Re: Synchronous replication

On Tue, Jul 27, 2010 at 10:12 PM, Joshua Tolley <eggyknap(at)gmail(dot)com> wrote:
> I don't think it can support the case you're interested in, though I'm not
> terribly expert on it. I'm definitely not arguing for the syntax Oracle uses,
> or something similar; I much prefer the flexibility we're proposing, and agree
> with Yeb Havinga in another email who suggests we spell out in documentation
> some recipes for achieving various possible scenarios given whatever GUCs we
> settle on.

Agreed. I'll add it to my TODO list.

> My concern is that in a quorum system, if the quorum number is less than the
> total number of replicas, there's no way to know *which* replicas composed the
> quorum for any given transaction, so we can't know which servers to fail to if
> the master dies.

What about checking the current WAL receive location of each standby by
using pg_last_xlog_receive_location()? The standby which has the newest
location should be failed over to.

> This isn't different from Oracle, where it looks like
> essentially the "quorum" value is always 1. Your scenario shows that all
> replicas are not created equal, and that sometimes we'll be interested in WAL
> getting committed on a specific subset of the available servers. If I had two
> nearby replicas called X and Y, and one at a remote site called Z, for
> instance, I'd set quorum to 2, but really I'd want to say "wait for server X
> and Y before committing, but don't worry about Z".
>
> I have no idea how to set up our GUCs to encode a situation like that :)

Yeah, quorum commit alone cannot cover that situation. I think that
current approach (i.e., quorum commit plus replication mode per standby)
would cover that. In your example, you can choose "recv", "fsync" or
"replay" as replication_mode in X and Y, and choose "async" in Z.

Regards,

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