C# – SELECT and UPDATE table so there is no overlap of Threads

cconcurrencysqlsql-server-2005tsql

Say I have the following table:

ID|Read
-------
 1|true
 2|false
 3|false
 4|false

… and I need to read the smallest ID, that has [Read] == false; plus, update that I have now read it.

So if i execute my Stored Procedure dbo.getMinID, it will return ID: 2, and update [Read] -> true.

CREATE PROCEDURE [dbo].[getMinID]
(
  @QueryID INT OUTPUT 
)
BEGIN
  SELECT TOP 1 @QueryID = [ID] from Table
  UPDATE Table SET [Read] = 1 WHERE [ID] = @QueryID 
END

The problem is that I have ten (10) asynchronous Threads executing dbo.getMinID, at the same time, and I CANNOT have them select the SAME [ID] under any circumstances. I am worried that a second thread my execute between my SELECT and UPDATE statement, thus returning [ID]: 2 in both scenarios.

How can I ensure that I do not select/update the same record twice, no matter how many threads are acting upon the Stored Procedure? ALSO, please keep in mind that the table CONSTANTLY has new rows added, so I cannot lock the table!

Best Answer

If you mean a concurrency safe queue type locking, then use ROWLOCK, UPDLOCK, READPAST hints?

SQL Server Process Queue Race Condition

BEGIN TRAN

SELECT TOP 1 @QueryID = [ID] from Table WITH (ROWLOCK, UPDLOCK, READPAST)
UPDATE Table SET [Read] = 1 WHERE [ID] = @QueryID 

COMMIT TRAN -- TRAM

However, in one statement. something like

WITH T AS
(
    --ORDER BY with TOP , or perhaps MIN is better?
    SELECT TOP 1 [Read], [ID] from Table
    WITH (ROWLOCK, UPDLOCK, READPAST) ORDER BY [Read]
)
UPDATE
    T
SET
    [Read] = 1;
Related Topic