Re: [v9.3] Extra Daemons (Re: elegant and effective way for running jobs inside a database) - Mailing list pgsql-hackers

2012/6/29 Boszormenyi Zoltan <zb@cybertec.at>:
> 2012-04-25 11:40 keltezéssel, Kohei KaiGai írta:
>
>> 2012/3/10 Simon Riggs <simon@2ndquadrant.com>:
>>>
>>> On Fri, Mar 9, 2012 at 6:51 PM, Andrew Dunstan <andrew@dunslane.net>
>>> wrote:
>>>>
>>>>
>>>> On 03/09/2012 01:40 PM, Robert Haas wrote:
>>>>>
>>>>> On Fri, Mar 9, 2012 at 12:02 PM, David E.
>>>>> Wheeler<david@justatheory.com>
>>>>>  wrote:
>>>>>>
>>>>>> On Mar 9, 2012, at 7:55 AM, Merlin Moncure wrote:
>>>>>>>
>>>>>>> 100% agree  (having re-read the thread and Alvaro's idea having sunk
>>>>>>> in).  Being able to set up daemon processes side by side with the
>>>>>>> postmaster would fit the bill nicely.  It's pretty interesting to
>>>>>>> think of all the places you could go with it.
>>>>>>
>>>>>> pgAgent could use it *right now*. I keep forgetting to restart it
>>>>>> after
>>>>>> restarting PostgreSQL and finding after a day or so that no jobs have
>>>>>> run.
>>>>>
>>>>> That can and should be fixed by teaching pgAgent that failing to
>>>>> connect to the server, or getting disconnected, is not a fatal error,
>>>>> but a reason to sleep and retry.
>>>>
>>>>
>>>> Yeah. It's still not entirely clear to me what a postmaster-controlled
>>>> daemon is going to be able to do that an external daemon can't.
>>>
>>> Start and stop at the same time as postmaster, without any pain.
>>>
>>> It's a considerable convenience to be able to design this aspect once
>>> and then have all things linked to the postmaster follow that. It
>>> means people will be able to write code that runs on all OS easily,
>>> without everybody having similar but slightly different code about
>>> starting up, reading parameters, following security rules etc.. Tight
>>> integration, with good usability.
>>>
>> I tried to implement a patch according to the idea. It allows extensions
>> to register an entry point of the self-managed daemon processes,
>> then postmaster start and stop them according to the normal manner.
>>
>> [kaigai@iwashi patch]$ ps ax | grep postgres
>> 27784 pts/0    S      0:00 /usr/local/pgsql/bin/postgres
>> 27786 ?        Ss     0:00 postgres: writer process
>> 27787 ?        Ss     0:00 postgres: checkpointer process
>> 27788 ?        Ss     0:00 postgres: wal writer process
>> 27789 ?        Ss     0:00 postgres: autovacuum launcher process
>> 27790 ?        Ss     0:00 postgres: stats collector process
>> 27791 ?        Ss     0:00 postgres: auth_counter              <== (*)
>>
>> The auth_counter being included in this patch is just an example of
>> this functionality. It does not have significant meanings. It just logs
>> number of authentication success and fails every intervals.
>>
>> I'm motivated to define an extra daemon that attach shared memory
>> segment of PostgreSQL as a computing server to avoid limitation of
>> number of GPU code that we can load concurrently.
>>
>> Thanks,
>
>
> I have tested this original version. The patch has a single trivial reject,
> after fixing it, it compiled nicely.
>
> After adding shared_preload_libraries='$libdir/auth_counter', the extra
> daemon start and stops nicely with pg_ctl start/stop. The auth_counter.c
> code is a fine minimalistic example on writing one's own daemon.
>
Thanks for your testing.

According to Simon's comment, I'm waiting for his integration of this patch
with another implementation by him.

The auth_counter is just an proof-of-concept patch, so, it is helpful if you
could provide another use case that can make sense.

Best regards,
--
KaiGai Kohei <kaigai@kaigai.gr.jp>