Re: Configuring synchronous replication - Mailing list pgsql-hackers

On Fri, 2010-09-24 at 11:08 +0300, Heikki Linnakangas wrote:
> On 24/09/10 01:11, Simon Riggs wrote:
> >> But that's not what I call synchronous replication, it doesn't give
> >> you the guarantees that
> >> textbook synchronous replication does.
> >
> > Which textbook?
> 
> I was using that word metaphorically, but for example:
> 
> Wikipedia
>   http://en.wikipedia.org/wiki/Replication_%28computer_science%29
>   (includes a caveat that many commercial systems skimp on it)

Yes, I read that. The example it uses shows only one standby, which does
suffer from the problem/caveat it describes. Two standbys resolves that
problem, yet there is no mention of multiple standbys in Wikipedia.

> Oracle docs
>  
> http://download.oracle.com/docs/cd/B10500_01/server.920/a96567/repoverview.htm
>   Scroll to "Synchronous Replication"

That document refers to sync rep *only* in the context of multimaster
replication. We aren't discussing that here and so that link is not
relevant at all.

Oracle Data Guard in Maximum availability mode is roughly where I think
we should be aiming
http://download.oracle.com/docs/cd/B10500_01/server.920/a96653/concepts.htm#1033871

But I disagree with consulting other companies' copyrighted material,
and I definitely don't like their overcomplicated configuration. And
they have not yet thought of per-transaction controls. So I believe we
should learn many lessons from them, but actually ignore and surpass
them. Easily.

> Googling for "synchronous replication textbook" also turns up this 
> actual textbook:
>    Database Management Systems by R. Ramakrishnan & others
> which uses synchronous replication with this meaning, although in the 
> context of multi-master replication.
> 
> Interestingly, "Transaction Processing: Concepts and techniques" by 
> Grey, Reuter, chapter 12.6.3, defines three levels:
> 
> 1-safe - what we call asynchronous
> 2-safe - commit is acknowledged after the slave acknowledges it, but if 
> the slave is down, fall back to asynchronous mode.
> 3-safe - commit is acknowledged only after slave acknowledges it. If it 
> is down, refuse to commit

Which again is a one-standby viewpoint on the problem. Wikipedia is
right that there is a problem when using just one server.

"3-safe" mode is not more safe than "2-safe" mode when you have 2
standbys.

If you want high availability you need N+1 redundancy. If you want a
standby server that is N=1. If you want a highly available standby
configuration then N+1 = 2.

Show me the textbook that describes what happens with 2 standbys. If one
exists, I'm certain it would agree with my analysis.

(I'll read and comment on your other points later today.)

-- Simon Riggs           www.2ndQuadrant.comPostgreSQL Development, 24x7 Support, Training and Services