Re: Cascade replication

On Mon, Jul 4, 2011 at 6:24 PM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
> On Tue, Jun 14, 2011 at 6:08 AM, Fujii Masao <masao(dot)fujii(at)gmail(dot)com> wrote:
>
>>> The standby must not accept replication connection from that standby itself.
>>> Otherwise, since any new WAL data would not appear in that standby,
>>> replication cannot advance any more. As a safeguard against this, I introduced
>>> new ID to identify each instance. The walsender sends that ID as the fourth
>>> field of the reply of IDENTIFY_SYSTEM, and then walreceiver checks whether
>>> the IDs are the same between two servers. If they are the same, which means
>>> that the standby is just connecting to that standby itself, so walreceiver
>>> emits ERROR.
>
> Thanks for waiting for review.

Thanks for the review!

> This part of the patch is troubling me. I think you have identified an
> important problem, but this solution doesn't work fully.
>
> If we allow standbys to connect to other standbys then we have
> problems with standbys not being connected to master. This can occur
> with a 1-step connection, as you point out, but it could also occur
> with a 2-step, 3-step or more connection, where a circle of standbys
> are all depending upon each other. Your solution only works for 1-step
> connections. "Solving" that problem in a general sense might be more
> dangerous than leaving it alone. I think we should think some more
> about the issues there and approach them as a separate problem.
>
> I think we should remove that and just focus on the main problem, for
> now. That will make it a simpler patch and easier to commit.

I agree to focus on the main problem first. I removed that. Attached
is the updated version.

Regards,

--
Fujii Masao
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center