CHANGE DATA CAPTURE: A BRIEF OVERVIEW PRESENTED BY TIM WEIGEL

CHANGE DATA CAPTURE: WHAT IS IT? Broadly speaking, Change Data Capture (CDC) is a set of methods used to identify and track changes made to data There are a number of ways to track changes in a database I’m just going to give you the Wikipedia link:

CHANGE DATA CAPTURE IN SQL SERVER First implemented in SQL Server 2008 Uses a our friend the Transaction Log to get change information Enterprise-only feature –Change Tracking is available in all editions, but only tells you that something changed – it doesn’t tell you what! –There are community-based alternatives to CDC, but they rely on different mechanisms

WHAT MAKES CHANGE DATA CAPTURE TICK Our old friend, sp_replcmds –CDC will use the same Log Reader Agent to populate the change tables and the distribution tables if the table is published –If not, the cdc. _capture job executes sp_MScdc_capture_job which executes sp_cdc_scan which then executes sp_replcmds And a few other things, too… –CDC also has a cleanup job which executes sp_MScdc_cleanup_job which gathers retention information and then executes sp_cdc_cleanup_change_table The first rule of data warehouse is don’t talk about data warehouse… So we’re not gonna.

ENABLING CHANGE DATA CAPTURE CDC must be enabled at the database level first: – EXEC sys.sp_cdc_enable_db CDC must then be enabled on the tables you wish to track: – EXEC = N' = N' = N' ' or = N' ' – = -- = N' ' – = N' ' or NULL – = N' ' – = -- optional

CHANGE DATA CAPTURE METADATA AND SUPPORT TABLES Created when CDC is enabled on the DB Metadata tables –cdc.change_tables –cdc.index_columns –cdc.captured_columns Other tables of note –cdc.lsn_time_mapping –cdc.ddl_history –dbo.systranschemas

CHANGE DATA CAPTURE CHANGE TABLES The default naming convention is cdc. _ _CT Common columns: –__$start_lsn and __$end_lsn –__$seqval –__$operation 1 = delete, 2 = insert, 3 = update (before), and 4 = update (after) –__$update_mask Variable bitmap, one bit per column INSERTs and DELETEs will have all bits set, but UPDATES will only set the bits corresponding to the updated columns

WORKING WITH CHANGE DATA CAPTURE How to get data out –Query the change table directly Well, Microsoft says you oughtn’t do this… But you’re totally going to do this anyway, at least once –There are several built-in functions and procs! cdc.fn_cdc_get_all_changes_ and cdc.fn_cdc_get_net_changes_ sys.fn_cdc_map_lsn_to_time and sys.fn_cdc_map_time_to_lsn sys.fn_cdc_has_column_changed and sys.fn_cdc_get_column_ordinal sys.sp_cdc_get_captured_columns and sys.sp_cdc_get_ddl_history And many more!

WORKING WITH CHANGE DATA CAPTURE How to tune it –Customize sp_cdc_scan ? You can! – msdb.dbo.cdc_jobs –And of course, procs to work with it! sys.sp_cdc_help_jobs sys.sp_cdc_change_job allows you to change the parameters How to keep an eye on it – sys.sp_cdc_help_change_data_capture – sys.dm_cdc_log_scan_sessions lets you know what’s currently going on Where to find the errors – sys.dm_cdc_errors

DISABLING CHANGE DATA CAPTURE At the table level: – EXEC = N' = N' = N' ' At the database level: – EXEC sys.sp_cdc_disable_db

A FEW COMMON ISSUES CDC assumes you know what you’re doing – it doesn’t give you warnings. Rather, it gives you rope. Triggers on CDC tables: recipe for performance issues, if not outright disaster CDC tables are not persistent if CDC is disabled Replication and CDC: best frenemies –They use the same log reader if the same table(s) are published and tracked –New snapshots will disable CDC if a subscriber is tracked Yes, without warning –Proc exec isn’t available in transactional replication if CDC is also enabled New columns aren’t automatically added to CDC when added to a tracked table –It’s possible to add them without disabling and re-enabling CDC, but it’s kinda clunky Computed columns always have a value of NULL if captured, and some other unexpected behaviors

DEMO TIME! Enabling CDC Disabling CDC Status Errors

QUESTION TIME! Thank you very