Binary Replication and Slony

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.

--
---
John L Cheng

johnlicheng(at)gmail(dot)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.
--
output = reverse("ofni.secnanifxunil" "@" "enworbbc")
"What you said you want to do is roughly equivalent to nailing
horseshoes to the tires of your Buick." -- danceswithcrows(at)usa(dot)net on
the question "Why can't Linux use Windows Drivers?"

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.

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ It's impossible for everything to be true. +

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.

Much thanks to everyone! The mailing list, as usual, has been extremely
helpful.

On Mon, Sep 20, 2010 at 10:33 AM, Brad Nicholson
<bnichols(at)ca(dot)afilias(dot)info>wrote:

> 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.
>
>
>
>
> --
> Sent via pgsql-general mailing list (pgsql-general(at)postgresql(dot)org)
> To make changes to your subscription:
> http://www.postgresql.org/mailpref/pgsql-general
>

--
---
John L Cheng

I have applied the attached documentation improvement to better
highlight Slony's abilities.

---------------------------------------------------------------------------

Brad Nicholson wrote:
> 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.
>
>
>
> --
> Sent via pgsql-general mailing list (pgsql-general(at)postgresql(dot)org)
> To make changes to your subscription:
> http://www.postgresql.org/mailpref/pgsql-general

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ It's impossible for everything to be true. +