Best way of locking item during update

Question

I am creating a website (c#/SQL Server 2008, .net) that needs to allow updating of some common information that can be shared by users.

For instance, I have 2 users and a table that contains a number of items. Much like a moderation queue.

Lets say it contains rows with ID's #1, #2, #3

If user 1 selects #1 first this needs to lock immediately until it is released/and available.

I have a couple of questions as this is uncharted territory for me.

• What's the best way of locking this row in SQL?
• Being stateless, how can user 2 be made aware that user 1 has claimed #1? Is it better to use a 'next available' approach?

Thanks in advance.

Answer 1

You have something that is called optimistic locking. But it doesnt, per say, solve your problem of 'reporting' that the row is locked. For most cases reporting is usless, but it can be solved programaticly.

That locking is kinda 'fun' depend if you are making desktop app, or web one, but mostly it is solved with using of extra column (timestamp) and lock a row for a period of time.

I made one project where i reported a user a row is locked, and i solved it with 2 fields (start time, end time) due i wanted to give variable amount of time to the person to update row.

sample:

query: select a, b, c, d, start_time, end_time FROM table

Program : if ((datetime.now()-start time > 1min) or (end_time <> null))
{
update table
SET start_time = datetime.now()
set end_time = null
where A = a and B = b and C= c and D = d, and START_TIME = start_time 
--big caps is current value, small caps value from last select ( and that is OPTIMISTIC concurency).
}
else
{
messagebox.show ("You suck, cant be changed") --personal touch :-)
}

when you update row, dont forget to place end_time.

OFC, check for time, within update is possible

Edit: i got that idea from row_scn in oracle, so it isnt anything revolutionary