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Change Data Capture - sys.dm_cdc_log_scan_sessions

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Applies to: SQL Server Azure SQL Database Azure SQL Managed Instance

Returns one row for each log scan session in the current database. The last row returned represents the current session. You can use this view to return status information about the current log scan session, or aggregated information about all sessions since the instance of SQL Server was last started.

The records in sys.dm_cdc_log_scan_sessions are reset whenever the instance of SQL Server is restarted or after a failover occurs. Additionally, if the queried database is in the secondary role of an Always On availability group, then no records are returned.

For more information, review What is change data capture (CDC)?

Column name Data type Description
session_id int ID of the session.

0 = the data returned in this row is an aggregate of all sessions since the instance of SQL Server was last started.
start_time datetime Time the session began.

When session_id = 0, the time aggregated data collection began.
end_time datetime Time the session ended.

NULL = session is active.

When session_id = 0, the time the last session ended.
duration int The duration (in seconds) of the session.

0 = the session does not contain change data capture transactions.

When session_id = 0, the sum of the duration (in seconds) of all sessions with change data capture transactions.
scan_phase nvarchar(200) The current phase of the session. The following are the possible values and their descriptions:

1: Reading configuration
2: First scan, building hash table
3: Second scan
4: Second scan
5: Second scan
6: Schema versioning
7: Last scan
8: Done

When session_id = 0, this value is always "Aggregate".
error_count int Number of errors encountered.

When session_id = 0, the total number of errors in all sessions.
start_lsn nvarchar(23) Starting LSN for the session.

When session_id = 0, the starting LSN for the last session.
current_lsn nvarchar(23) Current LSN being scanned.

When session_id = 0, the current LSN is 0.
end_lsn nvarchar(23) Ending LSN for the session.

NULL = session is active.

When session_id = 0, the ending LSN for the last session.
tran_count bigint Number of change data capture transactions processed. This counter is populated in phase 2.

When session_id = 0, the number of processed transactions in all sessions.
last_commit_lsn nvarchar(23) LSN of the last commit log record processed.

When session_id = 0, the last commit log record LSN for any session.
last_commit_time datetime Time the last commit log record was processed.

When session_id = 0, the time the last commit log record for any session.
log_record_count bigint Number of log records scanned.

When session_id = 0, number of records scanned for all sessions.
schema_change_count int Number of data definition language (DDL) operations detected. This counter is populated in phase 6.

When session_id = 0, the number of DDL operations processed in all sessions.
command_count bigint Number of commands processed.

When session_id = 0, the number of commands processed in all sessions.
first_begin_cdc_lsn nvarchar(23) First LSN that contained change data capture transactions.

When session_id = 0, the first LSN that contained change data capture transactions.
last_commit_cdc_lsn nvarchar(23) LSN of the last commit log record that contained change data capture transactions.

When session_id = 0, the last commit log record LSN for any session that contained change data capture transactions
last_commit_cdc_time datetime Time the last commit log record was processed that contained change data capture transactions.

When session_id = 0, the time the last commit log record for any session that contained change data capture transactions.
latency int The difference, in seconds, between end_time and last_commit_cdc_time in the session. This counter is populated at the end of phase 7.

When session_id = 0, the last nonzero latency value recorded by a session.
empty_scan_count int Number of consecutive sessions that contained no change data capture transactions.
failed_sessions_count int Number of sessions that failed.

Remarks

The sys.dm_cdc_log_scan_sessions DMV contains up to 32 scan sessions and an aggregate of all the scan sessions with session_id= 0. So, at any given time, this dynamic management view can contain a maximum of 33 rows.

Permissions

Requires VIEW DATABASE STATE permission to query the sys.dm_cdc_log_scan_sessions dynamic management view. For more information about permissions on dynamic management views, see Dynamic Management Views and Functions (Transact-SQL).

Permissions for SQL Server 2022 and later

Requires VIEW DATABASE PERFORMANCE STATE permission on the database.

Examples

The following example returns information for the most current session.

USE AdventureWorks2022;  
GO  
SELECT session_id, start_time, end_time, duration, scan_phase,  
    error_count, start_lsn, current_lsn, end_lsn, tran_count,  
    last_commit_lsn, last_commit_time, log_record_count, schema_change_count,  
    command_count, first_begin_cdc_lsn, last_commit_cdc_lsn,   
    last_commit_cdc_time, latency, empty_scan_count, failed_sessions_count  
FROM sys.dm_cdc_log_scan_sessions  
WHERE session_id = (SELECT MAX(b.session_id) FROM sys.dm_cdc_log_scan_sessions AS b);  
GO

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