At 10:52 AM 2/10/2006, Alec.Cawley@stripped wrote:
>Patrick Duda <pduda@stripped> wrote on 10/02/2006 16:28:56:
>
> > I guess I don't understand this locking stuff. I have a InnoDB table
>that
> > has one thing in it, a counter. All I want to do is have multiple
> > instances of the code read this counter and increment it. I want to
>make
> > sure that each one is unique.
> >
> > Here is what I am doing in java:
> >
> > c.setAutoCommit(false);
> > ...
> > rs = statement.executeQuery("select request_id from requestid_innodb for
>
> > update");
> > ...
> > String updateQuery = "update requestid_innodb set request_id=";
> > updateQuery = updateQuery + nextRequestId;
> > tempStatement = c.createStatement();
> > tempStatement.executeUpdate(updateQuery);
> > ...
> > c.commit();
> > c.setAutoCommit(true);
> >
> > If I have multiple instances of this code running I end up with
>duplicate
> > keys. I thought this was suppose to lock the table so that would not
>happen.
> >
> > What am I not doing right? What am I not understanding about locking?
>
>I think this problem is explained in detail at
>http://dev.mysql.com/doc/refman/5.0/en/innodb-locking-reads.html
>
>Alec
>
Yes, I know that, I have read that and that is why I am asking what it is
that I am not doing right. It talks about a counter specifically:
"2) read the counter first with a lock mode FOR UPDATE, and increment after
that. The latter approach can be implemented as follows:
SELECT counter_field FROM child_codes FOR UPDATE;
UPDATE child_codes SET counter_field = counter_field + 1;
A SELECT
FOR UPDATE reads the latest available data, setting exclusive
locks on each row it reads. Thus, it sets the same locks a searched SQL
UPDATE would set on the rows. "
Isn't that what my Java code is doing?
I start a transaction by turning off autocommit, I then do a select for
update. Then I do the update and I commit. From the way I read this, no
one else should be able to read the table until I commit.
Yet, that is not what I am seeing. When I start several instances of the
program running I get lots and lots of:
Error inserting records into database [Caused by: Duplicate entry '152' for
key 1]
That is what has me confused. I thought I was doing things they way the
manual said to.
Thanks
Patrick
