[drupal-support] How to use MySQL last_insert_id function with locking?

[KZ Mai]

I was under the impression that when drupal uses a single shared MySQL
connection, using last_insert_id to get the newly created rows in
tables may cause problems.

Say in my drupal module,
User A tries to create a new row in a table, before my module calls
last_insert_id.  User B creates a new row in the same table.

Is it possible that urser A gets back the id which is associated with
the row User B created?


Yes.  I know last_insert_id works for MySQL only.  I don't have much
time to refactor my codes to use db_next_id.  so I just want to get
the easiest solution.

Thanks.
Kai


On 7/21/05, Chris Johnson <chris at tinpixel.com> wrote:
> KZ Mai wrote:
> > Hi,
> > I am developing my own module which uses tables that have auto-increment fields.
> >
> > As far as I can tell, drupal uses a single connection for my default
> > configuration.
> >
> > I don't think i think use last_insert_id without locking because the
> > MySQL DB connection is shared accross requests.
> >
> > so what should I lock in order to get the correct result from last_insert_id?
> 
> I'm not positive I understand what you are asking, but Drupal's use of a
> single database connection does not preclude you from using last_insert_id().
>   Each thread or process of your web server will have its own copy of the
> database connection used by Drupal, and MySQL will properly keep track of the
> last ID used in an auto_increment field for each connection.  Even if the
> thread running your code is interrupted by the operating system between the
> insert and your call to last_insert_id(), no other thread will use that
> connection and that information in last_insert_id() will remain valid when
> your thread is resumed.
> 
> --
> Chris Johnson
> 
> --
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