[Pgpool-hackers] Promoting any node as master with SR slaves follow

Tue Jan 25 08:24:10 UTC 2011

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Le 25/01/2011 01:06, Tatsuo Ishii a écrit :
>> Hi,
>>
>> Le 24/01/2011 15:38, Tatsuo Ishii a écrit :
>>>> Le 20/01/2011 20:15, Gilles Darold a écrit :
>>>>> Hi,
>>>>>
>>>>> [...]
>>>>>
>>>>> All this make me do some work on PgPool source code as I think it may
>>>>> handle that easily. The patch do the following:
>>>>>
>>>>> Add a PCP command to promote any node in the pgpool line as the new
>>>>> master : pcp_promote_node
>>>>> Add a follow master configuration option to call an external command
>>>>> when the failover is over.
>>>>>
>>>>> So that if I run :
>>>>>
>>>>>     pcp_promote_node 10 192.168.1.11 9898 postgres postgres 3
>>>>>
>>>>> Node 3 is promoted as master and all other nodes are degenerated.
>>>> great. +1
>>> Thinking a little bit more, there seems to be an use case where it's
>>> not a good idea. Consider a heavyly read system rarely updated. In a
>>> case data is not so often updated, maybe once a day. Nodes that were
>>> not promoted do not have old data until daily updation occurs. So
>>> there's no need to degenerate those nodes until that. If you do not
>>> like this, you could always use "delay_threthold". I admit that
>>> there's a case immediate degeration is appropreate. So we may need a
>>> switch to control the behavior.
>> The way I've coded that is very simple : automatic slave degeneration
>> and recovery over the new master is done only if you have filled the
>> "follow_master_command" configuration directive. In the other case
>> nothing is done than the failover and you have all your time to manually
>> handle slaves detach and recovery stage once the update happens :-)
> 
> I'm not sure your implementation details because you did not show us
> your patches.
> 
>> Using a threshold value can be used to delay degeneration during a
>> certain laps of time, but I'm not sure that this is something valuable.
>> When a cluster is broken what we may want is to be back asap in a stable
>> situation and not waiting an hypothetical update on the new master.
> 
> That is *one* of the use cases.
> 
>> The worse situation with this delay_threshold is when a failover happen
>> and wait some minutes/hours the update query. In this laps of time the
>> fresh new master crash so that a new failover is done on an other node, etc.
> 
> Then just another standby is prmoted to primary. I see no problem here?
> 
>>>>> When the failover is ended PgPool fork a child to execute the command
>>>>> given in the follow_master_command successively for each node
>>>>> degenerated. The follow master command is simply a call to the
>>>>> pcp_recovery_node.
>>>> IIRC, we talked a bit about that together a week or two ago.
>>>> it seems to me it would be cleaner and simpler to use the parameter
>>>> "recovery_end_command" from the postgresql recovery.conf file to send the
>>>> command follow_master_command to the pgpool service as soon as the new master is
>>>> effectively up and ready.
>>> I'll catch up the discussion.
> 
> I couldn't find discussion on "recovery_end_command". Could you please
> give me a pointer?

Erf, sorry, there's no public log about that discussion because it stud actually
on IRC...

>>> BTW how can we determine the promoting standby has finished the
>>> reconvery and become ok to accept write queries?
>>
>> I'm not agree jehan-guillaume, I think that the context can be fully and
>> better handled with PgPool. PostgreSQL on a standby node doesn't  have
>> any idea of who are the others standby nodes and then how it can
>> advertised them to follow ? PgPool always maintained the list and the
>> states of the standby nodes.
> --
> Tatsuo Ishii
> SRA OSS, Inc. Japan
> English: http://www.sraoss.co.jp/index_en.php
> Japanese: http://www.sraoss.co.jp

- -- 
Jehan-Guillaume de Rorthais
DBA
http://www.dalibo.com
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