Re: 100% failover + replication solution

On Mon, Oct 30, 2006 at 06:41:54PM +0530, Moiz Kothari wrote:
> I agree that PGCluster might be a better option, i dont want to go with
> Slony because of primary key constraints.

I don't know what the "primary key constraints" issue you have is,
but Slony would be inappropriate for a "100% failover" system anyway:
you can't know you haven't trapped data on the origin. This is in
fact true for the WAL shipping you suggested, also. The only way to
achieve 100% reliable failover today, with guaranteed no data loss,
is to use a system that commits all the data on two machines at the
same time in the same transaction. I haven't seen any argument so
far that there is any such system "out of the box", although with two
phase commit support available, it would seem that some systems could
be extended in that direction.

The other answer for all of this is to do it with hardware, but
that's a shared-disk system, so if your disk blows up, you have a
problem. Or, if you're using the operating system of people who
don't know how fsck works. I don't know anyone who has that problem;
certainly not any vendors whose name starts with 'I' and ends with
'M'.

> 1) It might slow down the process a bit. as confirmation happens after
> transaction gets comitted to all the nodes.

Anyone who tells you that you can have completely reliable data
replication with no performance hit is trying to sell you a bridge in
Brooklyn. If you want reliable data replication that guarantees you
can have automatic failover, you are going to pay for it somehow; the
question is which compromise you want to make. That seems to be
something you'll need to decide.

> 2) Its difficult to convince, as it is an external project and if support
> for the same stops or future versions of postgres does not work, it might be
> a problem.

If you have this problem, probably free software isn't for you.
PostgreSQL is a modular system, and people use different components
together in deployed systems. This happens to be true of commercial
offerings too (if not, you could buy the cheapest version of, say,
Oracle and get RAC in the bargain), but they _sell_ it to you as
though it were one big package. To the extent your managers don't
understand this, you're always going to have a problem using free
software.

A
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