Can't they put in a where condition in the update double checking that
the locked_by is null?

-steve

Kjell Rilbe wrote:

>
> Steve Wiser wrote:
> > Just add a new field to the table called LOCKED_BY_USER or something and
> > when a user wants to lock a record you just update the table so it is
> > filled in with their user id. Other instances of the program should
> > check that field and not allow the user to touch the record if it is not
> > null. Of course once the original user is done they need to update the
> > table again and null out the field...
>
> If the intention is to update that field and then commit, then the app
> has to select with lock when checking the field. Otherwise you end up
> with a race condition:
>
> 1. User1: select
> 2. User2: select
> 3. U1: OK, no lock!
> 4. U2: OK, no lock!
> 5. U1: Update & commit
> 6. U2: Update & commit, overwriting U1's lock
>
> Now both users think they've got the lock.
>
> With select with lock, user 2 will get an exception at step 2, which is
> what we want.
>
> Kjell
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> Kjell Rilbe
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