How It Works: Attention, Attention or Should I say Cancel the Query and Be Sure to Process Your Results
I ran into this issue the other day and decided it would make a good post. Since I have seen this issue more times that I can count I will again attempt to provide details.
The attention event in SQL Server trace output indicates a query cancellation. There are three common ways this occurs.
1. Asynchronous cancellation (ODBC SQLCancel for example)
2. Query Timeout Exceeded and the client driver submits the attention
3. Results are not processed (very dangerous)
The attention is submitted to the SQL Server anytime the query is canceled and the client detects there are still pending results. If the results stream has been processed there is no need to tell SQL Server to cancel the query because SQL Server is already done with the query processing. So if you see the attention the results are not all processed.
An attention is treated by the SQL Server as an immediate processing termination. Stop right where you are and terminate the results.
select (becomes blocked - attention arrives at the server)
At the time of the attention the transaction is not rolled back unless transaction abort (XACT_ABORT) has been enabled. The client needs to submit a rollback. If the client does not submit the rollback and continues other processing the transaction remains open and the behavior scope of the application becomes undefined.
Not Processing All Rows
I often see this by a pattern in a trace showing that the attention arrived right after the batch started. Generally milli-seconds afterwards and not something like a 30 second query timeout. This is often a clear indication that the client processed the first row or first few rows and discarded the rest of the result set unsafely. Take the following procedure for example.
create procedure spMyProc
insert into tbl2 select ... with output ....inserted....
The procedure uses the OUTPUT clause of the insert to return the inserted rows to the client and let's say that the results from the output clause took more than a single network packet. The client library gets the first TDS packet and the first set of rows. If if client only processes the first result set the commit tran may never execute. Let's look at this in more detail with the actual ODBC client calls.
SQLExecute("spMyProc") <-------------- Returns as soon as the first result set if available (first set of rows in this case)
SQLFetch() <---------------- Retrieves the a row (should be done in a loop)
SQLFreeStmt() <------------ Releases the statement and if results are pending issues a SQLCancel.
So if the application only processes the first result it thinks the procedure ran successfully but because it did not process all results the SQLFreeStmt will issue the attention and cancel the procedure execution. Since we have not completed the insert (still streaming output rows) the commit tran is never executed (attention stops processing immediately). Data and application behavior becomes inconsistent. I have even seen customers increase the default packet size to allow the procedure to stream all rows in the first packet and complete until they get their application corrected.
The application should look like the following.
Here is an example of a bad application pattern as shown in an ODBC trace. Notice the SQLFetch is not called until it returns NO_MORE_ROWS and no call to SQLMoreResults is made. The SQLFreeStmt will submit the attention to the server and may create unexpected behavior for the application.
lord003 16 1000 fe4-13e0 EXIT SQLBindParameter with return code 0 (SQL_SUCCESS)
SWORD 1 <SQL_PARAM_INPUT>
SWORD 1 <SQL_C_CHAR>
SWORD 12 <SQL_VARCHAR>
SQLLEN * 0x00000000
lord003 16 1000 fe4-13e0 EXIT SQLExecDirect with return code 0 (SQL_SUCCESS)
UCHAR * 0x00B15DA8 [ -3] "SELECT * FROM xx00Online.dbo.loan (INDEX=rowno_loan) WHERE xx00Online.dbo.loan.rowno>? AND ((xx00Online.dbo.loan.balance < ?) AND (xx00Online.dbo.loan.paydue < ?)) ORDER BY xx00Online.dbo.loan.rowno 0"
lord003 16 1000 fe4-13e0 EXIT SQLFetch with return code 0 (SQL_SUCCESS)
lord003 16 1000 fe4-13e0 ENTER SQLGetData
HSTMT 00B55530 <-------------------- STMT HANDLE
UWORD 1 <------------------- COLUMN NUMBER
SWORD -2 <SQL_C_BINARY>
PTR <unknown type>
SQLLEN * 0x00B1B12C
lord003 16 1000 fe4-13e0 ENTER SQLFreeStmt
UWORD 0 <SQL_CLOSE>
lord003 16 1000 fe4-13e0 EXIT SQLFreeStmt with return code 0 (SQL_SUCCESS)
UWORD 0 <SQL_CLOSE>
SQL Server Senior Escalation Engineer