Re: queueing via database table?

Mark --

As others have indicated, there may be some blocking issues with the approach you outlined.

A variant I have seen used in the past uses a table with a unique id for the job, the work queue it is in, a status
flag,priority and at least one time stamp (and perhaps space for a process id). 

Each client that wants work issues a request (SELECT FOR UPDATE) to get the next job in its queue that has a status
flagof "Available" ordered by priority or initial time of creation, etc.; update that entry with the current timestamp
(andperhaps the process id of the client) and set the status flag to show the job is now being worked on all in one
transaction.

This releases the job but now with a changed status flag so other processes pulling work from the same queue won't see
itanymore. 

When the job finishes it selects its entry and updates the status flag and timestamp (and probably clears its process
id).Logic for how to bump a job to the next step can be embedded in the client or in another process, depending on your
needs.

It is useful to have a daemon or some other process to sweep the queue table and at least send an alert about stale or
frozenjobs. 

HTH,

Greg Williamson
DBA
GlobeXplorer LLC
-----Original Message-----
From:    pgsql-general-owner@postgresql.org on behalf of Mark Harrison
Sent:    Tue 1/2/2007 10:34 PM
To:    pgsql-general@postgresql.org
Cc:
Subject:    [GENERAL] queueing via database table?

I have a cluster of CPUs generating thumbnails for
a render farm.  I would like to place thumbnail
requests on a queue, and have the cluster of client
dequeue the requests and process them.

Of course, each request should be only dequeued once...
if a thumbnail is being processed by one CPU, it
shouldn't be processed by another CPU.

Does the following sound like a reasonable approach?
If not, what's a good way to go?

The processes generating the requests will insert into
a queue table.  They may add a priority and timestamp.

The several processes servicing the requests will do a
SELECT FOR UPDATE where ... limit 1, generate thumbnail,
delete the record and commit.

Comments and suggestions welcome,
Mark

--
Mark Harrison
Pixar Animation Studios

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