How It Works: sys.dm_tran_session_transactions

For some reason I have been looking at DMV output closely the last couple of weeks. I just blogged about the pending I/O requests and now I have a behavior to outline for dm_tran_session_transactions for todays blog.

select * from sys.dm_tran_session_transactions

The scenario I was looking at is as follows.

Expand table

Server A		Server B
Broker calls activation procedure
Begin Tran (local)
Linked Server Query	Transaction Promoted to DTC	Transaction Imported from Server A
xp_cmdshell	Looped back and blocked Separate Transaction	Blocking

• The process is based on SQL Service Broker. 
• The service broker session on Server A is 20s. 
• It starts a local transaction and eventually performs a linked server query.  
• This causes the transaction to be promoted to a DTC transaction.  

At this point in time the *active transaction* DMVs on both Server A and B show the enlisted UOW of the DTC transaction. That is, except the sys.dm_tran_session_transactions on server A.

After some digging I uncovered that the dm_tran_session_transactions DMV only outputs rows for sessions (s) that are NOT system level sessions. Since the broker activity is handled on a system session the DMV will not materialize a row for Session 20s on Server A in this example. Instead you have to use the additional *active transaction* tables to track the UOW across this system.

Note: I am able to use any transaction (local or DTC) as part of SQL Service Broker activation which will not show rows in the session transactions DMV because it is considered a system session.

Bob Dorr - Principal SQL Server Escalation Engineer