[pgpool-general: 6170] Re: Fwd: repeat the check of the trusted servers

[Piotr Gbyliczek]

On Tuesday, 24 July 2018 13:05:07 BST Muhammad Usama wrote:

Hi Muhammad,

> trusted_servers config could be used to mitigate the split-brain syndrome
> in older versions of Pgpool-II but
> if you are on 3.6.7 which has the quorum aware watchdog system you don't
> require to use trusted_servers config
> and leave it empty, of course if you don't have some other specific
> requirements as the one I described in above example.
> 
> Please let me know if you need further clarification or explanation.

Would that work in cluster with 2 pgpool nodes ? I'm guessing not, as "quorum" 
suggests 3+ servers to be effective in my mind. However after your explanation 
it seems to me that pgpool does not behave logically in regards to 
"trusted_servers" and  introduces some risks by this. 

I mean, why would any pgpool process stop itself due to failure to communicate 
the trusted servers when there is communication with peer over the watchdog 
channel ? I understand your hypothetical environment, and indeed, we don't 
want to have application/clients connecting to the database via multiple 
routes due to delegate IP being up on more than one pgpool server after 
network partition. 

But that suggest the communication between nodes is severed, and ping to 
trusted servers helps establish if given pgpool still has access to network, 
in which case it is plausible that it is expected to have VIP on itself. In 
case that it can't ping outside, there is even no point of bringing up the 
VIP, as there is network issue and little expectation of traffic from app/
clients will get to it. 

Thinking of it, does this opens us up for a case where network partition runs 
between pgpool nodes, and outside connectivity is not affected ?  Both nodes 
will bring up VIP then, and potentially make a bit of mess..

Anyway, in case that watchdog is communicating fine with other nodes, but 
trusted servers can't be contacted, watchdog should take priority, and dictate 
that nothing should be done, imho. 

I think that it may be an edge case, and solution for that would be to get 
another pgpool node running. is there a way to make a witness node in pgpool ? 
I'm unable to add third pgpool node due to cost, but could install pgpool to 
another server to make it 3 nodes. However, that server would be unable to 
take production traffic, so should be never able to bring up VIP.

Regards,
Piotr

--

Piotr Gbyliczek
Solutions Engineer

Node4 Ltd, The POD, 10 Bottle Ln, Nottingham NG1 2HL
ddi. 08454 210444 | t. 0845 123 2222
e. P.Gbyliczek at node4.co.uk



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