Thread: Replication on the backend

What about replication or data distribution inside the backend. This is a valid issue?<br /><br /> Thanks,<br />
Gustavo.<br/> 

gustavotonini@gmail.com (Gustavo Tonini) writes:
> What about replication or data distribution inside the backend.  This
> is a valid issue?

I'm not sure what your question is...
-- 
(reverse (concatenate 'string "gro.gultn" "@" "enworbbc"))
http://www.ntlug.org/~cbbrowne/x.html
"Love is like a snowmobile flying over the frozen tundra that suddenly
flips, pinning you underneath.  At night, the ice weasels come."
-- Matt Groening

Gustavo Tonini wrote:
> replication (master/slave, multi-master, etc) implemented inside 
> postgres...I would like to know what has been make in this area.
http://www.commandprompt.com/ - Master/Slave

Joshua D. Drake


>
> Gustavo.
>
> P.S. Sorry for my bad English.
>
> 2005/12/5, Chris Browne <cbbrowne@acm.org <mailto:cbbrowne@acm.org>>:
>
>     gustavotonini@gmail.com <mailto:gustavotonini@gmail.com> (Gustavo
>     Tonini) writes:
>     > What about replication or data distribution inside the
>     backend.  This
>     > is a valid issue?
>
>     I'm not sure what your question is...
>     --
>     (reverse (concatenate 'string "gro.gultn" "@" "enworbbc"))
>     http://www.ntlug.org/~cbbrowne/x.html
>     <http://www.ntlug.org/%7Ecbbrowne/x.html>
>     "Love is like a snowmobile flying over the frozen tundra that
>     suddenly
>     flips, pinning you underneath.  At night, the ice weasels come."
>     -- Matt Groening
>
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>
>

> replication (master/slave, multi-master, etc) implemented inside 
> postgres...I would like to know what has been make in this area.

It's not in the backend, check out things like Slony (www.slony.info) 
and various other commercial solutions.

Chris

On 12/5/2005 8:18 PM, Gustavo Tonini wrote:

> replication (master/slave, multi-master, etc) implemented inside
> postgres...I would like to know what has been make in this area.

We do not plan to implement replication inside the backend. Replication 
needs are so diverse that pluggable replication support makes a lot more 
sense. To me it even makes more sense than keeping transaction support 
outside of the database itself and add it via pluggable storage add-on.


Jan


> 
> Gustavo.
> 
> P.S. Sorry for my bad English.
> 
> 2005/12/5, Chris Browne <cbbrowne@acm.org>:
>>
>> gustavotonini@gmail.com (Gustavo Tonini) writes:
>> > What about replication or data distribution inside the backend.  This
>> > is a valid issue?
>>
>> I'm not sure what your question is...
>> --
>> (reverse (concatenate 'string "gro.gultn" "@" "enworbbc"))
>> http://www.ntlug.org/~cbbrowne/x.html
>> "Love is like a snowmobile flying over the frozen tundra that suddenly
>> flips, pinning you underneath.  At night, the ice weasels come."
>> -- Matt Groening
>>
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>> TIP 1: if posting/reading through Usenet, please send an appropriate
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>>
> 


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#================================================== JanWieck@Yahoo.com #

On Tue, 2005-12-06 at 10:03 -0200, Gustavo Tonini wrote:
> But,  wouldn't the performance be better? And wouldn't asynchronous
> messages be better processed?

At least for synchronous multi-master replication, the performance
bottelneck is going to be the interconnect between the nodes -
integration of the replication logic into the backend most probably
doesn't affect performance that much.

I'd rather like to ask Jan what different needs for replication he
discovered so far. And how he came to the conclusion, that it's not
possible to provide a general solution.

My point for integration into the backend is flexibility: obviously the
replication code can influence the database much more from within the
backend than from the outside. For example running one complex query on
several nodes. I know, this a very advanced feature - currently it's not
even possible to run one query on multiple backends (i.e. processors of
a multi-core system) - but I like to plan ahead instead of throwing away
code later. For such advanced features you simply have to dig around in
the backend code one day. Of course you can always add hooks, but IMHO
that only complicates matters.

Is there some discussion going on about such topics somewhere? What's up
with slony-2? The wiki on slony2.org still doesn't provide a lot of
technical information (and obviously got spammed BTW).

Regards
Markus

On 12/6/2005 8:10 AM, Markus Schiltknecht wrote:

> On Tue, 2005-12-06 at 10:03 -0200, Gustavo Tonini wrote:
>> But,  wouldn't the performance be better? And wouldn't asynchronous
>> messages be better processed?
> 
> At least for synchronous multi-master replication, the performance
> bottelneck is going to be the interconnect between the nodes -
> integration of the replication logic into the backend most probably
> doesn't affect performance that much.

That is exactly right. Thus far, processor, memory and disk speeds have 
allways advanced on a higher pace than network speeds. Thus, the few 
percent of performance gain we'd get from moving things into the backend 
will be irrelevant tomorrow with 4x-core and 16x-core CPU's.

> I'd rather like to ask Jan what different needs for replication he
> discovered so far. And how he came to the conclusion, that it's not
> possible to provide a general solution.
  - Asynchronous master to multi-slave. We have a few of those with    Mommoth-Replicator and Slony-I being the top
players.Slony-I does    need some cleanup and/or reimplementation after we have a general    pluggable replication API
inplace.
 
  - Synchronous multimaster. There are certain attempts out there, like    Postgres-R, pgcluster, Slony-II. Some more
advanced,some less. But    certainly nothing I would send into the ring against Oracle-Grid.
 
  - Asynchronous multimaster with conflict resolution. I have not seen    any reasonable attempt on this one yet. Plus,
itdivides again into    two camps. One is the idea to have one central system with thousands    of satellites (salesman
onthe street), the other being two or more    central systems doing load balancing (although this competes with
sync-mm).

> My point for integration into the backend is flexibility: obviously the
> replication code can influence the database much more from within the

We need a general API. It should be possible to define on a per-database 
level which shared replication module to load on connect. The init 
function of that replication module then installs all the required 
callbacks at strategic points (like heap_update(), at_commit() ...) and 
the rest is hidden in the module.

> Is there some discussion going on about such topics somewhere? What's up
> with slony-2? The wiki on slony2.org still doesn't provide a lot of
> technical information (and obviously got spammed BTW).

Slony-II has been slow lately in the Eastern timezone.


Jan

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# It's easier to get forgiveness for being wrong than for being right. #
# Let's break this rule - forgive me.                                  #
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Hello Jan,

On Tue, 2005-12-06 at 10:10 -0500, Jan Wieck wrote:
> We need a general API. It should be possible to define on a per-database 
> level which shared replication module to load on connect. The init 
> function of that replication module then installs all the required 
> callbacks at strategic points (like heap_update(), at_commit() ...) and 
> the rest is hidden in the module.

thank you for your list of replication types. Those still have some
things in common. Thus your approach of providing hooks for different
modules might make sense. Only I fear that I would need way to many
hooks for what I want ;)

> Slony-II has been slow lately in the Eastern timezone.

What is that supposed to mean? Who sits in the eastern timezone?

Regards
Markus

gustavotonini@gmail.com (Gustavo Tonini) writes:
> But,  wouldn't the performance be better? And wouldn't asynchronous
> messages be better processed?

Why do you think performance would be materially affected by this?

The MAJOR performance bottleneck is normally the slow network
connection between servers.

When looked at in the perspective of that bottleneck, pretty much
everything else is just noise.  (Sometimes pretty loud noise, but
still noise :-).)
-- 
let name="cbbrowne" and tld="cbbrowne.com" in name ^ "@" ^ tld;;
http://cbbrowne.com/info/spreadsheets.html
"When the grammar checker identifies an error, it suggests a
correction and can even makes some changes for you."  
-- Microsoft Word for Windows 2.0 User's Guide, p.35:

IMO this is not true. You can get affordable 10GBit network adapters, so you can have plenty of bandwith in a db server
pool(if they are located in the same area). Even 1GBit Ethernet greatly helps here, and would make it possible to
balanceread-intensive (and not write intensive) applications. We using linux bonding interface with 2 gbit NICs, and
200MBytes/sec throughput is something you need to have a quite some harddisks to reach that. Latency is not bad too. 

Regards,
Mario weilguni


-----Original Message-----
From: pgsql-hackers-owner@postgresql.org [mailto:pgsql-hackers-owner@postgresql.org] On Behalf Of Chris Browne
Sent: Tuesday, December 06, 2005 4:43 PM
To: pgsql-hackers@postgresql.org
Subject: Re: [HACKERS] Replication on the backend

gustavotonini@gmail.com (Gustavo Tonini) writes:
> But,  wouldn't the performance be better? And wouldn't asynchronous
> messages be better processed?

Why do you think performance would be materially affected by this?

The MAJOR performance bottleneck is normally the slow network
connection between servers.

When looked at in the perspective of that bottleneck, pretty much
everything else is just noise.  (Sometimes pretty loud noise, but
still noise :-).)
--
let name="cbbrowne" and tld="cbbrowne.com" in name ^ "@" ^ tld;;
http://cbbrowne.com/info/spreadsheets.html
"When the grammar checker identifies an error, it suggests a
correction and can even makes some changes for you."
-- Microsoft Word for Windows 2.0 User's Guide, p.35:

---------------------------(end of broadcast)---------------------------
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>      Postgres-R, pgcluster, Slony-II. Some more advanced, some less. But
>      certainly nothing I would send into the ring against Oracle-Grid.

Assuming that you mean Oracle Real Application Cluster (the Grid is more, 
right?) I wonder if this technology technically still counts as replication. 
AFAIK they do not replicate data but share a common data pool among different 
servers. You still have communication overhead but you write a tuple only 
once for all servers involved. Takes away a lot of overhead on a system 
that's heavily written too.

Michael
-- 
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Email: Michael at Fam-Meskes dot De, Michael at Meskes dot (De|Com|Net|Org)
ICQ: 179140304, AIM/Yahoo: michaelmeskes, Jabber: meskes@jabber.org
Go SF 49ers! Go Rhein Fire! Use Debian GNU/Linux! Use PostgreSQL

Just like MySql!

On Dec 5, 2005, at 10:35 PM, Jan Wieck wrote:

> On 12/5/2005 8:18 PM, Gustavo Tonini wrote:
>
>> replication (master/slave, multi-master, etc) implemented inside
>> postgres...I would like to know what has been make in this area.
>
> We do not plan to implement replication inside the backend.  
> Replication needs are so diverse that pluggable replication support  
> makes a lot more sense. To me it even makes more sense than keeping  
> transaction support outside of the database itself and add it via  
> pluggable storage add-on.
>
>
> Jan
>
>
>> Gustavo.
>> P.S. Sorry for my bad English.
>> 2005/12/5, Chris Browne <cbbrowne@acm.org>:
>>>
>>> gustavotonini@gmail.com (Gustavo Tonini) writes:
>>> > What about replication or data distribution inside the  
>>> backend.  This
>>> > is a valid issue?
>>>
>>> I'm not sure what your question is...
>>> --
>>> (reverse (concatenate 'string "gro.gultn" "@" "enworbbc"))
>>> http://www.ntlug.org/~cbbrowne/x.html
>>> "Love is like a snowmobile flying over the frozen tundra that  
>>> suddenly
>>> flips, pinning you underneath.  At night, the ice weasels come."
>>> -- Matt Groening
>>>
>>> ---------------------------(end of  
>>> broadcast)---------------------------
>>> TIP 1: if posting/reading through Usenet, please send an appropriate
>>>        subscribe-nomail command to majordomo@postgresql.org so  
>>> that your
>>>        message can get through to the mailing list cleanly
>>>
>
>
> -- 
> #===================================================================== 
> =#
> # It's easier to get forgiveness for being wrong than for being  
> right. #
> # Let's break this rule - forgive  
> me.                                  #
> #==================================================  
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>

>   - Asynchronous master to multi-slave. We have a few of those with
>     Mommoth-Replicator and Slony-I being the top players. Slony-I does
>     need some cleanup and/or reimplementation after we have a general
>     pluggable replication API in place.

Is this API actually have people working on it or just something on  
the todo list?

I don't see anything in the TODO list. I'm very interesting in work that. If is possible...<br /><br /> Gustavo.<br />

I would classify it as a clustered database system (Oracle 10g that is).
Clustered meaning more than one node in the cluster.

ALy.

On Tue, 6 Dec 2005, Michael Meskes wrote:

>>      Postgres-R, pgcluster, Slony-II. Some more advanced, some less. But
>>      certainly nothing I would send into the ring against Oracle-Grid.
>
>Assuming that you mean Oracle Real Application Cluster (the Grid is more,
>right?) I wonder if this technology technically still counts as replication.
>AFAIK they do not replicate data but share a common data pool among different
>servers. You still have communication overhead but you write a tuple only
>once for all servers involved. Takes away a lot of overhead on a system
>that's heavily written too.
>
>Michael
>

-- 
Aly S.P Dharshi
aly.dharshi@telus.net
 "A good speech is like a good dress  that's short enough to be interesting  and long enough to cover the subject"

On 12/6/2005 11:23 AM, Mario Weilguni wrote:

> IMO this is not true. You can get affordable 10GBit network adapters, so you can have plenty of bandwith in a db
serverpool (if they are located in the same area). Even 1GBit Ethernet greatly helps here, and would make it possible
tobalance read-intensive (and not write intensive) applications. We using linux bonding interface with 2 gbit NICs, and
200MBytes/sec throughput is something you need to have a quite some harddisks to reach that. Latency is not bad too.
 

It's not so much the bandwidth but more the roundtrips that limit your 
maximum transaction throughput. Remember, whatever the priority, you 
can't increase the speed of light.


Jan


> 
> Regards,
> Mario weilguni
> 
> 
> -----Original Message-----
> From: pgsql-hackers-owner@postgresql.org [mailto:pgsql-hackers-owner@postgresql.org] On Behalf Of Chris Browne
> Sent: Tuesday, December 06, 2005 4:43 PM
> To: pgsql-hackers@postgresql.org
> Subject: Re: [HACKERS] Replication on the backend
> 
> gustavotonini@gmail.com (Gustavo Tonini) writes:
>> But,  wouldn't the performance be better? And wouldn't asynchronous
>> messages be better processed?
> 
> Why do you think performance would be materially affected by this?
> 
> The MAJOR performance bottleneck is normally the slow network
> connection between servers.
> 
> When looked at in the perspective of that bottleneck, pretty much
> everything else is just noise.  (Sometimes pretty loud noise, but
> still noise :-).)


-- 
#======================================================================#
# It's easier to get forgiveness for being wrong than for being right. #
# Let's break this rule - forgive me.                                  #
#================================================== JanWieck@Yahoo.com #

On 12/6/05, Jan Wieck <JanWieck@yahoo.com> wrote:
> > IMO this is not true. You can get affordable 10GBit network adapters, so you can have plenty of bandwith in a db
serverpool (if they are located in the same area). Even 1GBit Ethernet greatly helps here, and would make it possible
tobalance read-intensive (and not write intensive) applications. We using linux bonding interface with 2 gbit NICs, and
200MBytes/sec throughput is something you need to have a quite some harddisks to reach that. Latency is not bad too. 
>
> It's not so much the bandwidth but more the roundtrips that limit your
> maximum transaction throughput. Remember, whatever the priority, you
> can't increase the speed of light.

Eh, why would light limited delay be any slower than a disk on FC the
same distance away? :)

In any case, performance of PG on iscsi is just fine. You can't blame
the network... Doing multimaster replication is hard because the
locking primitives that are fine on a simple multiprocessor system
(with a VERY high bandwidth very low latency interconnect between
processors) just don't work across a network, so you're left finding
other methods and making them work...

But again, multimaster isn't hard because there of some inherently
slow property of networks.

On Tue, 2005-12-06 at 23:19 -0500, Jan Wieck wrote:
> It's not so much the bandwidth but more the roundtrips that limit your 
> maximum transaction throughput. 

I completely agree that the latency is counting, not the bandwith.

Does anybody have latency / roundtrip measurements for current hardware?
I'm interested in:1Gb Ethernet,10 Gb Ethernet,InfiniBand,probably even p2p usb2 or firewire links?

At least Quadrics claims(1) to have measured only 1.38 microseconds.
Assuming real world condition would give you 5 microseconds, on a 3 GHz
processor that's 15'000 CPY cycles. Which is IMHO not that much any
more. Or am I wrong (mental arithmetic never was my favourite subject)?

RegardsMarkus

[1]
http://www.quadrics.com/quadrics/QuadricsHome.nsf/NewsByDate/98FFE60F799AC95180256FEA002A6D9D

On Wed, 2005-12-07 at 01:04 -0800, J. Andrew Rogers wrote:
> Opteron boards get pretty damn close to Big Iron SMP fabric  
> performance in a cheap package.  Given how many companies have  
> announced plans to produce Opteron server boards with Infiniband  
> fabrics directly integrated into HyperTransport, I would say that  
> this is the future of server boards.

InfiniBand on-board? Wow, seems very interesting. Thank you for your
hints and numbers, very helpfull!

> And if postgres could actually use an infiniband fabric for  
> clustering a single database instance across Opteron servers, that  
> would be very impressive...

full ACK

Regards
Markus

On Dec 6, 2005, at 9:09 PM, Gregory Maxwell wrote:
> Eh, why would light limited delay be any slower than a disk on FC the
> same distance away? :)
>
> In any case, performance of PG on iscsi is just fine. You can't blame
> the network... Doing multimaster replication is hard because the
> locking primitives that are fine on a simple multiprocessor system
> (with a VERY high bandwidth very low latency interconnect between
> processors) just don't work across a network, so you're left finding
> other methods and making them work...


Speed of light latency shows up pretty damn often in real networks,  
even relatively local ones.  The number of people that wonder why a  
transcontinental SLA of 10ms is not possible is astonishing.  The  
silicon fabrics are sufficiently fast that most well-designed  
networks are limited by how fast one can push photons through a  
fiber, which is significantly slower than photons through a vacuum.   
Silicon switch fabrics add latency measured in nanoseconds, which is  
effectively zero for many networks that leave the system board.

Compared to single system simple SMP, a local cluster built on a  
first-rate fabric will have about an order of magnitude higher  
latency but very similar bandwidth.  On the other hand, at those  
latencies you can increase the number of addressable processors with  
that kind of bandwidth by an order of magnitude, so it is a bit of a  
trade.  However, latency matters a lot such that one would have to be  
a lot smarter about partitioning synchronization across that fabric  
even though one would lose nothing in the bandwidth department.


> But again, multimaster isn't hard because there of some inherently
> slow property of networks.


Eh?  As far as I know, the difficulty of multi-master is almost  
entirely a product of the latency of real networks such that they are  
too slow for scalable distributed locks.  SMP is little more than a  
distributed lock manager implemented in silicon.  Therefore, multi- 
master is hard in practice because we cannot drive networks fast  
enough.  That said, current state-of-the-art network fabrics are  
within an order of magnitude of SMP fabrics such that they could be  
real contenders, particularly once you get north of 8-16 processors.

The really sweet potential is in Opteron system boards with  
Infiniband directly attached to HyperTransport.  At that level of  
bandwidth and latency, both per node and per switch fabric, the  
architecture possibilities start to become intriguing.


J. Andrew Rogers

On Dec 6, 2005, at 11:42 PM, Markus Schiltknecht wrote:
> Does anybody have latency / roundtrip measurements for current  
> hardware?
> I'm interested in:
>     1Gb Ethernet,
>     10 Gb Ethernet,
>     InfiniBand,
>     probably even p2p usb2 or firewire links?


In another secret life, I know a bit about supercomputing fabrics.   
The latency metrics have to be thoroughly qualified.

First, most of the RTT latency numbers for network fabrics are for 0  
byte packet sizes, which really does not apply to anyone shuffling  
real data around.  For small packets, high-performance fabrics (HTX  
Infiniband, Quadrics, etc) have approximately an order of magnitude  
less latency than vanilla Ethernet, though the performance specifics  
depend greatly on the actual usage.  For large packet sizes, the  
differences in latency become far less obvious.  However, for "real"  
packets a performant fabric will still look very good compared to  
disk systems.  Switched fiber fabrics have enough relatively  
inexpensive throughput now to saturate most disk systems and CPU I/O  
busses; only platforms like HyperTransport can really keep up.  It is  
worth pointing out that the latency of high-end network fabrics is  
similar to large NUMA fabrics, which exposes some of the limits of  
SMP scalability.  As a point of reference, an organization that knows  
what they are doing should have no problem getting 500 microsecond  
RTT on a vanilla metropolitan area GigE fiber network -- a few  
network operators actually do deliver this on a regional scale.  For  
a local cluster, a competent design can best this by orders of  
magnitude.

There are a number of silicon limitations, but a system that connects  
the fabric directly to HyperTransport can drive several GB/s with  
very respectable microsecond latencies if the rest of the system is  
up to it.  There are Opteron system boards now that will drive  
Infiniband directly from HyperTransport.  I know Arima/Rioworks makes  
some (great server boards generally), and several other companies are  
either making them or have announced them in the pipeline.  These  
Opteron boards get pretty damn close to Big Iron SMP fabric  
performance in a cheap package.  Given how many companies have  
announced plans to produce Opteron server boards with Infiniband  
fabrics directly integrated into HyperTransport, I would say that  
this is the future of server boards.

And if postgres could actually use an infiniband fabric for  
clustering a single database instance across Opteron servers, that  
would be very impressive...

J. Andrew Rogers

Andrew,
> And if postgres could actually use an infiniband fabric for
> clustering a single database instance across Opteron servers, that
> would be very impressive...

That's what we do with Bizgres MPP.  We've implemented an interconnect to do the data shuffling underneath the
optimizer/executorand we currently use TCP/IP, though we could haul out SDP over Infiniband should we need it. 
However, our optimizer effectively minimizes traffic over the interconnect now and that works well for all of the plans
we'verun so far.  It would be nice to characterize the improvements we could get from moving to 3x infiniband. 
Regarding a direct Hypertransport to Infiniband bridge, have you looked at Pathscale? http://www.pathscale.com/  I know
thefellow behind the scenes who designed it, and I think it's probably well thought out.  We were gunning for less than
1usRTT through the adapter and switch once, and I bet they are close. 
- Luke

On Tue, Dec 06, 2005 at 12:35:43AM -0500, Jan Wieck wrote:
> We do not plan to implement replication inside the backend. Replication 
> needs are so diverse that pluggable replication support makes a lot more 
> sense. To me it even makes more sense than keeping transaction support 
> outside of the database itself and add it via pluggable storage add-on.

And, as I say every single time this comes up, Oracle's and IBM's and
MS's and everybody else's replication systems are _also_ add ons.  If
you don't believe me, look at the license costs.  You can get a
system without it enabled, which means (by definition) it's a modular
extension.

A

-- 
Andrew Sullivan  | ajs@crankycanuck.ca
In the future this spectacle of the middle classes shocking the avant-
garde will probably become the textbook definition of Postmodernism.                --Brad Holland

What is the point of these questions? If you have a concrete, practical 
proposal to make, please do so. Otherwise, you have already got the 
answer from the people who are actually working on replication and 
understand it far beyond abstract considerations. If you think there is 
a good reason to do replication directly in the backend code rather than 
as an addon, possibly using an agreed API, then you need to provide hard 
evidence, not mere assertion or conjecture.

cheers

andrew

Gustavo Tonini wrote:

> Are you sure that no way to implement a generic aproach on the 
> backend? What does specification say? Does Oracle 10g have a core 
> implementation of replication (cluster)?
>
> Gustavo.
>
>
> 2005/12/7, Andrew Sullivan <ajs@crankycanuck.ca 
> <mailto:ajs@crankycanuck.ca>>:
>
>     On Tue, Dec 06, 2005 at 12:35:43AM -0500, Jan Wieck wrote:
>     > We do not plan to implement replication inside the backend.
>     Replication
>     > needs are so diverse that pluggable replication support makes a
>     lot more
>     > sense. To me it even makes more sense than keeping transaction
>     support
>     > outside of the database itself and add it via pluggable storage
>     add-on.
>
>     And, as I say every single time this comes up, Oracle's and IBM's and
>     MS's and everybody else's replication systems are _also_ add ons.  If
>     you don't believe me, look at the license costs.  You can get a
>     system without it enabled, which means (by definition) it's a modular
>     extension.
>
>     A
>
>     --
>     Andrew Sullivan  | ajs@crankycanuck.ca <mailto:ajs@crankycanuck.ca>
>     In the future this spectacle of the middle classes shocking the avant-
>     garde will probably become the textbook definition of Postmodernism.
>                     --Brad Holland
>
>     ---------------------------(end of
>     broadcast)---------------------------
>     TIP 9: In versions below 8.0, the planner will ignore your desire to
>            choose an index scan if your joining column's datatypes do not
>            match
>
>

On 12/8/2005 1:28 PM, Gustavo Tonini wrote:

> Are you sure that no way to implement a generic aproach on the backend? What

You mean "generic" as in a replication system that can do asynchronous 
master-slave, asynchronous multimaster with conflict resolution based on 
timestamps, system priority or user defined resolution stubs, can do 
synchronous predicate locking but also does support thousands of 
asynchronous, partial replica (salesman on the road), and last but not 
least can run as a synchronous cluster in a LAN segment. All the above 
can of course be mixed together ... like a central cluster of 8 load 
balanced, fault tolerant systems, with async multimaster replica in all 
external branch servers, partial multimaster replica for the road 
warriers and some slave leaf nodes for reporting.

If you can present a prototype for the above, I am sure that we will 
change our opinion and finally settle for one, builtin replication system.

> does specification say? Does Oracle 10g have a core implementation of
> replication (cluster)?

What specification?


Jan

-- 
#======================================================================#
# It's easier to get forgiveness for being wrong than for being right. #
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> Are you sure that no way to implement a generic aproach on the
> backend? What does specification say?

What specification are you talking about?

> Does Oracle 10g have a core implementation of replication (cluster)?

Since replication is sold as a separate product from Oracle 10g,
obviously not.

We *know* (particularly those of us that have had involvement in
actually implementing replication systems used in production
environments) that "user space" implementations of replication can
function satisfactorily.  We've implemented it.

You might be able to convince us that implementing replication inside
the backend is preferable, but you'll have to have exceedingly strong
evidence in order to overcome the already known factor that doing it
outside the backend works quite well.
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Hello,

On Fri, 2005-12-09 at 08:47 -0500, Christopher Browne wrote:
> We *know* (particularly those of us that have had involvement in
> actually implementing replication systems used in production
> environments) that "user space" implementations of replication can
> function satisfactorily.  We've implemented it.

While this might be true, allow me a sidenote: AFAIK the very first,
functional prototype we know of was Postgres-R for PostgreSQL 6.4.2 (1).
So the very same holds true for a replication solution integrated into
the backend: we know such an implementation can function satisfactorily.

As we mostly agree, the performance bottelneck is _not_ the CPU, but the
nodes interconnects (the network). Regarding communication between the
backends and the replication solution, performance isn't that much of an
issue, because the inter-node communication will allways be slower than
inter-process communication.

A different problem is how to distribute PostgreSQL with different
upcomming replication solutions. It seems to me that most people's main
concern is not being able to get a prebuilt PostgreSQL with
_just_one_replication_solution_that_works_(tm) For most users it really
doesn't matter _how_ exactly the solution technically got integrated. 

This problem gets solved with hooks and preloading a library: you could
simply provide _one_ PostgreSQL package which provides hooks for
replication solutions. Those could then provide a package with their
library. This of course is only doable if the number of hooks is kept
low.

Regards
Markus


[1] pgreplication project on gborg:
http://gborg.postgresql.org/project/pgreplication/projdisplay.php