Re: The plan for FDW-based sharding

On 01/03/16 18:18, Konstantin Knizhnik wrote:
>
> On 01.03.2016 19:03, Robert Haas wrote:
>> On Tue, Mar 1, 2016 at 10:37 AM, Bruce Momjian <bruce(at)momjian(dot)us> wrote:
>>> On Tue, Mar 1, 2016 at 10:19:45AM -0500, Robert Haas wrote:
>>>>> Two reasons:
>>>>> 1. There is no ideal implementation of DTM which will fit all
>>>>> possible needs
>>>>> and be efficient for all clusters.
>>>> Hmm, what is the reasoning behind that statement? I mean, it is
>>>> certainly true that there are some places where we have decided that
>>>> one-size-fits-all is not the right approach. Indexing, for example.
>>> Uh, is that even true of indexing? While the plug-in nature of indexing
>>> allows for easier development and testing, does anyone create plug-in
>>> indexing that isn't shipped by us? I thought WAL support was something
>>> that prevented external indexing solutions from working.
>> True. There is an API, though, and having pluggable WAL support seems
>> desirable too. At the same time, I don't think we know of anyone
>> maintaining a non-core index AM ... and there are probably good
>> reasons for that. We end up revising the index AM API pretty
>> regularly every time somebody wants to do something new, so it's not
>> really a stable API that extensions can just tap into. I suspect that
>> a transaction manager API would end up similarly situated.
>>
>
> IMHO non-stable API is better than lack of API.
> Just because it makes it possible to implement features in modular way.
> And refactoring of API is not so difficult thing...
>

Since this thread heavily discusses the XTM, I have question about the
XTM as proposed because one thing is very unclear to me - what happens
when user changes the XTM plugin on the server? I didn't see any xid
handover API which makes me wonder if changes of a plugin (or for
example failure to load previously used plugin due to admin error) will
send server to similar situation as xid wraparound.

--
Petr Jelinek http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services