Re: Problem with pg_try_advisory_lock and two connections (seemingly) getting the same lock

You could implement an optimistic lock strategy by placing a 'version'
column in the table and increment it on successful 'check-out' and test
against the value the user has as he/she tried to act on the record.  If
some else got there first the second user fails to check-out the queue
item.  Triggers could do the work.

On 07/19/2010 08:06 PM, Brett Hoerner wrote:
> Hi,
> 
> I currently have a simple queue written ontop of Postgres.  Jobs are
> inserted and workers periodically check for jobs they can do, do them,
> and then delete the rows.  pg_try_advisory_lock is used to (attempt
> to) stop two workers from doing the same job.
> 
> (I'm working on moving to a "real" messaging queue right now, this is
> more a point of curiosity and education now.)
> 
> Here is my queue table,
> 
> CREATE TABLE queue (
>     id serial NOT NULL PRIMARY KEY,
>     rcvd timestamp with time zone,
>     sent timestamp with time zone,
>     host character varying(32),
>     job character varying(32),
>     arg text
> );
> 
> Here is an example query,
> 
> SELECT q.*
> FROM (SELECT id, job, arg
>       FROM queue
>       WHERE job = 'foo' OR job = 'bar'
>       OFFSET 0) AS q
> WHERE pg_try_advisory_lock(1, q.id)
> LIMIT 10
> 
> (For information on OFFSET 0 see:
> http://blog.endpoint.com/2009/04/offset-0-ftw.html)
> 
> Now if I have two workers running I will periodically see that each
> worker gets a row with the same q.id (and thus does the work).  How is
> that possible?  The outer query seemingly does a WHERE on an
> advisory_lock.
> 
> Does anyone have any ideas?  Am I grossly misusing advisory_locks?
> 
> Thanks,
> Brett
>