You can also use the facilities of the database engine to help with this
problem. Oracle has sequences which guarantee a unique number even with
concurrent access. Sybase has autoincrement columns.
----- Original Message -----
From: Peter Routtier-Wone <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Sent: Tuesday, April 03, 2001 7:39 AM
Subject: Re: [JBoss-user] Writing a finder which returns max id used
> > Wouldn't the easiest solution be to make a select max(key) statement and
> > then add 1 to get your new key?
>
> This isn't an EJB question, it's a general concurrency issue.
>
> What you say is true for single user exclusive access to the database.
> However, processes seldom run in isolation.
>
> High performance systems supporting concurrent access run a constant risk
> that after process A reads the max value, but before it commits ++max to
the
> database, process B will read the database and get THE SAME max and
> therefore try to use the same value for a PK. This is even more possible
> with multi-processor arrangements. Not only the operation but the the
whole
> transaction becomes unserialisable and is therefore aborted by the
database
> server.
>
> You could write lock the table for the duration, but bye-bye scalability.
>
> > Is this possible and how would I write it?
>
> It's entirely possible and you wouldn't write it now that you know better.
> You would bite the bullet and implement another, more robust solution. For
> example, you might build a servlet that queues requests for a single
thread
> that dispenses PK values. This assumes that all processes writing into the
> table will use the dispenser.
>
> There are other issues relating to initialisation of the dispenser, and
the
> need for it to dispense in multiple contexts (for different entities).
With
> appropriate design it can maintain contiguity using your ++max trick when
it
> starts a context.
>
>
>
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