Re: Binary Replication and Slony

 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.

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