How to guarantee only one process picks up a processing task - sql

Question

I have multiple computers that have the task of sending out emails found in a table on a common SQL Server. Each computer polls the email table to look for messages it can send by looking at a status flag set to 0. If a computer does a
SELECT * FROM tblEmailQueue where StatusFlag=0
if it returns a record it immediately sets the StatusFlag to 1 which should cause the other computer polling the same table not to find this record. My fear is that if two computer find the record at the same time before either can update the StatusFlag, the email will be sent twice. Does anyone have ideas on how to ensure only one computer will get the record? I know I might be able to do a table lock but I would rather now have to do this.

Answer 1

Instead of using two queries which may cause a race condition, you can update the values and output the updated rows at once using the OUTPUT clause.
This will update the rows with statusflag=0 and output all of the updated ones;
UPDATE tblEmailQueue
SET statusflag=1
OUTPUT DELETED.*
WHERE statusflag=0;
An SQLfiddle to test with.
EDIT: If you're picking one row, you may want some ordering. Since the update itself can't order, you can use a common table expression to do the update;
WITH cte AS (
SELECT TOP 1 id, statusflag FROM tblEmailQueue
WHERE statusflag = 0 ORDER BY id
)
UPDATE cte SET statusflag=1 OUTPUT DELETED.*;
Another SQLfiddle.

Answer 2

You can perform select and send email in the same transaction. Also you can use ROWLOCK hint and don't commit transaction until you send email or set new value for StatusFlag. It means that nobody (exept transaction with hint NOLOCK or READ UNCOMMITED isolation level) can read this row as long as you commit transaction.
SELECT * FROM tblEmailQueue WITH(ROWLOCK) where StatusFlag=0
In addition you should check isolation level. For your case isolation level should be READ COMMITED or REPEATABLE READ.
See information about isolation levels here

Answer 3

Add another column to your table tblEmailQueue (say UserID), then try to pull one email such as
--Let flag an email and assign it to the application who made the request
--#CurrentUserID is an id unique to each application or user and supplied by the application
--or user who made the request, this will also ensures that the record is going to
--the right application and perhaps you can use it for other purpose such as monitoring.
UPDATE tblEmailQueue set UserID = #CurrentUserID, StatusFlag=1 where ID = isnull(
select top 1 ID from tblEmailQueue where StatusFlag=0 order by ID
), 0)
--Lets get an email that had a flag for the current user id
SELECT * FROM tblEmailQueue where StatusFlag=1 and UserID = #CurrentUserID

Answer 4

Here in Indianapolis, we are familiar with race conditions ;-)
Lets assume you actually have and ID field and a StatusFlag field and create a stored proc that includes
declare #id int
select top 1 #id = id from tblEmailQuaue where StatusFlag=0
if ##rowcount = 1
begin
update tblEmailQuaue set StatusFlag=1 where ID = #id AND StatusFlag=0
if ##rowcount = 1
begin
-- I won the race, continue processing
...
end
end
ADDED
An explicit handling like this is inferior to Joachim's method if all you want is the result of the select. But this method this method also works with old versions of SQL server as well as other databases.