[pgpool-hackers: 3898] Re: [pgpool-general: 7543] VIP with one node

[Tatsuo Ishii]

Hi Pgpool-II developers,

Recently we got a complain below from a user.

Currently Pgpool-II releases VIP if the quorum is lost.  This is
reasonable and safe so that we can prevent split-brain problems.

However, I feel it would be nice if there's a way to allow to hold VIP
even if the quorum is lost for emergency sake.

Suppose we have 3-node pgpool each in different 3 cities. Those 2
cities are break down by an earth quake, and user want to keep their
business relying on the remaining 1 node. Of course we could disable
watchdog and restart pgpool so that applications can directly connect
to pgpool. However in this case applications need to change the IP
which connect to.

Also as the user pointed out, with 2-node configuration the VIP can be
used by enabling enable_consensus_with_half_vote even if there is
only 1 node remains. It seems as if 2-node config is better than
3-node config in this regard. Of course this is not true since 3-node
config is much more resistant to split-brain problems.

I think there are multiple ways to deal with the problem:

1) invent a new config parameter so that pgpool keeps VIP even if the
quorum is lost.

2) add a new pcp command which re-attaches the VIP after VIP is lost
due to loss of the quorum.

#1 could easily creates duplicate VIPs. #2 looks better but when other
 nodes come up, it could be possible that duplicate VIPs are created.

Thoughts?

Best regards,
--
Tatsuo Ishii
SRA OSS, Inc. Japan
English: http://www.sraoss.co.jp/index_en.php
Japanese:http://www.sraoss.co.jp

> Dear all,
> 
> I have fairly common 3-node cluster, with each node running a PgPool
> and a PostreSQL instance.
> 
> I have set up priorities so that:
>   - when all 3 nodes are up, the 1st node is gonna have the VIP,
>   - when the 1st node is down, the 2nd node is gonna have the VIP, and
>   - when both the 1st and the 2nd nodes are down, then the 3rd node
> should get the VIP.
> 
> My problem is that when only 1 node is up, the VIP is not brought up,
> because there is no quorum.
> How can I get PgPool to bring up the VIP to the only remaining node,
> which still could and should serve requests?
> 
> Regards,
> 
> tamas
> 
> -- 
> Rébeli-Szabó Tamás
> 
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