does not already exist, a database role of that name is created automatically. Members of either the sysadmin or db_owner role have full access to the data in the change tables. All other users must have SELECT permission on all the captured columns of the source table. In addition, when a role is specified, users who are not members of either the sysadmin or db_owner role must also be members of the specified role.

If you do not want to use a gating role, explicitly set the @role_name parameter to NULL. See the Enable a Table Without Using a Gating Role template for an example of enabling a table without a gating role.

-- ===================================================

-- Enable a Table Without Using a Gating Role template

-- ===================================================

USE MyDB
GO

EXEC sys.sp_cdc_enable_table
@source_schema = N'dbo',
@source_name   = N'MyTable>',
@role_name     = NULL,
@supports_net_changes = 1
GO

A function to query for net changes.

A capture instance will always include a table valued function for returning all change table entries that occurred within a defined interval. This function is named by appending the capture instance name to "cdc.fn_cdc_get_all_changes_". For more information, see cdc.fn_cdc_get_all_changes_<capture_instance> (Transact-SQL).

If the parameter @supports_net_changes is set to 1, a net changes function is also generated for the capture instance. This function returns only one change for each distinct row changed in the interval specified in the call. For more information, see cdc.fn_cdc_get_net_changes_<capture_instance> (Transact-SQL).

To support net changes queries, the source table must have a primary key or unique index to uniquely identify rows. If a unique index is used, the name of the index must be specified using the @index_name parameter. The columns defined in the primary key or unique index must be included in the list of source columns to be captured.

See the Enable a Table for All and Net Changes Queries template for an example demonstrating the creation of a capture instance with both query functions.

=======================================================

-- Enable a Table for All and Net Changes Queries template

-- =======================================================

USE MyDB
GO

EXEC sys.sp_cdc_enable_table
@source_schema = N'dbo',
@source_name   = N'MyTable',
@role_name     = N'MyRole',
@supports_net_changes = 1
GO
NoteNote

If change data capture is enabled on a table with an existing primary key, and the @index_name parameter is not used to identify an alternative unique index, the change data capture feature will use the primary key. Subsequent changes to the primary key will not be allowed without first disabling change data capture for the table. This is true regardless of whether support for net changes queries was requested when change data capture was configured. If there is no primary key on a table at the time it is enabled for change data capture, the subsequent addition of a primary key is ignored by change data capture. Because change data capture will not use a primary key that is created after the table was enabled, the key and key columns can be removed without restrictions.

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