Re: HA and Replication - how to choose among all the available solutions

On Mon, Dec 03, 2007 at 09:16:40AM +0100, Pascal Cohen wrote:
>
> * For High-availability, I mainly studied PgPool and Log-shipping (and
> in fact forgot Slony).
> Until now I feel more comfortable with Log-shipping because it seems
> safer (I am not sure I can't get some problems with sequences and
> current_timestamp with PgPool - it is even mentioned in the doc).

Slony can do most things the current log shipping code can do (but it does
_not_ solve DDL changes automatically; you must control that differently).

Slony imposes a load on your origin server that log shipping does not.

> Do you mean that Log-shipping is well for a Standby server not handling
> request while Slony is good with "select" load balancing ?
> Why and what limits Log-shipping for that case ?

When you ship logs, you more or less have the "replica" in recovery mode all
the time (as though you pulled the plug on the server). So it is always
recovering the shipped logs. This is also how Oracle did this for years,
although they've recently addressed the limitation. There is some promise
of doing this in Postgres in future, but it's not done yet.

Also, have a look at hardware answers for HA. That's the real current
answer for always-on service, if you ask me.

A