How can i run once on time procedure?

Question

I wrote multi-threads application that every single thread call a sql server procedure.. i need to protect a row from getting by more than one thread i want every row take it only with by one thread i used this code:

CREATE PROCEDURE GetRow
    @ThreadID int
AS
BEGIN

    DECLARE @rowID int;
    SELECT TOP (1) @rowID=[ID] FROM [TestTable] WHERE [ThreadID] = 0 (AND some other parameters)

    IF @rowID is not NULL
    BEGIN
        UPDATE TOP (1) [TestTable] SET [CallCount] = [CallCount]+1,[ThreadID] = @ThreadID WHERE [ID] = @rowID

    --some other codes

    END
END

as i said i run application and then my application create 10 threads and i set threadid for taking a row by only one thread but i saw that a row called by more than 1 thread and about 4-5 threads get one row.. how could i do this the best way, i solved it by this code by i don`t like to have two same where in select and update:

CREATE PROCEDURE GetRow
    @ThreadID int
AS
BEGIN

    DECLARE @rowID int;

    UPDATE TOP (1) [TestTable] SET [CallCount] = [CallCount]+1,[ThreadID] = @ThreadID WHERE [ThreadID] = 0 (AND some other parameters)


    SELECT TOP (1) @rowID=[ID] FROM [TestTable] WHERE [ThreadID] = @ThreadID (AND some other parameters)

    IF @rowID is not NULL
    BEGIN

    --some other codes

    END
END

how can i do this with top code? and should i update first and then select like above code for fix it?

Answer 1

If you're using SQL Server 2005 or newer you may be able to combine the UPDATE and SELECT using the new OUTPUT clause .

However I'd suggest backing up and thinking about your database schema, and whether it really fits your problem -- especially with multi-threading!

I get the impression you want to count the calls from each thread. It would work better to have each call INSERT a row into a simple ThreadCallCount table that simply has one ThreadID column. Every call is a new row. Later you can Sum() this table by ThreadID to get the total calls per thread. That would be simple and easy to ensure it's thread-safe.

It sounds like there's some other data you're trying to retrieve, and it's not clear how/when it gets stored. But a similar analysis -- and more, simpler tables -- might help you there.

In general: Don't feel like you need to squeeze it all in one table. With multi-threading it will help to break things into smaller pieces. Feel free to use more tables, each of which has a single, clear purpose. SQL Server is designed to handle row transactions safely and efficiently, so try to leverage that as much as possible.

Answer 2

I would recommend writing thead safe code instead of doing that in stored procedure.

You did not mention your coding language, in C# one way is to wrap the code that calls this stored procedure inside lock block:

using System.Threading
...

public class ClassName {

    object syncLock = new object();
    ...

    SomeFunction(){

        lock(syncLock){

            // For example, com could be SqlCommand object that calls stored procedure
            int updatedRowCount = com.ExecuteNonQuery();
            ...
        }
    }

}