Message-ID: <4C979AEF.9030402@ca.afilias.info>
Date: Mon, 20 Sep 2010 13:33:35 -0400
From: Brad Nicholson <bnichols@ca.afilias.info>
User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US;
	rv:1.9.2.9) Gecko/20100916 Lightning/1.0b2 Thunderbird/3.1.4
MIME-Version: 1.0
To: pgsql-general@postgresql.org
Subject: Re: Binary Replication and Slony
References: <201009201649.o8KGnAn00996@momjian.us>
In-Reply-To: <201009201649.o8KGnAn00996@momjian.us>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit

  On 10-09-20 12:49 PM, Bruce Momjian wrote:
> John Cheng wrote:
>> 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.
> You have summarized the differences well.  Streaming replication has
> lower overhread, but doesn't allow per-table granularity or allow
> replication between different versions of Postgres.
>

Slony will also allow you to:

-run custom schema (like extra indexes) on replicas
-replicate between different hardware architectures and OS's
-run lengthy queries against replicas having to worry about trade offs 
surrounding query cancellation vs standby lagging.
-switch roles of two nodes without entering a degraded state or worrying 
about STONITH.  If you switch roles in a controlled manner, both nodes 
remain in the cluster.  Slony prevents writes against the replica.

I do agree that for most, Slony is overkill and streaming replication 
and hot standby will be the better choice.

-- 
Brad Nicholson  416-673-4106
Database Administrator, Afilias Canada Corp.