SQL Server 2008 introduces a new feature called Change Data Capture (CDC). CDC Captures DDL and DML activities on a SQL Server Table and places the changes in a separate SQL Server relational table. In this article we will see how DBA can administer and monitor CDC in SQL Server 2008. The CDC feature is disabled by default at the database level. A member of the sysadmin server role must enable a database for change data capture. Once the database is enabled for CDC any member of the dbo fixed database role can enable a table within the database for change data capture.
Overview of Change Data Capture
Once CDC is enabled at the database level, the next step is to enable CDC for a specific table for which the change needs to be captured. The CDC feature gathers the changed data from the database transaction log file and inserts the change information in an associated change table which is created during the setup and configuration process of CDC. There is a one to one relationship that exists between the source table and the change capture table. You can have a maximum of two change tables for a single source table. As the CDC feature needs to continuously read the transaction log file it’s obvious that for CDC to work SQL Server Agent should be running. As this feature in entrenched in the transaction log architecture and thus a lot of the metadata in CDC is related around the concept of a Log Sequence Number (LSN) Definition of Log Sequence Number (LSN)
Every record in the Microsoft SQL Server transaction log is uniquely identified by a log sequence number (LSN). LSN’s are ordered such that if LSN2 is greater than LSN1, the change described by the log record referred to by LSN2 occurs after the change described by the log record LSN. SQL Server Versions Supporting Change Data Capture
The Change Data Capture feature is available in SQL Server 2008 Enterprise, Developer Editions. How to Enable CDC for a SQL Server 2008 Database
1. Connect to a SQL Server 2008 instance using SQL Server Management Studio
2. In the query window, type the following TSQL Query to create a ChangeDataCapture Database: Use Master
IF EXISTS (SELECT name FROM sys.databases WHERE name = N’ChangeDataCapture’)
DROP DATABASE ChangeDataCapture
Create Database ChangeDataCapture
Go 3. Once the database is successfully created you need to enable the change data capture feature for the database, this can be done by executing the following TSQL Query: Use ChangeDataCapture
GO 4. Execute the query below to check whether the database is enabled for CDC: Select [name] as DBName, is_cdc_enabled from sys.databases The value of 1 for the is_cdc_enabled column means that the database is enabled for CDC and value of 0 means that the database is not enabled for CDC. 5. Once the database is enabled for CDC, you will see a new cdc schema, cdc user and a few metadata tables and other system objects created in the ChangeDataCapture database. The most important things which a DBA needs to keep in mind when enabling CDC for a database is to make sure that there are not a cdc schema or cdc user existing in the database before configuring CDC. If there was a cdc schema or cdc user existing in the database then the configuration of CDC will fail, so a DBA needs to remove or rename any previously existing cdc schema or user from the database before configuring CDC.