From: Chris Browne <cbbrowne@acm.org>
Subject: Re: Binary Replication and Slony
Date: Mon, 20 Sep 2010 12:22:14 -0400
Organization: cbbrowne Computing Inc
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To: pgsql-general@postgresql.org

johnlicheng@gmail.com (John Cheng) writes:
> Congrats on the 9.0 release of PostgreSQL. One of the features I am really
> interested in is the built-in binary replication.
>
> Our production environment has been using PostgreSQL for more than 5 years
> (since this project started). We have been using Slony-I as our replication
> mechanism. I am interested to find out the pros and cons of Slony vs the
> built-in replication in 9.0. Based on what I understand:
>
> * Slony has a higher overhead than the binary replication in 9.0
> * When using Slony, schema change must be applied via slonik (in most cases)
> * Unfortunately, IMO it is easy to make a mistake when applying schema changes
> in Slony, fortunately, it is easy to drop and recreate the replication sets
> * Slony is an asynchronous replication mechanism
> * Slony allows you to replication some tables, while ignoring others
>
> * PostgreSQL 9.0 with hot standby & streaming replication is an asynchronous
> replication mechanism
> * Overhead is low compared to Slony
>
> Are there some cases where it is better to use Slony, for example, when you
> must specifically exclude tables from replication? I believe our system will be
> better off using the built-in replication mechanism of 9.0, and I am guessing
> most people will be in the same boat.

There are three characteristic kinds of cases where you'll need
something like Slony-I, where the built-in WAL-based replication won't
work:

a) You need to interact between PostgreSQL versions.  Slony (and similar
   systems like Londiste and Bucardo) can cope with having nodes running
   different versions of PostgreSQL.

   WAL-based replication requires that all databases use *identical*
   versions of PostgreSQL, running on identical architectures.

b) You only want to replicate parts of the changes that are going on.

   WAL-based replication duplicates *absolutely everything*.

c) You need for there to be extra behaviours taking place on
   subscribers, for instance, populating cache management information.

   WAL-based replication duplicates *absolutely everything*, and nothing
   extra that changes data can run on a WAL-based replica.

If you don't need any of those things, then, yes, I'd think the built-in
replication is a good choice, quite likely preferable to using the
trigger-based replication systems like Slony.
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