Query remains active while calling a batch file from stored procedure in SQL Server2008 - sql

Question

I created the following stored procedure:
CREATE PROCEDURE [dbo].[_StartWebcamStream]
AS
BEGIN
declare #command varchar(200)
set #command = 'C:\startStream.bat'
exec master..xp_cmdshell #command
END
for executing a the batch file startStream.bat. This batch contain the following code:
"C:\Program Files (x86)\VideoLAN\VLC\vlc.exe" -I dummy -vvv rtsp://mycamaddress/live_mpeg4.sdp --network-caching=4096 --sout=#transcode{vcodec=mp4v,fps=15,vb=512,scale=1,height=240,width=320,acodec=mp4a,ab=128,channels=2}:duplicate{dst=http{mux=asf,dst=:11345/},dst=display} :sout-keep}
The batch file is launched correctly, but the query continues to run until the vlc is stopped.
What can I do for stopping the query letting the vlc running?

Answer 1

DISCLAIMER: Don't run these examples in production!
SQL Server watches all processes spawned by xp_cmdshell and waits for them to finish. to see that you can just run this statement:
EXEC xp_cmdshell 'cmd /C "start cmd /K dir"'
This starts a command shell that in-turn starts another command shell to execute the dir command. The first shell terminates right away after spawning the second (/C switch) while the second executes the "dir" and then does not terminate (/K switch). Because of that second lingering process, even though the process that SQL Server started directly is gone, the query won't return. You cannot even cancel it. Even if you close the window in SSMS, the request continues to run. You can check that with
SELECT r.session_id, t.text
FROM sys.dm_exec_requests r
CROSS APPLY sys.dm_exec_sql_text(r.sql_handle) t
The NO_OUTPUT parameter does not help either:
EXEC xp_cmdshell 'cmd /C "start cmd /K dir"', NO_OUTPUT;
You will see the same "sticky" behavior.
The only way to get rid of these is to restart the computer or manually kill the processes (requires taskmanager to be executed as administrator). Restarting the SQL Server service does not stop the spawned processes.
As a solution you can use the SQL Agent. It has a "Operating System (CmdExec)" step type that you can use to run your program. You can create the job and then start it using sp_start_job.
Either way, your process actually needs to finish at some point. Otherwise you will create a pile of process-"bodies" that will cause performance problems in the long run.