1. Israeli SQL Server Users Group Change Data Capture Yossi MihailoviciSQL Server Consultantyossi@madeira.co.il

2. Agenda Introduction to CDC How does CDC work? DDL changes on a tracked table CDC objects overview What is it useful for? Performance issues

3. Mechanism that records any DML changes (Insert,Update,Delete) on a table Changes are captured for each row in a table All changes are recorded into designated table This feature is available only in SQL 2008 Enterprise Introduction

4. CDC needs to be enabled on DB level and on table level When enabling CDC for the first time, it automatically generates 2 jobs: Job for capture* Job for cleanup The capture job runs every 5 seconds and scans the transaction log for changes in the tracked table Once a change has been recognized and commited in the log, it’s recorded into a designated table How does it work?

5. Data Flow

6. The tracking table is created when CDC is enabled for a table The table is named according to the captured instance name and is categorized as system table under cdc schema Like any regular table, it can be queried, and indexes can be created on it This table consists of 5 fixed columns + 1 column for each tracked column Tracking Table

7. Alter column is cascaded to the tracked table When dropping a tracked column, the tracking table shows null values for this column Adding a column isn’t supported in the captured instance All DDL changes on a tracked table are recorded in cdc.ddl_history DDL Changes

8. System Tables: cdc.captured_columns cdc.change_tables cdc.ddl_history cdc.index_columns cdc.lsn_time_mapping cdc.Schema_Name_CT (change tables) DMVs: sys.dm_cdc_log_scan_sessions sys.dm_repl_traninfo sys.dm_cdc_errors CDC Objects Overview

9. System Stored Procedures: sys.sp_cdc_cleanup_change_table sys.sp_cdc_disable_db_change_data_capture sys.sp_cdc_disable_table_change_data_capture sys.sp_cdc_enable_db_change_data_capture sys.sp_cdc_enable_table_change_data_capture sys.sp_cdc_get_ddl_history sys.sp_cdc_get_captured_columns sys.sp_cdc_help_change_data_capture sys.sp_cdc_help_jobs sys.sp_cdc_change_job CDC Objects Overview

10. System Functions: cdc.fn_cdc_get_all_changes_<capture_instance> cdc.fn_cdc_get_net_changes_<capture_instance> sys.fn_cdc_decrement_lsn sys.fn_cdc_get_column_ordinal sys.fn_cdc_get_max_lsn sys.fn_cdc_get_min_lsn sys.fn_cdc_has_column_changed sys.fn_cdc_increment_lsn sys.fn_cdc_is_bit_set sys.fn_cdc_map_lsn_to_time sys.fn_cdc_map_time_to_lsn CDC Objects Overview

11. ETL process to Data Warehouse – No need in “Date Last Change” column Trigger replacement Auditing Snapshot for a table Fixing human errors Evaluate workload on a table Analyzing data changes What is it useful for?

12. Doesn’t support new column types introduced in SQL 2008 Doesn’t support partition swicth operation Only 2 captured instances can be created on a table Transaction log can’t be truncated as long as there are changes marked for capture Capture and cleanup jobs have the same definitions for all capture tables Limitations

13. LOB data types that weren’t updated aren’t shown in the before update row CDC can’t be enabled on another CDC table In order to support net changes tracking, table must have PK or unique index Table tracked by CDC can’t be truncated Limitations

14. CDC overhead is mostly on the transaction log file and on the tracking table To increase performance, it is recommended to separate CDC table to a different data file If capture job is disabled, there is still some overhead, but it’s very minimal Effect on Performance

15. Thank You!