Re: Horizontal scalability/sharding - Mailing list pgsql-hackers

On 30 August 2015 at 03:17, Bruce Momjian <bruce@momjian.us> wrote:

I have recently increased my public statements about the idea of adding
horizontal scaling/sharding to Postgres.

Glad to see it. Many people have been pushing such things for years, so it is good to finally see some debate about this on Hackers.

 

I wanted to share with hackers
a timeline of how we got here, and where I think we are going in the
short term:

2012-2013:  As part of writing my scaling talk
(http://momjian.us/main/presentations/overview.html#scaling), studying
Oracle RAC, and talking to users, it became clear that an XC-like
architecture (sharding) was the only architecture that was going to allow
for write scaling.

What other architectures were discussed? Where was that discussion?

 

Users and conference attendees I talked to were increasingly concerned
about the ability of Postgres to scale for high write volumes.  They didn't
necessarily need that scale now, but they needed to know they could get
it if they wanted it, and wouldn't need to switch to a new database in
the future.  This is similar to wanting a car that can get you on a highway
on-ramp fast --- even if you don't need it, you want to know it is there.

+1

 

2014:  I started to shop around the idea that we could use FDWs,
parallelism, and a transaction/snapshot manager to get XC features
as built-in to Postgres.  (I don't remember where the original idea
came from.)  It was clear that having separate forks of the source code
in XC and XL was never going to achieve critical mass --- there just
aren't enough people who need high right scale right now, and the fork
maintenance overhead is a huge burden.

I personally support the view that we should put scalability features into Postgres core, rather than run separate forks.

 

I realized that we would never get community acceptance to dump the XC
(or XL) code needed for sharding into community Postgres

How or why did you realize that? There has never been any such discussion, AFAIK. Surely it can be possible to move required subsystems across?

 

, but with FDWs,
we could add the features as _part_ of improving FDWs, which would benefit
FDWs _and_ would be useful for sharding.  (We already see some of those
FDW features in 9.5.)

That is a huge presumption. Not discussed or technically analyzed in any way with the community.

 

October, 2014:  EDB and NTT started working together in the community
to start improving FDWs as a basis for an FDW-based sharding solution.
Many of the 9.5 FDW improvements that also benefit sharding were developed
by a combined EDB/NTT team.  The features improved FDWs independent of
sharding, so they didn't need community buy-in on sharding to get them
accepted.

June, 2015:  I attended the PGCon sharding unconference session and
there was a huge discussion about where we should go with sharding.
I think the big take-away was that most people liked the FDW approach,
but had business/customer reasons for wanting to work on XC or XL because
those would be production-ready faster.

Cough, cough. You must surely be joking that "most people liked the FDW approach"? How did we measure the acceptance of this approach? 

What actually is the FDW approach? Since its not been written down anywhere, or even explained verbally, how can anyone actually agree to it?

 

July, 2015:  Oleg Bartunov and his new company Postgres Professional (PP)
started to think about joining the FDW approach, rather than working on
XL, as they had stated at PGCon in June.  A joint NTT/EDB/PP phone-in
meeting is scheduled for September 1.

 

August, 2015:  While speaking at SFPUG, Citus Data approached me about
joining the FDW sharding team.  They have been invited to the September
1 meeting, as have the XC and XL people.

2ndQuadrant is working in this area, specifically bringing XL 9.5 forwards. Please can invites be posted to myself, Pavan Deolasee and Petr Jelinek also? I'll pass on to others also.

Koichi Suzuki is arranging a meeting in Hong Long for XC/XL discussions. Presumably EDB is invited also? If Koichi is a leading organizer of this, why are there two meetings?

October, 2015:  EDB is sponsoring a free 3-hour summit about FDW sharding
at the PG-EU conference in Vienna.   Everyone is invited, but it is hoped
most of the September 1 folks can attend.

 

February, 2016:  Oleg is planning a similar meeting at their February
Moscow conference.

 

Anyway, I wanted to explain the work that has been happening around
sharding.

Thanks 

  

As things move forward, I am increasingly convinced that write
scaling will be needed soon,

+1

 

that the XC approach is the only reasonable way to do it,

 

and that FDWs are the cleanest way to get it into community
Postgres.

Those two things aren't at all obvious to me.

Please don't presume my opposition. If the technical information were made public, I might understand and agree with "the FDW approach", perhaps others also. 2ndQuadrant is certainly happy to become involved in any team aiming to add features to Postgres core, as long as that makes sense. There may be areas we can all agree upon even if the full architecture remains in doubt.

Before the community commits to a long term venture together we should see the plan. Like all IT projects, expensive failure is possible and the lack of a design is a huge flashing red warning light for me at present. If that requires a meeting of all Developers, why are the meetings for this specifically not happening at the agreed Developer meetings?

--

Simon Riggs                http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services