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Re: Job control in sql

From: Svenne Krap <svenne(dot)lists(at)krap(dot)dk>
To: Ireneusz Pluta <ipluta(at)wp(dot)pl>
Cc: "pgsql-sql(at)postgresql(dot)org" <pgsql-sql(at)postgresql(dot)org>
Subject: Re: Job control in sql
Date: 2012-05-29 15:19:16
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-sql
On 29-05-2012 12:32, Ireneusz Pluta wrote:
> W dniu 2012-05-25 10:28, Svenne Krap pisze:
>> Hi.
>> I am building a system, where we have jobs that run at different
>> times (and takes widely different lengths of time).
>> Basically I have a jobs table:
>> create table jobs(
>>      id serial,
>>     ready boolean,
>>     job_begun timestamptz,
>>     job_done timestamptz,
>>     primary key (id)
>> );
>> This should run by cron, at it is my intention that the cronjob
>> (basically) consists of
>> /
>> psql -c "select run_jobs()"/
>> My problem is, that the job should ensure that it is not running
>> already, which would be to set job_begun when the job starts". That
>> can easily happen as jobs should be started every 15 minutes (to
>> lower latency from ready to done) but some jobs can run for hours..
>> The problem is that a later run of run_jobs() will not see the
>> job_begun has been set by a prior run (that is unfinished - as all
>> queries from the plpgsql-function runs in a single, huge transaction).
>> My intitial idea was to set the isolation level to "read uncommitted"
>> while doing the is-somebody-else-running-lookup, but I cannot change
>> that in the plpgsql function (it complains that the session has to be
>> empty - even when I have run nothing before it).
>> Any ideas on how to solve the issue?
>> I run it on Pgsql 9.1.
>> Svenne
> I think you might try in your run_jobs()
> SELECT job_begun FROM jobs WHERE id = job_to_run FOR UPDATE NOWAIT;
> This in case of conflict would throw the exception:
> 55P03 could not obtain lock on row in relation "jobs"
> and you handle it (or not, which might be OK too) in EXCEPTION block.
Hehe.. good idea...

In the mean time I had thought about using advisory locks for the same
thing, but the old-fashioned locks work fine too.


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