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.
>
I am not concerned with warm DDL changes (I would say currently because
one never knows ;) ).
In fact, if my master server only handles updates while request are
handled by one or more slaves, I can probably accept some overload on it
or if I adjust weights between Master and Slaves through PgPool, I
should be able to accept some load on Master.

>
>> 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.
>
What kind of hardware solutions do you know ? - I will look on my own
what I can find.

Thanks

Pascal